INTRODUCTION
⌅Ceratoscopelus maderensis (Lowe, 1839) is a lanternfish species (Myctophidae) that is generally found in mesopelagic waters at depths ranging from 200 to 1000 m between 50ºN and 30°N in the North Atlantic Ocean (Hulley 1984Hulley P.A. 1984. Myctophidae. In: Whitehead P.J.P., Bauchot M.L., et al. (eds), Fishes of the North-eastern Atlantic and the Mediterranean. UNESCO. 1: 429-483.) and throughout the Mediterranean Sea (Jonsson 1992Jonsson G. 1992. Islenskir fiskar. Fiolvi. Reykjavik, Iceland. pp. 568., Cavallaro et al. 2019Cavallaro M., Battaglia P., Guerrera M.C., et al. 2019. Structure and ultrastructure study on photophores of the Madeira lanternfish, Ceratoscopelus maderensis (Lowe, 1839), Pisces: Myctophidae. Acta Zool. 100: 89-95. https://doi.org/10.1111/azo.12236 ). Its larval stages inhabit the epipelagic layer, mostly concentrated in the first 50 m of the water column. When the transformation (metamorphic) stage is reached, individuals begin to move to the mesopelagic zone (Kendall et al. 1984Kendall A. W., Ahlstrom E.H., Moser H.G. 1984. Early life history stages of fishes and their characters. In: Moser H.G., Richards W.J., Cohen D.M et al. (eds), Ontogeny and Systematics of Fishes, Am. Soc. Ichthyol. Herpetol. 1: 11-22., Richards 2005Richards W.J. 2005. Early stages of Atlantic fishes: An identification guide for the western central North Atlantic. Taylor & Francis, Boca Raton, FL. 1: 524-525. https://doi.org/10.1201/9780203500217 , Sassa et al. 2007Sassa C., Kawaguchi K., Hirota Y., et al. 2007. Distribution depth of the transforming stage larvae of myctophid fishes in the subtropical-tropical waters of the western North Pacific. Deep-Sea Res. I 54: 2181-2193. https://doi.org/10.1016/j.dsr.2007.09.006 ). From this period onward, like most myctophid species, C. maderensis acquires a nictoepipelagic behaviour that implies diel vertical migrations between ca. 1000 m depth and the surface (Hulley 1984Hulley P.A. 1984. Myctophidae. In: Whitehead P.J.P., Bauchot M.L., et al. (eds), Fishes of the North-eastern Atlantic and the Mediterranean. UNESCO. 1: 429-483., Mytilineou et al. 2005Mytilineou C., Politou C.Y., Papaconstantinou C., et al. 2005. Deep-water fish fauna in the Eastern Ionian Sea. Belg. J. Zool. 135: 229-233., Olivar et al. 2012Olivar M.P., Bernal A., Molí B. et al. 2012. Vertical distribution, diversity and assemblages of mesopelagic fishes in the western Mediterranean. Deep-Sea Res. I 62: 53-69. https://doi.org/10.1016/j.dsr.2011.12.014 ).
Ceratoscopelus maderensis is one of the most abundant myctophid species in the northeastern Atlantic Ocean and the Mediterranean Sea (Goodyear et al. 1972Goodyear R.H., Gibbs R.H.Jr., Roper C.F.E., et al. 1972. Mediterranean biological studies. Smithson. Institution Washington DC Rep. 1-278., Hulley 1981Hulley P.A. 1981. Results of the research cruises of FRV “Walther Herwig” to South America. LVIII. Family Myctophidae. Archiv. Fischwiss. 31: 1-300., Olivar et al. 2012Olivar M.P., Bernal A., Molí B. et al. 2012. Vertical distribution, diversity and assemblages of mesopelagic fishes in the western Mediterranean. Deep-Sea Res. I 62: 53-69. https://doi.org/10.1016/j.dsr.2011.12.014 ). As such, this species occupies a key position in the food-web structure of the mesopelagic community feeding on zooplankton (Bernal et al. 2015Bernal A. Olivar M.P., Maynou F., et al. 2015. Diet and feeding strategies of mesopelagic fishes in the western Mediterranean. Progr. Oceanogr. 135: 1-17. https://doi.org/10.1016/j.pocean.2015.03.005 ), which highlights its integral role in the functioning of oceanic ecosystems as an intermediate link between primary consumers and top predators (Gjøsaeter and Kawaguchi 1980Gjøsaeter J., Kawaguchi K.A. 1980. A review of the world resources of mesopelagic fish. FAO Fisheries Technical Paper 193: 1-151., Sassa and Takahashi 2018Sassa C., Takahashi M. 2018. Comparative larval growth and mortality of mesopelagic fishes and their predatory impact on zooplankton in the Kuroshio region. Deep-Sea Res. I 131: 121-132. https://doi.org/10.1016/j.dsr.2017.11.007 , Anderson et al. 2019Anderson T.R., Martin A.P., Lampitt R.S., et al. 2019. Quantifying carbon fluxes from primary production to mesopelagic fish using a simple food web model. ICES J. Mar. Sci. 76: 690-701. https://doi.org/10.1093/icesjms/fsx234 ). Despite its important ecological role, the populations of C. maderensis remain one of the least well-studied components of marine ecosystems at both a regional and a global scale (St. John et al. 2016St. John M.A., Borja A., Chust G., et al. 2016. A dark hole in our understanding of marine ecosystems and their services: perspectives from the mesopelagic community. Front. Mar. Sci. 3: 31. https://doi.org/10.3389/fmars.2016.00031 ). Several studies have reported growth patterns in myctophid species (e.g. Gartner 1991aGartner J.V.Jr. 1991a. Life histories of three species of lanternfishes (Pisces: Myctophidae) from the eastern Gulf of Mexico. Mar. Biol. 111: 11-20. https://doi.org/10.1007/BF01986340 , Linkowski et al. 1993Linkowsky T.B., Radtke R.L., Lenz, P.H., 1993. Otolith microstructure, age and growth of two species of Ceratoscopelus (Osteichthyes: Myctophidae) from the eastern North Atlantic. Jour. Exp. Mar. Biol. Ecol. 167: 237-260. https://doi.org/10.1016/0022-0981(93)90033-K , Sassa et al. 2015Sassa C., Takahashi M., Tsukamoto Y. 2015. Distribution, hatch-date, growth, and mortality of larval Benthosema pterotum (Pisces: Myctophidae) in the shelf region of the East China Sea. J. Mar. Biol. Assoc. U.K. 95: 161-174. https://doi.org/10.1017/S0025315414001209 ), but despite their wide geographical distribution, there is still a lack of information on their growth rates and lifespan.
Accurate age determinations provide basic life-history information and are imperative for describing population dynamics at the species level. Most age-determination studies on myctophids are based on the counting of daily growth increments (Brothers et al. 1976Brothers E.B., Mathews C. P., Lasker R. 1976. Daily growth increments in otoliths from larval and adult fishes. Fish. Bull. 74: 1-8., Methot and Kramer 1981Methot R.D. Jr, Kramer D. 1981. Growth of northern anchovy, Engraulis mordax, and northern lampfish, Stenobrachius leucopsarus. Rapp. P-v. Rèun. Cons. Int. l’Explor. Mer 178: 424-431., Gjøsaeter 1987Gjøsaeter H. 1987. Primary growth increments in otoliths of six tropical myctophid species. Biol. Oceanogr. 4: 359-382.) or seasonal growth increments in sagittae otoliths (Sarmiento et al. 2018Sarmiento-Lezcano A., Triay-Portella R., Castro J.J., et al. 2018. Age-based life-history parameters of the mesopelagic fish Notoscopelus resplendens (Richardson, 1845) in the Central Eastern Atlantic. Fish. Res. 204: 412-423. https://doi.org/10.1016/j.fishres.2018.03.016 ), length-based analysis (Ricker 1975Ricker W.E. 1975. Computation and interpretation of biological statistics of fish populations. Bull. Fish. Res. Board Can. 191: 207-211. and references therein, Harvey et al. 2000Harvey J.T., Loughlin T.R., Perez M.A., et al. 2000. Relationship between fish size and otolith length for 63 species of fishes from the eastern North Pacific Ocean. NOAA/National Marine Fisheries Service, Seattle. NOAA Tech. Rep. NMFS 150: 1-36.) or a combination of these methods (Aguilar-Perera and Quijano-Puerto 2016Aguilar-Pereira A., Quijano-Puerto L. 2016. Relations between fish length to weight, and otolith length and weight, of the lionfish Pterois volitans in the Parque Nacional Arrecife Alacranes, southern Gulf of Mexico. Rev. Biol. Mar. Oceanogr. 51: 469-474. https://doi.org/10.4067/S0718-19572016000200025 , Saunders et al. 2020Saunders R.A., Lourenço S., Vieira R.P., et al. 2020. Age and growth of Brauer’s lanternfish Gymnoscopelus braueri and rhombic lanternfish Krefftichthys anderssoni (Family Myctophidae) in the Scotia Sea, Southern Ocean. J. Fish. Biol.96:364-377. https://doi.org/10.1111/jfb.14206 ). Microincrements in the otoliths of myctophids appear to be equivalent to daily rings that have been validated in other myctophids directly by considering their formation over periods of 24 hours (Gartner 1991aGartner J.V.Jr. 1991a. Life histories of three species of lanternfishes (Pisces: Myctophidae) from the eastern Gulf of Mexico. Mar. Biol. 111: 11-20. https://doi.org/10.1007/BF01986340 , bGartner J.V.Jr. 1991b. Life histories of three species of lanternfishes (Pisces: Myctophidae) from the eastern Gulf of Mexico (II). Age and growth patterns. Mar. Biol. 111: 21-27. https://doi.org/10.1007/BF01986340 , Moku et al. 2005Moku M., Hayashi A., Mori K., et al. 2005. Validation of daily otolith increment formation in the larval myctophid fish Diaphus slender-type spp. J. Fish Biol. 67: 1481-1485. https://doi.org/10.1111/j.0022-1112.2005.00824.x ) and indirectly by the back-calculation of birth dates from the age, date of capture and timing with their spawning season (Young et al. 1988Young J.W., Bulman C.M., Blaber S.J.M., et al. 1988. Age and growth of the lanternfish Lampanyctodes hectoris (Myctophidae) from eastern Tasmania, Australia. Mar. Biol. 99: 569-576. https://doi.org/10.1007/BF00392564 ). Therefore, most studies about the growth and age of myctophids have assumed a daily micro-increment deposition (Greely et al. 1999Greely T.M., Gartner J.V.Jr., J.J. Torres. 1999. Age and growth of Electrona Antarctica (Pisces: Myctophidae), the dominant mesopelagic fish of the Southern Ocean. Mar. Biol. 133: 145-158. https://doi.org/10.1007/s002270050453 , Hayashi et al. 2001Hayashi A., Kawaguchi K., Watanabe H., et al. 2001. Daily growth increment formation and its lunar periodicity in otoliths of the myctophid fish Myctophum asperum (Pisces: Myctophidae). Fish. Sci. 67: 811-817. https://doi.org/10.1046/j.1444-2906.2001.00327.x , Wang et al. 2018Wang Y., Zhang J., Chen Z., et al. 2018. Age and growth of Myctophum asperum in the South China Sea based on otolith microstructure analysis. Deep-Sea Res. II 167: 121-127. https://doi.org/10.1016/j.dsr2.2018.07.004 ).
Linkowsky et al. (1993)Linkowsky T.B., Radtke R.L., Lenz, P.H., 1993. Otolith microstructure, age and growth of two species of Ceratoscopelus (Osteichthyes: Myctophidae) from the eastern North Atlantic. Jour. Exp. Mar. Biol. Ecol. 167: 237-260. https://doi.org/10.1016/0022-0981(93)90033-K investigated the growth patterns of adult individuals of C. maderensis from the northeast Atlantic Ocean. However, the age and growth patterns of this species during its larval stage remained unknown so far, and no studies have been conducted in either early stages or adults from the Mediterranean Sea. The Mediterranean Sea is characterized by the near-constant water temperature below the thermocline (100m depth) at ca.13°C (Olivar et al. 2012Olivar M.P., Bernal A., Molí B. et al. 2012. Vertical distribution, diversity and assemblages of mesopelagic fishes in the western Mediterranean. Deep-Sea Res. I 62: 53-69. https://doi.org/10.1016/j.dsr.2011.12.014 , Houpert et al. 2015Houpert L., Testor P., Durrieu de Madron X., et al. 2015. Seasonal cycle of the mixed layer, the seasonal thermocline and the upper-ocean heat storage rate in the Mediterranean Sea derived from observations. Prog. Oceanogr. 132: 333-352. https://doi.org/10.1016/j.pocean.2014.11.004 ) and its relative oligotrophy (Estrada 1985Estrada M. 1985. Primary production at the deep chlorophyll maximum in the western Mediterranean. In: Gibbs P.E. (ed): Proc. Nineteenth Eur. Mar. Biol. Symp., pp. 135-143. https://doi.org/10.1007/978-1-4899-2248-9_12 , Morel and Andre 1991Morel A., André J.M. 1991. Pigment Distribution and Primary Production in the western Mediterranean as derived and modelled from coastal zone color scanner observations. J. Geophys. Res., 96 C7: 12685-12698. https://doi.org/10.1029/91JC00788 ). This contrasts with the colder and more productive waters of the northeastern Atlantic Ocean, where temperatures range between 8°C and 18°C from 500 m depth to the surface (Emery and Meincke 1986Emery W.J., Meincke J. 1986. Global water masses: summary and review. Oceanol. Acta, 9: 383-391.). This could lead to dissimilar growth patterns in C. maderensis between the two biomes. Hulley (1984)Hulley P.A. 1984. Myctophidae. In: Whitehead P.J.P., Bauchot M.L., et al. (eds), Fishes of the North-eastern Atlantic and the Mediterranean. UNESCO. 1: 429-483. showed that, in general, the maximum body size reached by mesopelagic fish species in the Atlantic Ocean was larger than that in the western Mediterranean Sea. However, several studies have also shown that higher temperature regimes can increase both otolith (Rountrey et al. 2014Rountrey A.N., Coulson P.G., Meeuwig J.J., et al. 2014. Water temperature and fish growth: otoliths predict growth patterns of a marine fish in a changing climate. Glob. Change Biol. 20: 2450-2458. https://doi.org/10.1111/gcb.12617 ) and fish growth (Handeland et al. 2008Handeland S.O., Imsland A.K., Stefansson S.O. 2008. The effect of temperature and fish size on growth, feed intake, food conversion efficiency and stomach evacuation rate of Atlantic salmon post-smolts. Aquaculture. 283: 36-42. https://doi.org/10.1016/j.aquaculture.2008.06.042 , Silva et al. 2008Silva A., Carrera P., Massé J., et al. 2008. Geographical variability of sardine growth across the northeastern Atlantic and the Mediterranean Sea. Fish. Res. 90: 56-69. https://doi.org/10.1016/j.fishres.2007.09.011 ). Further genetic studies on the populations of C. maderensis might confirm whether the species separated into two different stocks, one in the Atlantic Ocean and one in the Mediterranean Sea, with a biogeographical frontier in the Alboran Sea, as it has been demonstrated for many other fish species (Naciri et al. 1999Naciri M., Lemaire C., Borsa P. et al. 1999. Genetic study of the Atlantic/Mediterranean transition in sea bass (Dicentrarchus labrax), J.Hered. 90: 591-596. https://doi.org/10.1093/jhered/90.6.591 ).
The aims of the present study are i) to estimate the age and growth patterns of C. maderensis in the western Mediterranean Sea through the analysis of sagittae otoliths considering most of its entire lifecycle, and ii) to compare the growth patterns of this species in the western Mediterranean Sea with those reported by Linkowski et al. (1993)Linkowsky T.B., Radtke R.L., Lenz, P.H., 1993. Otolith microstructure, age and growth of two species of Ceratoscopelus (Osteichthyes: Myctophidae) from the eastern North Atlantic. Jour. Exp. Mar. Biol. Ecol. 167: 237-260. https://doi.org/10.1016/0022-0981(93)90033-K for this species with individuals from the northeastern Atlantic Ocean. We expect that the warmer temperatures in the Mediterranean Sea will make this species grow faster than in the northeastern Atlantic Ocean.
METHODS
⌅Fish sampling
⌅Sampling was performed on board the research vessel Sarmiento de Gamboa during a cruise carried out in December 2009 (late autumn). The study area was located to the northwest and southwest of Mallorca (Balearic Islands, western Mediterranean Sea, Fig. 1) at the shelf-break (>200 m depth) and the median slope (700-900 m depth). A 280 m2 midwater trawl (10 mm mesh size in the cod-end) and a 3 m2 (3 mm mesh size) Isaacs- Kidd Midwater Trawl (IKMT) were employed to capture adult and juvenile fish. The depth where the fishing nets operated was controlled using a SCANMAR sensor. Fishing operations were performed both near the surface (40-80 m) and near the Deep Scattering Layer (DSL) at 400 m depth, where the echosounder recorded the highest acoustic signal at 18 and 38 kHz (Olivar et al. 2012Olivar M.P., Bernal A., Molí B. et al. 2012. Vertical distribution, diversity and assemblages of mesopelagic fishes in the western Mediterranean. Deep-Sea Res. I 62: 53-69. https://doi.org/10.1016/j.dsr.2011.12.014 ). The vessel speed was maintained at 4 knots for the large midwater trawl and 3 knots for the IKMT. After on-board fish identification using specific literature (Hulley 1981Hulley P.A. 1981. Results of the research cruises of FRV “Walther Herwig” to South America. LVIII. Family Myctophidae. Archiv. Fischwiss. 31: 1-300., 1984Hulley P.A. 1984. Myctophidae. In: Whitehead P.J.P., Bauchot M.L., et al. (eds), Fishes of the North-eastern Atlantic and the Mediterranean. UNESCO. 1: 429-483.), the specimens were frozen and stored at -20°C to prevent otolith damage. The water column temperature was obtained with the SBE 911plus CTD at each station.
The collection of fish larvae and transformation stages was carried out at the same stations with a 0.25 m2 Hydro-Bios MultiNet with 0.3 mm mesh size. Oblique tows at five discrete depths from 200 m to the surface were performed during a 24-h cycle (8 in daytime and 8 at night). Ichthyoplankton samples were preserved in 5% buffered formalin and sorted once in the laboratory. The pH was maintained at ca. 8 to prevent otolith degradation.
Fish larvae were sorted and identified in the laboratory using relevant literature (Tåning 1918Tåning A.V. 1918. Mediterranean Scopelidae: (Saurus, Aulopus, Chlorophthalmus, and Myctophum). Rep. Danish oceanogr. Exped. 1908-1910, 2(A7): 1-154., Moser and Watson 2006Moser H.G., Watson W. 2006. Myctophidae. In: Richards W.J. (eds), Early Stages of Atlantic Fishes: An identification guide for the western central North Atlantic. Taylor and Francis Group, U.S., pp. 473-589.). Larvae were ascribed to preflexion, flexion and postflexion stages (according to the urostyle bending), or transformation stages (according to photophore development) (Kendall et al. 1984Kendall A. W., Ahlstrom E.H., Moser H.G. 1984. Early life history stages of fishes and their characters. In: Moser H.G., Richards W.J., Cohen D.M et al. (eds), Ontogeny and Systematics of Fishes, Am. Soc. Ichthyol. Herpetol. 1: 11-22.), and were measured to 0.1 mm precision under a microscope employing an ocular micrometre.
Otolith preparation
⌅The standard length (SL) was measured before otolith extraction for 208 individuals, of which 45 were larvae (<16 mm SL), 49 were individuals at the transformation stage (17-20 mm SL), and the remaining 114 were juveniles and adults (>20 mm SL). For each individual, the left and right sagittae otoliths were extracted from the ear cavity. In the larval and transformation stages, the otolith extraction was performed using a fine needle, with the help of polarized light of the microscope for a better location of the otoliths. After drying, a drop of Cristal Bond 590TM thermolabile resin was poured over the otolith for further preservation. In the juveniles and adults, we first made a cross-sectional incision at the back of the fish head, near the rear of the operculum. Then, the two sagittae were extracted with forceps according to Secor et al. (1991)Secor D.H., Dean J.M., Laban E.H. 1991. Manual for otolith removal and preparation for microstructure examination. Baruch Inst. Mar. Biol. Coast. Res. 85 pp.. After the extraction, both otoliths were cleaned with a 5% solution of KOH to remove the attached organic tissue and then rinsed with water. The mounting and polishing were performed according to the methods described by Morales-Nin (1992)Morales-Nin B.1992. Determination of growth in bony fishes from otolith microstructure. FAO Fisheries Technical Paper. No. 322. Rome, FAO. 51pp. http://www.fao.org/3/t0529e/T0529E00.htm and Stevenson and Campana (1992)Stevenson D.K., Campana S.E. 1992. Otolith Microstructure examination and Analysis. Can. B. Fish. Aquat. Sci. 117: 1-126. https://doi.org/10.1007/BF00043932 . An otolith of each fish was mounted on a slide to polish its sagittae section, fixing the inner face with thermolabile resin.
The otolith length was defined from maximum diameter in larvae and from rostrum to posterior margin according to Tuset et al. (2008)Tuset V.M., Lombarte A., Assis C.A. 2008. Otolith atlas for the western Mediterranean, north and Central Eastern Atlantic. Sci. Mar. 72S1: 7-198. https://doi.org/10.1007/s10228-006-0346-2 . In the smallest larvae (5-7 mm SL), the transparency of the otoliths allowed us to count growth increments without need for polishing. Otoliths from the transformation, juvenile and adult stages were manually polished with grinding lapping papers (0.1-3 μm). In otoliths from larger specimens, the polishing process was interspersed with micrographs of the increments located in the margin of the otolith (the narrowest ones) to ensure the identification and the count of these increments and prevent any loss in the case of over-polishing. To increase transparency in thicker otoliths, the polishing process was carried out on both sides, taking care not to affect the maximum diameter of the otolith. Finally, the otoliths were ultra-polished with an abrasive consisting of a MicroCloth PSA 10/ PK 8 disc previously impregnated with an abrasive solution with 0.05 μm of alumina. Daily increments were counted from images obtained with a digital camera mounted on an optical microscope (Zeiss Axioskop 2 Plus) and with the Image-Pro Plus v.5.0 image analysis software (magnification: 100×, 400×, 630× and 1000×). Double readings were performed in the postrostrum otolith section. If the first reading was not coincident with the second one, a third reading was performed to guarantee the correct interpretation of age.
To determine whether the increment width was outside the resolution threshold of the optical microscope, some otoliths from the transformation stage (n=1), and juvenile and adult stages (n=5) were also examined at high resolution. Polished otoliths were immersed in a 1% HCl solution for between 60 and 300 seconds, rinsed with water and allowed to air dry for 24 hours. Finally, the otoliths were mounted, covered with gold-palladium and then observed using a scanning electron microscope (SEM).
Growth estimation
⌅In the Mediterranean Sea, C. maderensis is not sexually mature until it reaches ≥40 mm SL (Hulley 1981Hulley P.A. 1981. Results of the research cruises of FRV “Walther Herwig” to South America. LVIII. Family Myctophidae. Archiv. Fischwiss. 31: 1-300., 1984Hulley P.A. 1984. Myctophidae. In: Whitehead P.J.P., Bauchot M.L., et al. (eds), Fishes of the North-eastern Atlantic and the Mediterranean. UNESCO. 1: 429-483.). Specimens collected in this study were not sexed, so we cannot confirm whether there are size differences between sexes in individuals of C. maderensis from the western Mediterranean Sea. Therefore, in this study, specimens of both sexes were considered together. Linkowski et al. (1993)Linkowsky T.B., Radtke R.L., Lenz, P.H., 1993. Otolith microstructure, age and growth of two species of Ceratoscopelus (Osteichthyes: Myctophidae) from the eastern North Atlantic. Jour. Exp. Mar. Biol. Ecol. 167: 237-260. https://doi.org/10.1016/0022-0981(93)90033-K observed that individuals of C. maderensis from the northeastern Atlantic Ocean showed no significant size differences between males and females. The relationship between the fish length and the otolith diameter was determined by linear regression to ensure that the growth of the otolith is proportional to the somatic growth of the species. As each increment is assumed to represent one-day growth, the relationship between the number of day-increments and SL was fitted by a von Bertalanffy growth curve based on Equation 1:
where Lt is standard length in mm, L∞ is the asymptotic size in mm, k in days−1 is the Brody growth rate coefficient, which measures the rate at which the growth rate declines, and t0 corresponds to the age at which L is 0 (von Bertalanffy 1938Von Bertalanffy L. 1938. A quantitative theory of organic growth. Hum. Biol. 10: 181-213.).
The absolute growth rate (gt) is given by gt =k(L∞ -Lt), and when t=0 (w), the absolute growth rate is w=k×L∞ . Parameters k and L∞ were fitted and estimated using R software.
Finally, a second von Bertalanffy growth curve considering adult specimens only (i.e. SL>19 mm) was fitted in order to assess for growth differences between our specimens from the western Mediterranean Sea and from the northeastern Atlantic Ocean measured by Linkowski et al. (1993)Linkowsky T.B., Radtke R.L., Lenz, P.H., 1993. Otolith microstructure, age and growth of two species of Ceratoscopelus (Osteichthyes: Myctophidae) from the eastern North Atlantic. Jour. Exp. Mar. Biol. Ecol. 167: 237-260. https://doi.org/10.1016/0022-0981(93)90033-K (individuals with SL<19 mm were not considered by these authors). The comparison of these two curves was performed without fixing parameters or fixing one, two or all three parameters using the fisheries stock-assessment method package (FSA; Ogle 2016Ogle D.H. 2016. Introductory fisheries Analyses with R. Chapman & Hall/CRC, Boca Raton. https://doi.org/10.1201/b19232 ) in R software. According to this method, when the best explanatory models are those in which most of the parameters have not been fixed, this indicates that there are differences between the curves. By contrast, when the best models are those in which most parameters have been fixed, this indicates similarities between the curves. The best models were selected using the Akaike information criterion (AIC, Burnham and Anderson 2002Burnham K.P., Anderson D.R. 2002. Model Selection and Multimodel Inference: A Practical Information-Theoretic Approach. 2nd ed. Springer-Verlag, New York.). Models with AIC differences ≤2 were considered equivalent (see Burnham and Anderson 2002Burnham K.P., Anderson D.R. 2002. Model Selection and Multimodel Inference: A Practical Information-Theoretic Approach. 2nd ed. Springer-Verlag, New York., 2004Burnham K.P., Anderson D.R. 2004. Multimodel inference: Understanding AIC and BIC in model selection. Sociol. Methods Res., 33: 261-304. https://doi.org/10.1177/0049124104268644 ).
RESULTS
⌅A total of 208 individuals of C. maderensis were selected to cover the entire size range of the species in the western Mediterranean Sea. However, only 59 individuals (28%) could be successfully processed because of difficulties in the preparation process. This is mainly because the counting of increments in the otoliths of larger specimens was only possible after obtaining extremely thin and delicate sheets that broke easily during polishing or when using the hot melting resin for fixing. Despite these difficulties, the number of otoliths successfully processed was large enough to constitute a representative sample of the size range of C. maderensis in the western Mediterranean Sea.
Larval otoliths were round and very small (otolith diameter in larval individuals ranged between 19.96 and 218.51 µm) but turned into an oval shape as the fish grows (Fig. 2). The size-frequency distribution of juvenile and adult stages showed two modal size classes (20 and 50 mm respectively) (Fig. S1).
Microstructure of the otoliths
⌅The observation and the counting of otolith increments were feasible using both optical microscope and SEM. Three different zones were differenciated in adult specimens: larval (Fig. 3A, B), metamorphic (Fig. 3C, D), and juvenile-adult (Fig. 3E, F). Table 1 shows the SL ranges and the number of increments for each of these regions. The larval region was composed of a core (Fig. 3B) followed by a succession of an average of 27 increments (n=9, σ=2.98). The mean radius between the centre of the core and the outer part of the first increment was 2.95 μm (n=3, σ=0.84), while the average radius of the larval zone was 125.34 μm (n=19, σ=30.46). Within the larval region, the increment thickness increased progressively following the exponential function:
where y corresponds to the thickness of the increments in μm and x is the radius in μm (r2=0.91). The mean increment thickness in this region ranged from 1.4 to 4.2 μm (n=14, σ=0.88). The end of the larval region was well defined by a dark band of transition between the larval and metamorphic zones (Fig. 3A). This band was 5-10 μm wide and could also be detected using an optical microscope.
| Development phase | SL (mm) | Increments | N |
|---|---|---|---|
| Larval | ≤ 14 | 7-43 | 12 |
| Metamorphic | 16-19 | 42-69 | 5 |
| Juvenile-adult | 19-64 | 61-332 | 42 |
The metamorphic zone (Fig. 3C, D) extended from the external limit of the larval region (Fig. 3A) to the beginning of the juvenile-adult region (Fig. 3E, F). Increments in this region showed no clear pattern and were sometimes even unnoticeable or incomplete and irregular. This greatly hindered counting of the number of increments in this area, so estimates for this region should be taken with caution. Additionally, it made it difficult to obtain accurate measures of the mean increment thickness in this region, which was 142.46 μm (n=14, σ=42.23). Finally, the juvenile-adult zone (Fig. 3E, F) consisted of well-defined increments that showed the typical radial structure and rhythmic growth patterns, as in other species. The mean increment thickness was 4.94 μm (n=22) and showed a decreasing trend towards the otolith edge (to <1μm) (Fig. 3F). In addition to the three development regions mentioned above, from one to three translucent regions were also observed along the radial axis of the otolith of adult individuals (Fig. 4). The radius of each of these translucent regions relating to both the fish SL and the number of increments in the otolith is shown in Table 2.
| Origin | Translucent zones | SL (mm) | Increments | Source | |||
|---|---|---|---|---|---|---|---|
| Mean | SD | Range | N | ||||
| Western Mediterranean Sea | I | 22.11 | 3.50 | 18-29 | 14 | 70 | This study |
| Western Mediterranean Sea | II | 39.87 | 4.05 | 33-46 | 14 | 165 | This study |
| Western Mediterranean Sea | III | 50.30 | 6.31 | 42-64 | 8 | 247 | This study |
| Atlantic Ocean | I | 25.14 | 6.88 | 15.8-39.5 | 41 | 84 | Linkowski et al. 1993Linkowsky T.B., Radtke R.L., Lenz, P.H., 1993. Otolith microstructure, age and growth of two species of Ceratoscopelus (Osteichthyes: Myctophidae) from the eastern North Atlantic. Jour. Exp. Mar. Biol. Ecol. 167: 237-260. https://doi.org/10.1016/0022-0981(93)90033-K |
| Atlantic Ocean | II | 35.27 | 6.25 | 23.9-46.1 | 27 | 130 | Linkowski et al. 1993Linkowsky T.B., Radtke R.L., Lenz, P.H., 1993. Otolith microstructure, age and growth of two species of Ceratoscopelus (Osteichthyes: Myctophidae) from the eastern North Atlantic. Jour. Exp. Mar. Biol. Ecol. 167: 237-260. https://doi.org/10.1016/0022-0981(93)90033-K |
| Atlantic Ocean | III | 41.82 | 6.86 | 34.4-59.5 | 11 | 169 | Linkowski et al. 1993Linkowsky T.B., Radtke R.L., Lenz, P.H., 1993. Otolith microstructure, age and growth of two species of Ceratoscopelus (Osteichthyes: Myctophidae) from the eastern North Atlantic. Jour. Exp. Mar. Biol. Ecol. 167: 237-260. https://doi.org/10.1016/0022-0981(93)90033-K |
Growth
⌅The age interpretation of the otoliths under the optical microscope with coincident readings was successful in 59 individuals between 3.5 mm and 64 mm SL (Table 1). Assuming that the frequency of increment formation in the otoliths of C. maderensis is daily, these individuals would be between 7 and 332 days old. The relationship between the number of increments and the SL, considering the full-size range of individuals, renders an asymptotic size of 70.59 mm and growth rates of 0.33 (for the smallest larvae) to 0.03 (for the largest adult specimen), with a growth rate decline through the development of 0.01mm (Fig. 5, Tables 3 and 4).
| CI (95%) | ||||
|---|---|---|---|---|
| Parameter | Estimated | Lower limit | Upper limit | P |
| L∞ (mm) | 70.59 | 61.1 | 80.08 | 0.0000 |
| k (days-1) | 0.01 | 0 | 0.01 | 0.0000 |
| t0 (days) | -2.67 | -10.61 | 5.27 | 0.5032 |
| Western Mediterranean Sea (this study) | Atlantic Ocean (Linkowski et al. 1993Linkowsky T.B., Radtke R.L., Lenz, P.H., 1993. Otolith microstructure, age and growth of two species of Ceratoscopelus (Osteichthyes: Myctophidae) from the eastern North Atlantic. Jour. Exp. Mar. Biol. Ecol. 167: 237-260. https://doi.org/10.1016/0022-0981(93)90033-K ) | |
|---|---|---|
| SL (mm) | gt | gt |
| 5 | 0.33 | |
| 10 | 0.30 | |
| 15 | 0.28 | |
| 20 | 0.25 | 0.21 |
| 25 | 0.23 | 0.19 |
| 30 | 0.20 | 0.17 |
| 35 | 0.18 | 0.16 |
| 40 | 0.15 | 0.14 |
| 45 | 0.13 | 0.20 |
| 50 | 0.10 | 0.10 |
| 55 | 0.08 | 0.08 |
| 60 | 0.05 | 0.07 |
| 65 | 0.03 | 0.05 |
| 70 | 0.03 | |
| 75 | 0.01 | |
| 80 | -0.01 |
Table S1 shows the estimates of the parameters of the von Bertalanffy growth curves that we obtained when considering adult specimens only (SL>19mm), in order to assess growth differences between our specimens and those measured by Linkowski et al. (1993)Linkowsky T.B., Radtke R.L., Lenz, P.H., 1993. Otolith microstructure, age and growth of two species of Ceratoscopelus (Osteichthyes: Myctophidae) from the eastern North Atlantic. Jour. Exp. Mar. Biol. Ecol. 167: 237-260. https://doi.org/10.1016/0022-0981(93)90033-K (Fig. 6) in the northeastern Atlantic Ocean. The comparison of these two curves revealed that the best explanatory model (Model 1, Table 5) was the one in which the three parameters of the growth curve (Linf, k and t0) had been fixed. This indicates that growth differences between our specimens and those measured by Linkowski et al. (1993)Linkowsky T.B., Radtke R.L., Lenz, P.H., 1993. Otolith microstructure, age and growth of two species of Ceratoscopelus (Osteichthyes: Myctophidae) from the eastern North Atlantic. Jour. Exp. Mar. Biol. Ecol. 167: 237-260. https://doi.org/10.1016/0022-0981(93)90033-K in the northeastern Atlantic Ocean are not statistically significant. Models 1, 2, 3 and 4 (Table 5) can be considered equivalent as their ∆AIC values were <2. For body size, von Bertalanffy models predicted a SL of 72.82 mm for the Mediterranean individuals and 77.94 mm for the Atlantic ones (Table S1). However, considering the standard error obtained, body size differences between the Mediterranean and Atlantic specimens should be considered with caution.
| Model | Fixed parameters | Deviance | AIC | ΔAIC | Wi |
|---|---|---|---|---|---|
| 1 | Linf, K, t0 | 1177.060 | 601.828 | 0.000 | 0.335 |
| 2 | K, t0 | 1174.704 | 603.597 | 1.770 | 0.138 |
| 3 | Linf, t0 | 1175.638 | 605.458 | 1.861 | 0.132 |
| 4 | Linf, K | 1176.968 | 607.449 | 1.991 | 0.124 |
| 5 | Linf | 1162.267 | 609.995 | 2.545 | 0.094 |
| 6 | K | 1165.683 | 612.878 | 2.883 | 0.079 |
| 7 | t0 | 1172.123 | 616.394 | 3.517 | 0.058 |
| 8 | 1159.323 | 620.648 | 4.254 | 0.040 | |
| Estimates for parameters ± SE | Model 1 | Model 2 | Model 3 | Model 4 | |
| Linf 1 | 76.16 ± 1.983 | 76.54 ± 2.157 | 76.43 ± 2.124 | 76.13 ± 2.003 | |
| Linf 2 | 77.16 ± 2.872 | ||||
| K1 | 0.004 ± 0.000 | 0.004 ± 0.000 | 0.004 ± 0.000 | 0.004 ± 0.000 | |
| K2 | 0.004 ± 0.000 | ||||
| t01 | 19.85 ± 7.152 | -20.83 ± 7.432 | -20.36 ± 7.295 | -19.93 ± 7.282 | |
| t02 | -19. 2 ± 7.870 | ||||
DISCUSSION
⌅This study provides for the first time the growth patterns of the lanternfish C. maderensis in the western Mediterranean Sea. Series of translucent bands alternating with more opaque ones were observed throughout the radius of the otolith across the ontogenetic development in agreement with observations by Linkowski et al. (1993)Linkowsky T.B., Radtke R.L., Lenz, P.H., 1993. Otolith microstructure, age and growth of two species of Ceratoscopelus (Osteichthyes: Myctophidae) from the eastern North Atlantic. Jour. Exp. Mar. Biol. Ecol. 167: 237-260. https://doi.org/10.1016/0022-0981(93)90033-K in Atlantic specimens. However, the age determination of C. maderensis had some uncertainties given the lack of clear increments during the metamorphic phase and the unknown period between hatching and the time at which the first increment is laid down. The complete reading of the number of increments in otoliths was particularly difficult in larger specimens (>40 mm SL) because the increments could not be read simultaneously across the entire otolith. In fact, only 28% of examined specimens could be successfully processed given these difficulties. While Linkowsky et al. (1993)Linkowsky T.B., Radtke R.L., Lenz, P.H., 1993. Otolith microstructure, age and growth of two species of Ceratoscopelus (Osteichthyes: Myctophidae) from the eastern North Atlantic. Jour. Exp. Mar. Biol. Ecol. 167: 237-260. https://doi.org/10.1016/0022-0981(93)90033-K applied the Ratke and Dean’s (1982)Ratke R.L., Dean J.M. 1982. Increment formation in the otoliths of embryos, larvae, and juveniles of the mummichog, Fundulus heteroclitus. Fish. Bull. 80: 201-215. regression model to predict the number of otolith increments in adults, in this study the preparation of the otoliths was optimized to enable the direct counting of increments from images obtained through the optical microscope. This allowed us to build a growth model using a single type of methodology to obtain all the data.
C. maderensis larvae inhabit the surface layer up to 100 m depth during the day and at night (Olivar et al. 2014Olivar M.P., Sabatés A., Alemany F., et al. 2014. Diel-depth distributions of fish larvae off the Balearic Islands (western Mediterranean) under two environmental scenarios. J. Mar. Syst. 138: 127-138. https://doi.org/10.1016/j.jmarsys.2013.10.009 ), but their feeding activity takes place only during daylight (Contreras et al. 2015Contreras T., Olivar M.P., Bernal A., et al. 2015. Comparative feeding patterns of early stages of mesopelagic fishes with vertical habitat partitioning. Mar. Biol. 162: 2265-2277. https://doi.org/10.1007/s00227-015-2749-y ). Thus, the photoperiod and the feeding activity of these early stages must play an important role in the deposition of calcium in the otoliths and the pattern of their increments (see Eckmann 2000Eckmann R. 2000. The influence of photoperiod and feeding frequency on the distinctness of daily otolith increments in larval whitefish (Coregonus lavaretus L.). Limnologica 30: 102-105. https://doi.org/10.1016/S0075-9511(00)80003-1 , Morales-Nin 2000Morales-Nin B. 2000. Review of the growth regulation processes of otolith daily increment formation. Fish. Res. 46: 53-67. https://doi.org/10.1016/S0165-7836(00)00133-8 ), which is in line with the regular deposition pattern that we observed in the larvae of our study. During the transformation phase, otoliths did not show such a definite deposition pattern, a finding that could probably be related to migration or feeding. This suggests that transformation individuals occur near the surface and at depths greater than 500 m, with non-defined daily feeding patterns (Contreras et al. 2015Contreras T., Olivar M.P., Bernal A., et al. 2015. Comparative feeding patterns of early stages of mesopelagic fishes with vertical habitat partitioning. Mar. Biol. 162: 2265-2277. https://doi.org/10.1007/s00227-015-2749-y , Olivar et al. 2018Olivar M.P., Contreras T., Hulley P.A., et al. 2018. Variation in the diel vertical distributions of larvae and transforming stages of oceanic fishes across the tropical and equatorial Atlantic. Progr. Oceanogr. 160: 83-100. https://doi.org/10.1016/j.pocean.2017.12.005 ). The absence of behavioural patterns during this developmental stage appears to be reflected in the otolith growth, in which the regular deposition pattern was drastically lost, as occurs in other myctophid species (e.g. Ozawa and Peñaflor 1990Ozawa T., Peñaflor C. 1990. Otolith Microstructure and early ontogeny of a myctophid species, Benthosema pterotum. Nippon Suisan Gakk. 56: 1987-1995. https://doi.org/10.2331/suisan.56.1987 , Gartner et al. 1991bGartner J.V.Jr. 1991b. Life histories of three species of lanternfishes (Pisces: Myctophidae) from the eastern Gulf of Mexico (II). Age and growth patterns. Mar. Biol. 111: 21-27. https://doi.org/10.1007/BF01986340 , Takagi et al. 2006Takagi K., Yatsu A., Moku M., et al. 2006. Age and growth of lanternfishes, Symbolophorus californiensis and Ceratoscopelus warmingii (Myctophidae), in the Kuroshio-Oyashio Transition Zone. Ichthyol. Res.53: 281-289. https://doi.org/10.1007/s10228-006-0346-2 ). As in other species, the end of this phase in C. maderensis is determined by the recovery of regularity in the deposition of the increments (e.g. Linkowski et al. 1993Linkowsky T.B., Radtke R.L., Lenz, P.H., 1993. Otolith microstructure, age and growth of two species of Ceratoscopelus (Osteichthyes: Myctophidae) from the eastern North Atlantic. Jour. Exp. Mar. Biol. Ecol. 167: 237-260. https://doi.org/10.1016/0022-0981(93)90033-K ).
The wide depth range associated with the habitat of C. maderensis encompasses major changes in seawater temperature, pressure, light and food availability. For instance, when the adult fishes migrate to near-surface layers (40-80m) at night, they are exposed to temperatures ~18°C. In contrast, during the daytime, adults remain in mesopelagic layers (200-1000 m) where the water temperature is practically homogeneous at ca. 13°C. Considering this marked contrast in water temperature, it makes sense to assume that the specimens experience a period of adaptation during transformation, in which they do not perform the customary vertical migrations observed in adults. Therefore, during the transformation, C. maderensis might be experiencing gradual contact with the physicochemical conditions that the species will have to face when reaching the adult phase. Ultimately, adults exhibit diel vertical migrations to feed near the surface (Bernal et al. 2015Bernal A. Olivar M.P., Maynou F., et al. 2015. Diet and feeding strategies of mesopelagic fishes in the western Mediterranean. Progr. Oceanogr. 135: 1-17. https://doi.org/10.1016/j.pocean.2015.03.005 ) and remain without feeding activity in the mesopelagic zone during the light hours, as mentioned above (Hulley 1984Hulley P.A. 1984. Myctophidae. In: Whitehead P.J.P., Bauchot M.L., et al. (eds), Fishes of the North-eastern Atlantic and the Mediterranean. UNESCO. 1: 429-483., Olivar et al. 2012Olivar M.P., Bernal A., Molí B. et al. 2012. Vertical distribution, diversity and assemblages of mesopelagic fishes in the western Mediterranean. Deep-Sea Res. I 62: 53-69. https://doi.org/10.1016/j.dsr.2011.12.014 ). Incremental deposition in the juvenile-adult phase involved well-defined increments, with a decreasing pattern in the increment thickness towards the otolith edge (to <1μm). This decreasing pattern could also be reversed in some cases, as described in other myctophids (e.g. Greely et al. 1999Greely T.M., Gartner J.V.Jr., J.J. Torres. 1999. Age and growth of Electrona Antarctica (Pisces: Myctophidae), the dominant mesopelagic fish of the Southern Ocean. Mar. Biol. 133: 145-158. https://doi.org/10.1007/s002270050453 , Tomás and Panfili 2000Tomás J., Panfili J. 2000. Otolith microstructure examination and growth patterns of Vinciguerria nimbaria (Photichthydae) in the tropical Atlantic Ocean. Fish. Res. 46:131-145. https://doi.org/10.1016/S0165-7836(00)00140-5 ). Therefore, the increment thickness in otoliths during the juvenile-adult phase might vary among individuals, probably as a result of short-term variations in food availability.
The temperature range in the sea surface and the mesopelagic waters is higher in the Mediterranean Sea than in the northeastern Atlantic Ocean (Salat et al. 2002Salat J., García M.A., Cruzado A., et al. 2002. Seasonal changes of water mass structure and shelf slope exchanges at the Ebro shelf (NW Mediterranean). Cont. Shelf Res.22: 327-346. https://doi.org/10.1016/S0278-4343(01)00031-0 , Olivar et al. 2012Olivar M.P., Bernal A., Molí B. et al. 2012. Vertical distribution, diversity and assemblages of mesopelagic fishes in the western Mediterranean. Deep-Sea Res. I 62: 53-69. https://doi.org/10.1016/j.dsr.2011.12.014 ). This is particularly interesting below the thermocline in the Mediterranean Sea, where the temperature does not drop below 13ºC. In general, higher water temperature regimes speed up metabolic processes increasing growth in organisms. Furthermore, these temperature differences can be observed in the otolith microstructure (Degens et al. 1969Degens E.T., Deuser W.G., Haedrich, R.L.1969. Molecular structure and composition of fish otoliths. Mar. Biol. 2: 105-113. https://doi.org/10.1007/BF00347005 ). This may help to explain why growth rates declined more slowly from a certain body size in individuals from the western Mediterranean Sea. However, growth differences between individuals from the western Mediterranean Sea and those from the northeastern Atlantic Ocean were not statistically significant.
Many aspects of the biology and ecology of myctophids are still poorly understood and require further investigation for managing ecosystems, particularly under uncertain climate changes. This study provides further knowledge of the biology of this abundant myctophid, taking into account not only its entire life cycle but also a different geographical scenario. However, a more complete time coverage over the year and the validation of the daily periodicity of the increments are still necessary to achieve a more profound knowledge of the age and growth of the species.