INTRODUCTIONTop

Pasiphaeid shrimps are a group of benthopelagic and pelagic species that currently comprises a total of 98 species grouped in seven genera: Alainopasiphaea (2), Eupasiphae (4), Glyphus (1), Leptochela (15), Parapasiphae (4), Pasiphaea (67), and Psathyrocaris (5) (De Grave and Fransen 2011De Grave S., Fransen C.H.J.M. 2011. Carideorum Catalogus: The Recent Species of the Dendrobranchiate, Stenopodidean, Procarididean and Caridean Shrimps (Crustacea: Decapoda). Zool. Meded. Leiden 85: 195-588., Komai et al. 2012Komai T., Lin C.W., Chan T.Y. 2012. Bathypelagic shrimps of the genus Pasiphaea (Decapoda: Caridea: Pasiphaeidae) from waters around Taiwan, with descriptions of four new species. J. Crust. Biol. 32: 295-325.). In particular, within the genus Pasiphaea up to four morphologically-related species groups have been recognized, including the Pasiphaea sivado group, to which a total of nine species belong (Hayashi 1999Hayashi K.I. 1999. Crustacea Decapoda: Revision of Pasiphaea sivado (Risso, 1816) and related species, with descriptions of one new genus and five new species (Pasiphaeidae). Mem. Mus. Nat. Hist. Nat. 180: 267-302., 2006Hayashi K.I. 2006. A new species of the Pasiphaea sivado species group from Taiwan (Decapoda, Caridea, Pasiphaeidae). Zoosystema 28: 341-346.). Species of this family are distributed along a wide latitudinal range, from tropical to Arctic and sub-Antarctic waters, mainly on and above continental slopes and in the open sea, including deep-sea habitats (Clarke and Holmes 1987Clarke A., Holmes L.J. 1987. Notes on the biology and distribution of Pasiphaea species from the Southern Ocean. Br. Antarct. Surv. Bull. 74: 17-30., Gibbons et al. 1994Gibbons M.J., Macpherson E., Barangé M. 1994. Some observations on the pelagic decapod Pasiphaea semispinosa Holthuis 1951 in the Benguela upwelling system. S. Afr. J. Marine. Sci. 14: 59-67., Tavares and Cardoso 2006Tavares C.R., Cardoso I.A. 2006. Deep-sea Pasiphaeidae (Crustacea: Decapoda: Caridea) from off the Brazilian central coast between 11 degrees and 22 degrees S, collected by the Revizee Program. Zootaxa 1174: 27-39.). Morphological features in this family, such as lateral compression of the body and pleopods, are considered to be the result of selective pressures of adaptation to the pelagic environment (Cartes et al. 1993Cartes J.E., Sardá F., Company J.B., et al. 1993. Day-night migrations by deep-sea decapod crustaceans in experimental sampling in the Western Mediterranean sea. J. Exp. Mar. Biol. Ecol. 171: 63-73., Johnsen 2005Johnsen S. 2005. The red and the black: Bioluminescence and the color of animals in the deep sea. Integr. Comp. Biol. 45: 234-246, Aguzzi et al. 2007Aguzzi J., Company J.B., Abelló P., et al. 2007. Ontogenetic changes in vertical migratory rhythms of benthopelagic shrimps Pasiphaea multidentata and P. sivado. Mar. Ecol. Progr. Ser. 335: 167-174.). Notwithstanding the relatively large amount of systematics and faunistics work done on this family (e.g. Zariquiey Álvarez 1957Zariquiey Álvarez R. 1957. Decápodos españoles XIII - Las Pasiphaeas del Mediterráneo occidental. Trabajos del Museo de Zoología Nueva Serie Zoológica 2: 1-41., Iwasaki 1990Iwasaki N. 1990. Pasiphaeid shrimps from the eastern North Atlantic and the Caribbean Sea, with the description of a new species of Pasiphae (Crustacea, Decapoda, Pasiphaeidae). Zool. Meded. Leiden 63: 187-203., Hayashi 1999Hayashi K.I. 1999. Crustacea Decapoda: Revision of Pasiphaea sivado (Risso, 1816) and related species, with descriptions of one new genus and five new species (Pasiphaeidae). Mem. Mus. Nat. Hist. Nat. 180: 267-302.), there are still few detailed studies on biological aspects of species of this family. They refer mainly to the European species Pasiphaea sivado and Pasiphaea multidentata (Cartes 1993Cartes J.E. 1993. Feeding habits of pasiphaeid shrimps close to the bottom on the western Mediterranean slope. Mar. Biol. 117:459-468., Frank 2000Frank T.M. 2000. Temporal resolution in mesopelagic crustaceans. Phil. Trans. R. Soc. Lond. B 355: 1195-1198., Ramirez-Llodra et al. 2007Ramirez-Llodra E., Company J.B., Camps M., et al. 2007. Spatio-temporal variations in reproductive patterns and population structure of Pasiphaea multidentata (Decapoda: Caridea) in the Blanes canyon and adjacent margin, Northwestern Mediterranean Sea. Mar. Ecol. Evol. Perspect. 28: 470-479.), but also to Pacific Ocean species such as Pasiphaea japonica and Leptochela gracilis (Nanjo 2007Nanjo N. 2007. Feeding habits of the glass shrimp Pasiphaea japonica in Toyama Bay of the Sea of Japan. Crustac. Res. 36: 45-51., Oh et al. 2006Oh C.W., Kim J.Y., Jeong I.J., et al. 2006. Reproduction and population dynamics of Leptochela gracilis (Decapoda: Pasiphaeidae) on the western coast of Korea, Yellow Sea. J. Mar. Biol. Ass. UK. 86: 113-120.).

No reference is made to the occurrence of macroscopic differences in shape in relation to gender in this family (Zariquiey Álvarez 1968Zariquiey Álvarez R. 1968. Crustáceos decápodos ibéricos. Inv. Pesq. 32: 1-510., Iwasaki 1990Iwasaki N. 1990. Pasiphaeid shrimps from the eastern North Atlantic and the Caribbean Sea, with the description of a new species of Pasiphae (Crustacea, Decapoda, Pasiphaeidae). Zool. Meded. Leiden 63: 187-203., Hayashi 1999Hayashi K.I. 1999. Crustacea Decapoda: Revision of Pasiphaea sivado (Risso, 1816) and related species, with descriptions of one new genus and five new species (Pasiphaeidae). Mem. Mus. Nat. Hist. Nat. 180: 267-302.), except for the noted occurrence of differences in pleopod shape in Pasiphaea japonica (Doi 1975Doi S. 1975. Notes on Pasiphaea in Toyama Bay. Japan Sea Reg. Fish. Res. News 285: 1-6, Nanjo and Ohtomi 2009Nanjo N., Ohtomi J. 2009. Reproductive biology of Pasiphaea japonica females in Toyama Bay, central Japan. Fisheries Sci. 75: 1189-1195.). Concerning body size dimorphism, based on overall size frequency distributions, males have been reported to be slightly larger than females in P. sivado, and slightly smaller than females in P. multidentata (Company et al. 2001Company J.B., Cartes J.E., Sardá F. 2001. Biological patterns and near-bottom population characteristics of two pasiphaeid decapod crustacean species, Pasiphaea sivado and P. multidentata, in the north-western Mediterranean Sea. Mar. Biol. 139: 61-73.), but sexual dimorphism has so far been considered insignificant in both P. sivado nor P. multidentata.

We herein aim to report sexual dimorphism in P. sivado, quantify the occurrence of morphological dimorphism, and analyse size-related population structure and sex ratio in relation to geographic and bathymetric characteristics.

MATERIALS AND METHODSTop

Samples of P. sivado were collected during a series of demersal fisheries research cruises (MEDITS_ES) between 2004 and 2011 at depths of around 40 to 800 m along the Mediterranean coasts of the Iberian Peninsula (Bertrand et al. 2002Bertrand J., Gil de Sola L., Papaconstantinou C., et al. 2002. The general specifications of the MEDITS surveys. Sci. Mar. 66S2: 9-17.). On these cruises, P. sivado was only collected at depths of 150-750 m (Abelló et al. 2002Abelló P., Carbonell A., Torres P. 2002. Biogeography of epibenthic crustaceans on the shelf and upper slope off the Iberian Peninsula Mediterranean coasts: implications for the establishment of natural management areas. Sci. Mar. 66 (Suppl. 2): 183-198.). All P. sivado individuals were sexed by checking the presence/absence of appendix masculina on the endopod of the second pleopod, as is usual in caridean shrimps.

Pleopod morphology

For morphometric analyses, ten adult specimens of Pasiphaea sivado (five males and five females) were selected from a sample taken by demersal trawling in the western Mediterranean Sea (Alboran Sea: 36.4908°N, 4.2885°W) at 424-428 m depth during daytime on 17/05/2010. The haul was performed on board the R/V Cornide de Saavedra at a speed of 3 knots for 1 h using a demersal GOC-73 otter trawl with a 2.5- to 3.0-m vertical opening and a codend stretched mesh size of 20 mm (Fiorentini et al. 1999Fiorentini L., Dremière P.Y., Leonori I., et al. 1999. Efficiency of the bottom trawl used for the Mediterranean international trawl survey (MEDITS). Aquat. Living. Resour. 12: 187-205.). The cruise was part of the MEDITS_ES project (Bertrand et al. 2000Bertrand J.A., Gil de Sola L., Papaconstantinou C., et al. 2000. An international bottom trawl survey in the Mediterranean: the MEDITS programme. Actes de Colloques IFREMER 26:76-93., 2002Bertrand J., Gil de Sola L., Papaconstantinou C., et al. 2002. The general specifications of the MEDITS surveys. Sci. Mar. 66S2: 9-17.).

The selected specimens had a cephalothorax length (CL) >18 mm and were clearly morphologically considered as adults (Company et al. 2001Company J.B., Cartes J.E., Sardá F. 2001. Biological patterns and near-bottom population characteristics of two pasiphaeid decapod crustacean species, Pasiphaea sivado and P. multidentata, in the north-western Mediterranean Sea. Mar. Biol. 139: 61-73., present observations). CL was measured using a digital calliper rule with a precision of 0.01 mm. Sizes of the examined specimens ranged between 18.8 and 23.5 mm CL in females, and between 21.6 and 24.0 mm CL in males.

In order to graphically show the a priori differences observed between genders, one adult male and one female of similar size were figured and measured. In a first step, the external view of all five right pleopod protopods of each individual were drawn using a camera lucida coupled to a Leica MZ12 magnifier provided with an ocular micrometer. After that, the length and maximum width of the rest of the individuals were measured to obtain a first index of pleopod shape based on the ratio between these perpendicular measures.

Geometric morphometrics

To further assess the occurrence of sexual dimorphism using geometric morphometric techniques, pictures were also taken from the same external view of the right second pleopod of all five males and females selected using the same magnifier, which had an integrated Leica DC100 video camera and a DC200 digital imaging system. The first landmark was placed on the dorsal maximum convex curvature of the protopod, the second and fourth were placed at the anterior maximum curvature where the exopod and endopod are inserted in the protopod, the third at the maximum concavity between these two landmarks, the fifth at the maximum perpendicular width of the protopod in relation to the first landmark, the sixth at the ventral maximum curvature in the posterior part of the protopod and the seventh at the maximum concave curvature on the posterior dorsal margin of the protopod (Fig. 1).

Using Tps.Dig2 v2.16 software (Rohlf 2010aRohlf F.J. 2010a. TpsDig, digitize landmarks and outlines, version v2.16. Department of Ecology and Evolution, State University of New York at Stony Brook.), the x,y coordinates of the landmark dataset were obtained. Generalized procrustes superimposition analysis was performed to align and to eliminate scale and orientation distortions before shape analysis using tpsRelw v1.49 software (Rohlf 2010bRohlf F.J. 2010b. TpsRelw, relative warps analysis, version 1.49. Department of Ecology and Evolution, State University of New York at Stony Brook.). Statistical analysis of shape variance was assessed using the MorphoJ software (Klingenberg 2011Klingenberg C.P. 2011. MorphoJ: an integrated software package for geometric morphometrics. Mol. Ecol. Resour. 11: 353-357.). We then applied procrustes ANOVA, which is a two factorial ANOVA that performs analysis of both size (univariate) and shape (multivariate). This procedure is therefore a useful method for discovering localized variation (Klingenberg and McIntyre 1998Klingenberg C.P., McIntyre G.S. 1998. Geometric morphometrics of developmental instability: analyzing patterns of fluctuating asymmetry with procrustes methods. Evolution 52: 1363-1375.). Finally, a principal component analysis (PCA) was applied to quantify the contribution of the principal factor of landmark displacement that explains the shape variance between genders. This PCA was run using the covariance matrix generated from shape variation components, which were previously derived from the coordinates landmark dataset, pooled within-group with sex as a covariate (Viscosi and Cardini 2011Viscosi V., Cardini A. 2011. Leaf morphology, taxonomy and geometric morphometrics: a simplified protocol for beginners. PLoS ONE 6: e25630.).

Gender assignment by visual inspection

We assessed the power of telling apart the gender in adult individuals by just visual observation, since this would facilitate sex identification in field work. A total of 187 individuals of sizes comprised between 8 and 24 mm CL were visually examined for sex identification based on pleopod morphology during one fishery research cruise. Three volunteers were asked to identify the sex visually based on the information provided by the morphological drawings obtained in the section on pleopod morphology. A posteriori one of the authors checked whether each assignation was correct based on the presence of appendix masculina under a stereoscope microscope, as usually performed for caridean shrimps. The proportion of correct assignation by size class was then calculated.

Gender differences in size and sex ratio

Sex ratio, as the proportion of males, was calculated within each 1 mm CL size class separately for samples taken in the Alboran Sea (the westernmost basin of the Mediterranean Sea), the intermediate Alacant area (between Cape Palos and Cape La Nao) and the Catalan Sea (in the NW Mediterranean). These sectors were established according to previous results (Simão et al. 2015Simão D.S., Zas E., Carbonell A., et al. 2015. Pasiphaeid shrimps in the western Mediterranean: geographical variability in distribution and population patterns. Sci. Mar. 79: 199-209.), in which differences in size at maturity were observed between Alboran Sea populations and the rest of the sampled populations along the Iberian Peninsula Mediterranean coasts. Sex ratio was not calculated in size classes with fewer than 25 individuals. The significance of the deviation from an expected 1:1 sex ratio was calculated using a heterogeneity G-test (Sokal and Rohlf 1981Sokal R.R., Rohlf F.J. 1981. Biometry, 2nd edition. W.H. Freeman & Co., New York., Abelló et al. 1990Abelló P., Pertierra J.P., Reid D.G. 1990. Sexual size dimorphism, relative growth and handedness in Liocarcinus depurator and Macropipus tuberculatus (Brachyura: Portunidae). Sci. Mar. 54: 195-202.). The relationship of sex ratio with depth was also calculated; only samples containing more than 25 sexed individuals larger than 15 mm CL were considered for this analysis; this cut-off size was chosen to ensure that sex ratio was calculated over adult specimens to minimize error in gender assessment (see below), and also to encompass the whole adult cohort as visually assessed through size frequency distributions.

In order to verify whether size frequency distributions (SFDs) differed between geographic populations and between depth strata, the Kolmogorov-Smirnov test within VITMAN fisheries software (Lleonart and Salat 1992Lleonart J., Salat J. 1992. VIT, programa de análisis de pesquerías. Inf. Téc. Sci. Mar. 168-169: 1-116.) was used. Combinations of geographic area and depth strata resulting in less than 100 individuals measured (i.e. at both extremes of the bathymetric range of the species in each area) were not used in the analyses.

RESULTSTop

A total of 4010 P. sivado individuals were sexed and measured. This allowed the assessment of any size-related differences between genders. These specimens were sexed as usual for caridean shrimps (i.e. by checking the presence of appendix masculina). In the present work, males were classified as juveniles when the appendix masculina was not fully formed but present as small buds. This was evident in individuals with sizes ranging between 9 and 16 mm CL. However, the proportion of individuals showing appendix masculina buds was very small at sizes smaller than 11-12 mm CL, when it was probably still undeveloped in a larger proportion of males, so gender could not be correctly assigned based on this character at these small sizes. Therefore, sex ratio analysis was restricted to individuals larger than 15 mm CL to ensure a higher reliability in properly assessing gender.

Pleopod morphology

The difference in protopod morphology between genders of Pasiphaea sivado was quite readily identified by sight (Fig. 2). Overall, in all five pleopod pairs, adult female protopods were clearly much thinner than those in adult males. Additionally, the anterior edge of the male protopods was clearly much more convex than in females, while the posterior edge was approximately straight in both males and females. As shown in Figure 3, the ratio between pleopod protopod length and width (L/W), which can be considered a protopod thinness index, was much higher in females than in males, thus characterizing the thinner, rectangular, elongated shape in females versus the more robust shape present in males. A significant difference between genders was obtained for all five pleopod pairs (Table 1). Additionally, no setae were present on the anterior edge of the first pleopod protopod in any of the adult males examined, and in the rest of the pleopods anterior setae were much scarcer in males than in females.

The use of geometric morphometrics techniques further evidenced the differences between genders in pleopod protopod shape. Both P. sivado males and females grouped as two well-differentiated clusters in the analysis of the shape of their second pleopod protopods (Fig. 4A). The overall consensus shape (Fig. 4B) of the pleopod protopods, which is based on the geometric mean position of each landmark, clearly differentiated between the thinner, elongated shape of the females versus the broader shape found in males. PCA showed that the first principal component (PC1), which accounted for 56.1% of the total shape variability, clearly differentiated males from females. The shape variance between genders explained by the PC1 was significantly different (ANOVA F_10,80=27.24 p<0.0001) andwas not affected by the centroid size effect (ANOVA F_1,8=1.87 p=0.209). The next five principal components explained 16.5%, 10.4%, 6.2%, 3.5% and 1.26% of the total shape variance, respectively.

Regarding the visual sex assessment on board, a total of 542 gender assignations were made on the examined individuals by three independent observers. Figure 5 shows the percentage of correct sex identification by pleopod morphology based on independent visual assessment by all observers. Total reliability was attained in individuals larger than 18 mm CL, and the percentage of correct assignation decreased with decreasing size.

Comparison of sizes and size-related sex ratio among geographic areas

The overall SFDs of sexed individuals by geographic area (Fig. 6) were biased towards adult individuals in all populations. The Alboran Sea population showed a higher proportion of individuals in the largest sizes than in those obtained in the Alacant area and the Catalan Sea. The comparison between male and female SFDs within each area showed that they were not significantly different in any case (main diagonal in Table 2). The comparison of male or female SFDs among areas also showed non-significant values (Table 2).

The analysis of sex ratio by size class showed that in all sectors females were dominant at sizes smaller than around 18-19 mm CL, while the proportion of males was significantly higher at larger sizes, therefore evidencing the occurrence of a slight sexual size dimorphism in P. sivado (Fig. 6). When special attention was paid to the precise size at which the dominance of males occurred, a decreasing gradient was found in the study area, with the proportion of males being significantly higher at CL sizes larger than 21 mm in the Alboran Sea, 20 mm in the Alacant area and 19 mm in the Catalan Sea (Fig. 6).

Size structure with depth by geographic area

When the shape of the SFDs was analysed by depth strata, a high frequency of juveniles in relation to adults was evident in the shallowest depth stratum (200-300 m), especially in the Alboran Sea (Fig. 7), where the population showed a marked SFD displacement towards larger sizes with increasing depth. No noticeable variation in SFD displacement was evident in the Alacant area and Catalan Sea populations (Fig. 7 and Table 3). The comparison of the SFDs with the Kolmogorov-Smirnov test showed the occurrence of significant differences by depth only in the Alboran Sea population, in both males and females (Table 4). No significant differences in SFD variation with depth were detected in the Alacant area and Catalan Sea populations. It is worth noting that despite intensive sampling, the densities of P. sivado at certain depths and areas are very low (Simão et al 2015Simão D.S., Zas E., Carbonell A., et al. 2015. Pasiphaeid shrimps in the western Mediterranean: geographical variability in distribution and population patterns. Sci. Mar. 79: 199-209.): the species is hardly found shallower than 200 m, except in the Alboran Sea, and deeper than 600 m throughout the study area.

Sex ratio with depth

A total of 84 samples were used to analyse the sex ratio across the bathymetric distribution of the species (Fig. 8). Most of the samples (70.2%) did not show a significant deviation from a 1:1 sex ratio. However, sex segregation was identified in the rest of samples, with 11.9% of them being biased towards females and 17.9% towards males. The highest proportion of significant deviation towards females was found at the shallowest extreme of the bathymetric distribution of the species but also in the deepest strata. On the other hand, samples with a significantly higher proportion of males were located in the central depth strata. Overall, no significant tendency of gender segregation across the bathymetric distribution was found (r=0.0803, p=0.4677).

DISCUSSIONTop

Pleopod morphology

The occurrence of sexual dimorphism in pleopod shape was detected in Pasiphaea sivado by both visualization and measurement methods: the traditional linear measurement and geometric morphometric analysis. An important, useful application of the present observations and the description of the occurrence of sexual dimorphism in pleopod shape in adult P. sivado is, from now on, the ability to quickly assess the gender of individuals when population biology studies are performed in the field.

A similar sexual dimorphism in pleopod shape has also been reported in Pasiphaea japonica, but it also seems to occur in Alainopasiphaea nudipeda, as evidenced by Figures 1 and 2d in Hayashi (1999)Hayashi K.I. 1999. Crustacea Decapoda: Revision of Pasiphaea sivado (Risso, 1816) and related species, with descriptions of one new genus and five new species (Pasiphaeidae). Mem. Mus. Nat. Hist. Nat. 180: 267-302., as well as in Pasiphaea semispinosa (authors’ personal observations based on specimens deposited in collections: CBR at the ICM-CSIC in Barcelona and CCDE at the IEO-Cádiz). Other personal observations, not yet quantified, on the congeneric species Pasiphaea multidentata—a species that coexists with P. sivado across the Mediterranean Sea (Company et al. 2001Company J.B., Cartes J.E., Sardá F. 2001. Biological patterns and near-bottom population characteristics of two pasiphaeid decapod crustacean species, Pasiphaea sivado and P. multidentata, in the north-western Mediterranean Sea. Mar. Biol. 139: 61-73., Abelló et al. 2002Abelló P., Carbonell A., Torres P. 2002. Biogeography of epibenthic crustaceans on the shelf and upper slope off the Iberian Peninsula Mediterranean coasts: implications for the establishment of natural management areas. Sci. Mar. 66 (Suppl. 2): 183-198.)—indicate that this species does not show this dimorphism.

The phylogenetic relationships within the family Pasiphaeidae are far from being properly understood (Hayashi 1999Hayashi K.I. 1999. Crustacea Decapoda: Revision of Pasiphaea sivado (Risso, 1816) and related species, with descriptions of one new genus and five new species (Pasiphaeidae). Mem. Mus. Nat. Hist. Nat. 180: 267-302., 2006Hayashi K.I. 2006. A new species of the Pasiphaea sivado species group from Taiwan (Decapoda, Caridea, Pasiphaeidae). Zoosystema 28: 341-346., Matzen da Silva et al. 2011Matzen da Silva J., Creer S., Dos Santos A., et al. 2011. Systematic and evolutionary insights derived from mtDNA COI barcode diversity in the Decapoda (Crustacea: Malacostraca). PLoS ONE 6: e19449.). Based on present findings, the occurrence of sexual dimorphism in pleopod shape may well have a significant role in determining phylogenetic relationships within the family Pasiphaeidae, in particular within the Pasiphaea sivado group (Hayashi 1999Hayashi K.I. 1999. Crustacea Decapoda: Revision of Pasiphaea sivado (Risso, 1816) and related species, with descriptions of one new genus and five new species (Pasiphaeidae). Mem. Mus. Nat. Hist. Nat. 180: 267-302., 2006Hayashi K.I. 2006. A new species of the Pasiphaea sivado species group from Taiwan (Decapoda, Caridea, Pasiphaeidae). Zoosystema 28: 341-346.), since both P. sivado and P. japonica (which also belongs to this group) have been shown to display this morphological characteristic (Doi 1975Doi S. 1975. Notes on Pasiphaea in Toyama Bay. Japan Sea Reg. Fish. Res. News 285: 1-6, Nanjo and Ohtomi 2009Nanjo N., Ohtomi J. 2009. Reproductive biology of Pasiphaea japonica females in Toyama Bay, central Japan. Fisheries Sci. 75: 1189-1195.). Insufficient information is as yet available concerning other species in the genus or the family.

Size dimorphism and sex ratio

The present study also provides evidence on the occurrence of significant size dimorphism in P. sivado between genders, a fact previously unreported in this species, with females accumulating in size classes smaller than around 20 mm CL and males accumulating in the largest size classes. Sex ratio as a function of size clearly suggests the occurrence of differential growth rates between genders in P. sivado, as reported in many species of crustaceans (Abelló and Cartes 1992Abelló P., Cartes J.E. 1992. Population characteristics of the deep-sea lobsters Polycheles typhlops and Stereomastis sculpta (Decapoda: Polychelidae) in a bathyal mud community of the Mediterranean Sea. Mar. Biol. 114: 109-117., Kevrekidis and Thessalou-Legaki 2006Kevrekidis K., Thessalou-Legaki M. 2006. Catch rates, size structure and sex ratio of Melicertus kerathurus (Decapoda: Penaeidae) from an Aegean Sea trawl fishery. Fish Res. 80: 270-279., Oh et al. 2006Oh C.W., Kim J.Y., Jeong I.J., et al. 2006. Reproduction and population dynamics of Leptochela gracilis (Decapoda: Pasiphaeidae) on the western coast of Korea, Yellow Sea. J. Mar. Biol. Ass. UK. 86: 113-120.). This fact is usually attributed to a higher investment of females in reproduction rather than growth. This pattern appears to follow the so-called anomalous pattern described by Wenner (1972)Wenner A.M. 1972. Sex ratio as a function of size in marine Crustacea. Am. Nat. 106: 321-350., in which no differences in sex ratio occur in juveniles, so a predominance of females is due to a lower female growth rate, with males finally accumulating their frequency in the largest sizes. Several explanations may account for this pattern, with different growth rates between the genders from the puberty moult or size at maturity being the most likely (Ben Mariem 2004Ben Mariem S. 2004. Première approche de la croissance de Penaeus kerathurus (Decapoda Penaeidae) dans le golfe de Gabes, Tunisie. Crustaceana 68: 583-596, Fantucci et al. 2009Fantucci M.Z., Biagi R., Mantelatto F.L. 2009. Use of pleopod morphology to determine sexual dimorphism and maturity in hermit crabs: Isocheles sawayai as a model. Helgol. Mar. Res. 63: 169–175.). In the present study, mature females were identified at sizes larger than around 15 mm CL, coinciding with results from Company et al. (2001)Company J.B., Cartes J.E., Sardá F. 2001. Biological patterns and near-bottom population characteristics of two pasiphaeid decapod crustacean species, Pasiphaea sivado and P. multidentata, in the north-western Mediterranean Sea. Mar. Biol. 139: 61-73., and size at 50% maturity was found to vary geographically between 20.6 mm CL in the Catalan Sea and 23.0 mm CL in the Alboran Sea. Some biological characters estimated by Company et al. (2001)Company J.B., Cartes J.E., Sardá F. 2001. Biological patterns and near-bottom population characteristics of two pasiphaeid decapod crustacean species, Pasiphaea sivado and P. multidentata, in the north-western Mediterranean Sea. Mar. Biol. 139: 61-73. for P. sivado in the Catalan Sea support the sex ratio pattern with size reported in the present study. In that work males showed a faster growth rate (k=0.62), a larger asymptotic size (L_∞=27.5 mm) and a higher growth performance index (Φ=0.13) than females (0.55, 26 mm and 0.06, respectively). Reproductive aspects of the species, such as the continuous reproduction throughout the year and the higher volumetric index of females (2.92) compared with males (2.76) (Company et al. 2001Company J.B., Cartes J.E., Sardá F. 2001. Biological patterns and near-bottom population characteristics of two pasiphaeid decapod crustacean species, Pasiphaea sivado and P. multidentata, in the north-western Mediterranean Sea. Mar. Biol. 139: 61-73.), also suggest the smaller amount of energy investment in size growth by P. sivado females in relation to males. Another factor that may also limit growth in females is the moulting delay of ovigerous females in order to avoid loss of attached embryos, a phenomenon characteristic of crustacean species with egg-carrying biological features (Guerao et al. 1994Guerao G., Pertez-Baquera J., Ribera C. 1994. Growth and reproductive biology of Palaemon xiphias Risso, 1816 (Decapoda: Caridea: Palaemonidae). J. Crustac. Biol. 14: 280-288., Oh et al. 1999Oh C.W., Hartnoll R.G., Nash R.D.M. 1999. Population dynamics of the common shrimp, Crangon crangon (L.), in Port Erin Bay, Isle of Man, Irish Sea. ICES J. Mar. Sci. 56: 718–733., Viegas et al. 2007Viegas I., Martinho F., Neto J., et al. 2007. Population dynamics, distribution and secondary production of the brown shrimp Crangon crangon (L.) in a southern European estuary. Latitudinal variations. Sci. Mar. 71: 451-460.), such as caridean shrimps, lobsters, and brachyuran and anomuran crabs.

The size at which differences in sex ratio become significant has also been suggested to be an indirect indicator of the size at puberty moult and/or maturity size/age of the species (Abelló et al. 1990Abelló P., Pertierra J.P., Reid D.G. 1990. Sexual size dimorphism, relative growth and handedness in Liocarcinus depurator and Macropipus tuberculatus (Brachyura: Portunidae). Sci. Mar. 54: 195-202.). In the present study, the precise size at which males outnumbered females progressively decreased from the Alboran Sea to the Alacant area and the Catalan Sea, corroborating the patterns of size structure and size at maturity observed in this area (Simão et al. 2015Simão D.S., Zas E., Carbonell A., et al. 2015. Pasiphaeid shrimps in the western Mediterranean: geographical variability in distribution and population patterns. Sci. Mar. 79: 199-209.) and implying the occurrence of environmental and ecological processes affecting growth rates.

Overall, no significant gender segregation in P. sivado populations as a function of depth was found, suggesting that this species does not perform differential migration between sexes over the sea bottom. In the Catalan Sea, the absence of gender segregation by depth was also reported by Company et al. (2001)Company J.B., Cartes J.E., Sardá F. 2001. Biological patterns and near-bottom population characteristics of two pasiphaeid decapod crustacean species, Pasiphaea sivado and P. multidentata, in the north-western Mediterranean Sea. Mar. Biol. 139: 61-73. throughout the year, and could be related to the gregarious behaviour of macroplankton/micronekton organisms (McManus and Woodson 2012McManus M.A., Woodson C.B. 2012. Plankton distribution and ocean dispersal. J. Exp. Biol. 215: 1008-1016.). During the daytime, P. sivado inhabits the epi-/suprabenthic environment over the upper and medium slope, with high densities recorded when sampled by demersal trawl nets during fishery research surveys (Cartes et al. 2007Cartes J.E., Serrano A., Velasco F., et al. 2007. Community structure and dynamics of deep-water decapod assemblages from Le Danois Bank (Cantabrian Sea, NE Atlantic): Influence of environmental variables and food availability. Prog. Oceanog. 75: 797-816., García-Rodríguez et al. 2011García-Rodríguez M., Abelló P., Fernández A., et al. 2011. Demersal assemblages on the soft bottoms off the Catalan-Levante coast of the Spanish Mediterranean. J. Mar. Biol. 2011: 1-16., Simão et al. 2015Simão D.S., Zas E., Carbonell A., et al. 2015. Pasiphaeid shrimps in the western Mediterranean: geographical variability in distribution and population patterns. Sci. Mar. 79: 199-209.), while during the night the species has been reported to migrate several hundred metres up the water column to waters near the surface to feed on plankton (Froglia and Giannini 1982Froglia C., Giannini S. 1982. Osservazioni sugli spost amenti verticali nictomerali di Sergestes arcticus Kröyer e Sergia robusta (Smith) in Mediterraneo (Decapoda, Sergestidae). Atti Convegno UU.OO. afferenti ai Sottopragetti Risorse biologiche e Inquinamento, P.F. Oceanografia e fondi Marini: 311-319., Cartes 1993Cartes J.E. 1993. Feeding habits of pasiphaeid shrimps close to the bottom on the western Mediterranean slope. Mar. Biol. 117: 459-468., Simão et al. 2014Simão D.S., Torres A.P., Olivar M.P., et al. 2014. Vertical and temporal distribution of pelagic decapod crustaceans over the shelf-break and middle slope in two contrasting zones around Mallorca (western Mediterranean Sea). J. Mar. Syst. 138: 139-149.).

Controversially, a relatively high number of samples showed biased sex ratios, towards both a higher proportion of males and of females. In the absence of direct observations, sex ratio–deviated groups, indicative of aggregative behaviour within genders, are suggestive of differential reproductive behaviour and/or selective feeding in relation to the energetics and quality needs depending on gonadal developmental stages. An increase in stomach fullness and dietary energy intake has been recorded in pre-reproductive females of the deep-sea shrimp Aristeus antennatus, parallel to a significant increase of the gonadosomatic index and fecundity (Cartes et al. 2008Cartes J.E., Papiol V., Guijarro B. 2008. The feeding and diet of the deep-sea shrimp Aristeus antennatus off the Balearic Islands (Western Mediterranean): Influence of environmental factors and relationship with the biological cycle. Prog. Oceanogr. 79: 37-54.). Differences in diet composition between genders and maturity stages have also been found in other shrimps, Plesionika heterocarpus and Plesionika martia (Fanelli and Cartes 2008Fanelli E., Cartes J.E. 2008. Spatio-temporal changes in gut contents and stable isotopes in two deep Mediterranean pandalids: influence on the reproductive cycle. Mar. Ecol. Prog. Ser. 355: 219-233.). Aggregative behaviour within genders has also been related to other causes, such as the need to look for appropriate habitat for moulting (Reid et al. 1994Reid D.G., Abelló P., Warman C.G., et al. 1994. Size-related mating success in the shore crab Carcinus maenas (Crustacea: Brachyura). J. Zool. Lond. 232: 397-407., 1997Reid D.G., Abelló P., Kaiser M.J., et al. 1997. Carapace colour, intermoult duration and the behavioural and physiological ecology of the shore crab Carcinus maenas. Est. Coast. Shelf Sci. 44: 203-211., Sampedro and González-Gurriarán 2004Sampedro M.P., González-Gurriarán E. 2004. Aggregating behaviour of the spider crab Maja squinado in shallow waters. J. Crust. Biol. 24: 168-177.) or for hatching (i.e. females look for suitable sites to improve dispersal and survival of recently hatched larvae) (Hicks 1985Hicks J.W. 1985. The breeding behaviour and migrations of the terrestrial crab Gecarcoidea natalis (Decapoda, Brachyura). Aust. J. Zool. 33: 127-142., Zeng and Naylor 1997Zeng C., Naylor E. 1997. Rhythms of larval release in the shore crab Carcinus maenas (Decapoda: Brachyura). J. Mar. Biol. Ass. U.K. 77: 451-461., Carr et al. 2004Carr S.D., Tankersley R.A., Hench J.L., et al. 2004. Movement patterns and trajectories of ovigerous blue crabs Callinectes sapidus during the spawning migration. Estuar. Coast. Shelf S. 60: 567-579.). Survival of larvae may also be related to larval aggregation behaviour in addition to dispersal patterns, which are assumed to be low given the large size of the eggs and lower number of embryos that P. sivado females carry, when compared with other caridean shrimps (Company et al. 2001Company J.B., Cartes J.E., Sardá F. 2001. Biological patterns and near-bottom population characteristics of two pasiphaeid decapod crustacean species, Pasiphaea sivado and P. multidentata, in the north-western Mediterranean Sea. Mar. Biol. 139: 61-73.).

In attention to the concept of operational sex ratio (OSR), defined as the proportion of receptive females to sexually active males (Correa and Thiel 2003Correa C., Thiel M. 2003. Mating systems in caridean shrimp (Decapoda Caridea) and their evolutionary consequences for sexual dimorphism and reproductive biology. Rev. Chil. Hist. Nat. 76: 187-203.), the results of the present study suggest that the populations of P. sivado may overall be close to the OSR optima. The histograms of Figure 6 show that the overall shape of the SFDs of males and females is very similar, but males are displaced towards the right (largest sizes) by just one size class. If we quantify the total adult males and females bigger than the size class preceding the one in which the sex ratio starts to be biased towards males, we will find a sex ratio close to equilibrium. Even with the existence of a size sexual dimorphism or some samples with biased sex ratio, no overall sex segregation at population level was found.

The present study has shown the occurrence of morphological differences between genders in the benthopelagic shrimp P. sivado, namely in the pleopod shape of adult individuals. This characteristic has been shown to be useful for determining sex in field studies. Additionally a slight size dimorphism has been identified, as well as geographic population differences in size structure. This knowledge may also be useful for studying population biology patterns in still poorly-known species inhabiting exploited demersal ecosystems.

ACKNOWLEDGEMENTSTop

The authors are grateful to all participants in the MEDITS_ES cruises for all the help and facilities provided. Special thanks are due to A. Carbonell, H. Gallardo, E. León, E. Marco-Herrero, E. Mayo, M. Rufino, P. Torres, E. Zas and many others who provided assistance on all cruises. We also thank Drs. G. Guerao, and J.E. Cartes for their comments. D.S. Simão acknowledges the pre-doctoral fellowship support by the Spanish Agency for International Cooperation and Development (AECID), Ministry of Foreign Affairs and Cooperation.

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