Age, growth and maturation in the mesopelagic squid Abralia andamanica (Cephalopoda: Enoploteuthidae) from the Arabian Sea

INTRODUCTION

Cephalopods are common components of the mesopelagic zones of the world oceans and occupy crucial roles in marine ecosystems both as predators and as prey (Clarke 1996Clarke M.R. 1996. The role of cephalopods in the world’s oceans: general conclusion and the future In: Clarke M.R. (ed), The Role of Cephalopods in the World’s Oceans. Phil. Trans. R. Soc. Lond. B. 351: 1105-1112. https://doi.org/10.1098/rstb.1996.0096 , Piatkowski et al. 2001Piatkowski U., Pierce G.J., Cunha M.M. 2001. Impact of cephalopods in the food chain and their interaction with the environment and fisheries: an overview. Fish. Res. 52: 5-10. https://doi.org/10.1016/S0165-7836(01)00226-0 , Boyle and Rodhouse 2005Boyle P., Rodhouse P.G. 2005. Cephalopods: ecology and fisheries. Blackwell Science, Oxford. https://doi.org/10.1002/9780470995310 ). Enoploteuthid squids, known as “myctophid fishes” of the squid world (Young and Harman 1985Young R.E., Harman R.F. 1985. Early life history stages of enoploteuthid squids (Cephalopoda: Teuthoidea: Enoploteuthidae) from Hawaiian waters. Vie Milieu. 35: 181-201.), generally occur at middle depths and occasionally on continental shelves (Norman 2003Norman M.D. 2003. Order Teuthida. In: Debelius H (ed). Cephalopods: a World Guide. Harxheim: Conchbooks, Germany. p.320.). The genus Abralia comprises small species associated with shallow bottoms on steep slopes, which as such are members of the mesopelagic-boundary fauna (Reid et al. 1991Reid S.B., Hirota J., Young R.E., et al. 1991. Mesopelagic-boundary community in Hawaii: Micronekton at the interface between neritic and oceanic ecosystems. Mar. Biol. 109: 427-440. https://doi.org/10.1007/BF01313508 , Roper and Jereb 2010Roper C.F.E., Jereb P. 2010. Family Enoploteuthidae. In: Jereb P., Roper C.F.E. (eds), Cephalopods of the world. An annotated and illustrated catalogue of species known to date. Vol. 2. Myopsid and Oegopsid Squids. FAO Species Catalogue for Fishery Purposes. No. 4, Vol. 2. Rome, FAO. pp. 223-236.). The genus contains 20 nominal species (Young and Tsuchiya 2018Young R.E., Tsuchiya K. 2018. Abralia Gray 1849. Version 29 March 2018 (under construction). http://tolweb.org/Abralia/19642/2018.03.29 in The Tree of Life Web Project, http://tolweb.org/ [Accessed 15 April 2018].).

Abralia andamanica Goodrich, 1896 is a small (mantle length up to 60 mm) mesopelagic-boundary species distributed in the Indo-West Pacific, the Arabian Sea, the Seychelles, the Yellow Sea, Japan, Hawaii, and off Australia’s northwest shelf (Wood and Day 1998Wood J.B., Day C.L. 1998. CephBase. http://www.cephbase.utmb.edu/ [Accessed 26/01/06]., Roper and Jereb 2010Roper C.F.E., Jereb P. 2010. Family Enoploteuthidae. In: Jereb P., Roper C.F.E. (eds), Cephalopods of the world. An annotated and illustrated catalogue of species known to date. Vol. 2. Myopsid and Oegopsid Squids. FAO Species Catalogue for Fishery Purposes. No. 4, Vol. 2. Rome, FAO. pp. 223-236.). A detailed study in the Arabian Sea showed that it is distributed along the continental shelf edge (Silas 1968Silas E.G. 1968. Cephalopoda of the west coast of India collected during the cruises of the research vessel Varuna, with a catalogue of the species known from the Indian Ocean. In: Proceedings of the Symposium on Mollusca. Marine Biological Association of India, Bangalore, pp. 277-359.). Its body morphology has been described, and its distribution is known, but little is known about its ecology and nothing about its age, growth or maturation. Enoploteuthids are an important component of the Arabian Sea ecosystem (Silas 1968Silas E.G. 1968. Cephalopoda of the west coast of India collected during the cruises of the research vessel Varuna, with a catalogue of the species known from the Indian Ocean. In: Proceedings of the Symposium on Mollusca. Marine Biological Association of India, Bangalore, pp. 277-359., Varghese et al. 2013Varghese S.P., Somvanshi V.S., Gulati D.K. 2013. Ontogenetic and seasonal variations in the feeding ecology of Indo-Pacific sailfish, Istiophorus platypterus (Shaw, 1792), of the eastern Arabian Sea. Indian J. Geo-Mar. Sci. 42:593-605.). According to the IUCN Red List of threatened species, it is as a species of Least-Concern and further research is recommended to better understand its population dynamics, life history, ecology and potential threats affecting it (Barratt and Allcock 2014Barratt I., Allcock L. 2014. Abralia andamanica. The IUCN Red List of Threatened Species 2014: e.T163184A981263. https://doi.org/10.2305/IUCN.UK.2014-1.RLTS.T163184A981263.en ).

Statolith growth increment analysis is a specialized method used for age and growth studies of squids (Arkhipkin 2004Arkhipkin A.I. 2004. Diversity in growth and longevity in short lived animals: squid of the suborder Oegopsina. Mar. Freshw. Res. 55: 341-355. https://doi.org/10.1071/MF03202 , Jackson 2004Jackson G.D. 2004. Advances in defining the life histories of myopsid squid. Mar. Freshw. Res. 55: 357-365. https://doi.org/10.1071/MF03152 ). Life-cycle characteristics of squids include rapid growth and a relatively short lifespan (Arkhipkin 2004Arkhipkin A.I. 2004. Diversity in growth and longevity in short lived animals: squid of the suborder Oegopsina. Mar. Freshw. Res. 55: 341-355. https://doi.org/10.1071/MF03202 ). However, there have been few studies of age and growth using statoliths in squids from the Arabian Sea (Sajikumar et al. 2019aSajikumar K.K, Sasikumar G, Mohan G., et al. 2019a. Age and growth of the little Indian squid, Loliolus hardwickei (Gray, 1849) in the Arabian Sea. J. Mar. Biol. Assoc. UK. 99:1621-1625. https://doi.org/10.1017/S0025315419000560 , 2020Sajikumar, K.K, Sasikumar G., Venkatesan V., et al. 2020. Distribution, age and growth of the diamondback squid, Thysanoteuthis rhombus (Cephalopoda: Thysanoteuthidae) from the tropical Arabian Sea. Fish. Res. 224: 105478. https://doi.org/10.1016/j.fishres.2019.105478 ). Furthermore, little is known about the biology, length-weight relationships and growth of mesopelagic squids from the Arabian Sea in general and of A. andamanica in particular. Though the family Enoploteuthidae comprises 41 species globally, only five have had their age and growth studied using statoliths: Abralia trigonura Berry, 1913, Abraliopsis atlantica Nesis, 1982, Abraliposis morisii Verany, 1839, Enoploteuthis leptura Leach, 1817 and Pterygioteuthis gemmata Chun, 1908 (Arkhipkin and Murzov 1990Arkhipkin A.I., Murzov S.A. 1990. Age and growth pattern of the micronectonic squid, Abraliopsis atlantica (Enoploteuthidae). Biol. Morya. 11:19-25., Bigelow 1992Bigelow K. 1992. Age and growth in paralarvae of the mesopelagic squid Abralia trigonura based on daily growth increments in statoliths. Mar. Ecol. Prog. Ser. 82: 31-40. https://doi.org/10.3354/meps082031 , Young and Mangold 1994Young R.E., Mangold K. 1994. Growth and reproduction in the mesopelagic boundary squid Abralia trigonura. Mar. Biol. 119: 413-421., Arkhipkin 1994Arkhipkin A.I. 1994. Age, growth and maturation of the squid Enoploteuthis leptura (Oegopsina: Enoploteuthidae) from the central-east Atlantic. J. Molluscan Stud. 60: 1-8. https://doi.org/10.1093/mollus/60.1.1 , 1996aArkhipkin A.I. 1996a. Age and growth of the squid Abraliopsis pfefferi (Oegopsina: Enoploteuthidae) from the Central-East Atlantic based on statolith microstructure. Sci. Mar. 60: 325-330., 1997Arkhipkin AI. 1997. Age of the micronektonic squid Pterygioteuthis gemmata (Cephalopoda: Pyroteuthidae) from the central-east Atlantic based on statolith growth increments, J. Molluscan Stud. 63: 287-290, https://doi.org/10.1093/mollus/63.2.287 ). These species are slow-growing, have a short lifespan, and mature at 3-4 months.

The biology of A. andamanica is poorly known, and nothing is known about the age and growth of enoploteuthids from the Arabian Sea. The objective of the present study was to determine the age and growth of A. andamanica from the tropical Arabian Sea based on statolith increment analysis.

MATERIALS AND METHODS

Sampling

 ⌅

Individuals of A. andamanica were collected from commercial bottom trawls towed at 300 m depth in the Arabian Sea off Kollam (8°45’N; 73°52’E) on 26 December 2019. The squid were caught in trawls targeting the deep-sea shrimp Aristeus alcockiRamadan, 1938 and were identified as A. andamanica based on the key provided by Tsuchiya (2009)Tsuchiya K. 2009. Abralia andamanica (Goodrich 1896). Version 26 July 2009 (under construction). http://tolweb.org/Abralia_andamanica/19655/2009.07.26 in The Tree of Life Web Project, http://tolweb.org/ . The species was identified based on the structure of five photophores (two large terminal opaque organs and three intermediate silvery organs) on the ventral side of the eyeball (Fig. 1B). In this species, the mantle is conical and wide at the anterior margin (Fig. 1A), and tapers in the posterior half forming a long mantle apex (tail). The arm formula is 4<2<3<1.

Fig. 1.  - Dorsal and ventral view of Abralia andamanica collected from Arabian Sea (scale bar=2 cm) (A); view of ocular photophore in the ventral region of the eyeball (scale bar=1 cm) (B).

A total of 140 specimens comprising 119 females and 21 males were examined. The samples were preserved in ice immediately after capture and transported to the laboratory, where the dorsal mantle length (DML) of each specimen was measured to the nearest mm, as defined by Roper and Voss (1983)Roper C.F.E., Voss G.L. 1983. Guidelines for taxonomic descriptions of cephalopod species. Mem. Mus. Vic. 44: 48-63. https://doi.org/10.24199/j.mmv.1983.44.03 , and sexual maturity was assessed using the scale described in Lipinski (1979)Lipinski M.R. 1979. Universal maturity scale for the commercially-important squids (Cephalopoda: Teuthoidea). The results of maturity classification of the Illex illecebrosus (LeSuer, 1821) populations for the years 1973-1977. International Commission for the Northwest Atlantic Fisheries 5364: 1-14.. Body weights of thawed specimens were determined using an electronic balance.

RESULTS

Length-weight relationship

 ⌅

DMLs ranged from 15 to 26 mm (mean=22 mm, SD±5.6) in males and 23 to 60 mm (mean=31 mm, SD±7.7) in females. Body weights ranged from 0.58 to 2.1 g (mean=1.28 g, SD±0.61) in males and 1.5 to 5.7 g (mean=2.9, SD±1.4 g) in females. The length-weight relationship was BW= 0.313 DML^1.808 for females and BW= 0.308 DML^1.718 for males (Fig. 2). The coefficient of determination (R²) was 0.88 for females and 0.85 for males. The ANCOVA showed no significant difference between the sexes (P=0.354). Therefore, the length-weight relationship of pooled samples (combined sexes) was determined as W=0.278 DML^1.884 (R²=0.93). Negative allometric growth was observed for males, for females and for the pooled data. The value of the exponent, b, was significantly different from 3 (P<0.01).

Fig. 2.  - Length-weight relationship of Abralia andamanica males (yellow circle) and females (red circle) from the Arabian Sea.

Age and growth

 ⌅

The total lengths of statoliths ranged from 815 to 960 µm (mean= 876 µm, SD±31). The statolith shape has a round lateral dome, with a straight rostrum and a dorsal dome and a large wing (Fig. 3A). Growth increments were observed in all statoliths. They were deposited around an oval-shaped nucleus with a diameter ranging from 14.26 to 18.46 µm (mean=15.46 µm, SD±4.1). Growth increments were grouped into two zones based on the increment width (Fig. 3B). The first 30-40 increments from the outside of the nucleus (inner zone) had an average width of 2.63 µm. Towards the margin (outer zone), increment widths gradually decreased (mean= 0.70 µm) (Fig. 3B).

Fig. 3.  - Light micrograph of the statolith of Abralia andamanica (DML=38 mm) (A) and lateral dome region of statolith with increments (B). N, nucleus; IN, inner zone; and OZ, outer zone of the statolith (scale bar, A=200 µm; B=100 µm).

Based on the increment counts, ages ranged from 83 to 125 (mean =99, SD±12) days in males and 79 to 177 (mean=126, SD±18) days in females (Fig. 4A). The youngest (79 days) was a maturing female with a DML of 24 mm, and the oldest (177 days) was a mature female of 60 mm DML. The smallest female measured had a DML of 23 mm and was 82 days old, while the smallest male measured was 15 mm DML and was 83 days old.

Fig.4.  - Relationship between age with dorsal mantle length (A) and total weight (B) of males (yellow circles) and females (red circles) Abralia andamanica from the Arabian Sea.

The growth rate in body weight ranged from 0.006 to 0.02 g/day in males and 0.01 to 0.05 g/day in females (Fig. 4B). The maximum growth rate recorded was 0.05 g/day in a female of 43 mm DML at an age of 106 days. The daily growth rate in length ranged from 0.16 to 0.30 mm (mean=0.24, SD±0.03) in males and from 0.23 to 0.43 mm (mean=0.33, SD±0.04) in females (Fig. 5). For both sexes, the relationship between DML and growth rate was best described as a power function (Fig. 5).

Fig. 5.  - Relationship between dorsal mantle length and growth rate of males (yellow circles) and females (red circles) Abralia andamanica from the Arabian Sea.

The length (DML) at age data were described by a linear function for males and a power function for females (Table.1). The relationship between DML and age was expressed as DML=0.395+13.552AGE (R²=0.79) for males and DML=0.241AGE^1.069 (R²=0.65) for females. The relationship between BW and age was expressed as a power function for both sexes: BW=6E-06AGE^2.715 (R²=0.65) for males and BW=0.0004AGE^1.916 (R²=0.57) for females. The correlation coefficient (R²) was higher for males than for females for both the DML and BW age relationships.

Table 1.  Models of growth curves fitted to the male and female Abralia andamanica from the Arabian Sea (bold type face indicates best fit model).

		DML/number of increments			BW/number of increments
Sex	Function	a	b	R2	a	b	R2
Male	Linear	0.395	13.552	0.79	0.0386	2.198	0.622
	Power	0.016	1.594	0.78	6E-06	2.715	0.650
	Exponential	5.261	0.016	0.76	0.107e	0.026	0.625
Female	Linear	0.323	2.055	0.601	0.0538	2.288	0.490
	Power	0.241	1.069	0.656	0.0004	1.916	0.570
	Exponential	14.221	0.0086162	0.609	0.605e	0.015	0.526

DISCUSSION

This report of A. andamanica from the southeast Arabian Sea was made after a 50-year hiatus. Silas (1968)Silas E.G. 1968. Cephalopoda of the west coast of India collected during the cruises of the research vessel Varuna, with a catalogue of the species known from the Indian Ocean. In: Proceedings of the Symposium on Mollusca. Marine Biological Association of India, Bangalore, pp. 277-359. recorded A. andamanica from the southeast Arabian Sea in all months of 1967 except January, September and October during the Varuna expedition. The individuals in the present study were caught from continental shelf waters, and earlier findings (Silas 1968Silas E.G. 1968. Cephalopoda of the west coast of India collected during the cruises of the research vessel Varuna, with a catalogue of the species known from the Indian Ocean. In: Proceedings of the Symposium on Mollusca. Marine Biological Association of India, Bangalore, pp. 277-359.) showed that A. andamanica is a member of the mesopelagic boundary community like its closely related congener A. trigonura from Hawaiian waters (Bower et al. 1999Bower J.R., Nakamura Y., Mori K., et al. 1999. Distribution of Todarodes pacificus (Cephalopoda: Ommasteriphidae) paralarvae near the Kuroshio off southern Kyushu, Japan. Mar. Bio. 135: 65-71 https://doi.org/10.1007/s002270050606 ).

There were no apparent differences between males and females in the dorsal mantle length - body weight relationship. The exponent (b) of the DML-BW relationship was significantly lower than 3, showing allometric growth. This general change in body shape with growth, in which the mantle length increases allometrically as juveniles develop into slender, more streamlined adults, has been previously recorded in other squid species (Coelho et al. 1994Coelho M.L., Quintela J., Bettencourt V., et al. 1994. Population structure, maturation patterns and fecundity of the squid Loligo vulgaris from southern Portugal. Fish. Res. 21: 87-102. https://doi.org/10.1016/0165-7836(94)90097-3 , Sifner and Vrgoc 2004Sifner S.K., Vrgoc N. 2004. Population structure, maturation and reproduction of the Common squid, Loligo vulgaris, in the Central Adriatic Sea. Fish. Res. 69: 239-249. https://doi.org/10.1016/j.fishres.2004.04.011 ).

Statoliths of A. andamanica are similar to those of other species of the genus Abralia (Young and Mangold 1994Young R.E., Mangold K. 1994. Growth and reproduction in the mesopelagic boundary squid Abralia trigonura. Mar. Biol. 119: 413-421., Sajikumar et al. 2019bSajikumar K.K., Lipinski M.R., Venkatesan V., et al. 2019b. New record of Abralia (Heterabralia) siedleckyi Lipinski, 1983 (Cephalopoda: Enoploteuthidae) from south-eastern Arabian Sea with some remarks about its biology. J. Mar. Biol. Assoc. India. https://doi.org/10.6024/jmbai.2018.60.2.2060-15 ). Characteristic morphological features include the short rostrum and well-developed dorsal dome, as well as the broad wing. The statolith increments are well resolved, unambiguous and similar to those in A. trigonura (Young and Mangold 1994Young R.E., Mangold K. 1994. Growth and reproduction in the mesopelagic boundary squid Abralia trigonura. Mar. Biol. 119: 413-421.).

The average diameter of the statolith nucleus of A. andamanica is larger than that of A. trigonura (12.7 µm) from Hawaiian waters (Bigelow 1992Bigelow K. 1992. Age and growth in paralarvae of the mesopelagic squid Abralia trigonura based on daily growth increments in statoliths. Mar. Ecol. Prog. Ser. 82: 31-40. https://doi.org/10.3354/meps082031 ). The two growth zones of the statolith indicate slow and fast statolith growth corresponding to two life-history phases. A. trigonura paralarvae occur at 15-30 m during the night and 50-70 m during the day (Young and Harman 1985Young R.E., Harman R.F. 1985. Early life history stages of enoploteuthid squids (Cephalopoda: Teuthoidea: Enoploteuthidae) from Hawaiian waters. Vie Milieu. 35: 181-201.), while adults are epipelagic (<200 m during the night) to mesopelagic (~600 m during the night). The early stages of the enoploteuthids occur in pelagic warm waters, later start their vertical migrations ascending the epipelagic zone at night and plunge during the day into mesopelagic waters (Arkhipkin and Schetinnikov 1989Arkhipkin.A.I., Schetinnikov A.S. 1989. Fauna and distribution of the young and adult pelagic cephalopods of Gulf of Guinea. ZooL Zh. 68: 26-32.), where the temperature is normally much lower than in surface waters. Bigelow (1992)Bigelow K. 1992. Age and growth in paralarvae of the mesopelagic squid Abralia trigonura based on daily growth increments in statoliths. Mar. Ecol. Prog. Ser. 82: 31-40. https://doi.org/10.3354/meps082031 and Arkhipkin (1996a)Arkhipkin A.I. 1996a. Age and growth of the squid Abraliopsis pfefferi (Oegopsina: Enoploteuthidae) from the Central-East Atlantic based on statolith microstructure. Sci. Mar. 60: 325-330. hypothesized a sharp decrease in the growth rate during the transition from holopelagic to mesopelagic lifestyle seems to be similar for all enoploteuthids.

Females grew faster than males. Earlier, length at age data were described as a linear function in P. gemmata from the central-east Atlantic (Arkhipkin 1997Arkhipkin AI. 1997. Age of the micronektonic squid Pterygioteuthis gemmata (Cephalopoda: Pyroteuthidae) from the central-east Atlantic based on statolith growth increments, J. Molluscan Stud. 63: 287-290, https://doi.org/10.1093/mollus/63.2.287 ). The relationship between body mass and age was best described by an exponential and linear growth curve for females and males, respectively, in A. andamanica from the Arabian Sea.

The oldest mature female was 177 days old, suggesting that A. andamanica may have a lifespan of about 6 months in the tropical Arabian Sea, which is similar to that reported for A. trigonura from the Hawaiian waters (Young and Mangold 1994Young R.E., Mangold K. 1994. Growth and reproduction in the mesopelagic boundary squid Abralia trigonura. Mar. Biol. 119: 413-421.). A mature female individual of Abralia siedleckyi Lipinski, 1983 (29 mm DML) from the Arabian Sea had an age of 93 days with a daily growth rate of 0.31 mm DML/day (Sajikumar et al. 2019bSajikumar K.K., Lipinski M.R., Venkatesan V., et al. 2019b. New record of Abralia (Heterabralia) siedleckyi Lipinski, 1983 (Cephalopoda: Enoploteuthidae) from south-eastern Arabian Sea with some remarks about its biology. J. Mar. Biol. Assoc. India. https://doi.org/10.6024/jmbai.2018.60.2.2060-15 ). It has been found that enoploteuthids including Abraliopsis atlantica, Abraliopsis morisii and Enoploteuthis leptura grow slowly and have short lifespans (Arkhipkin and Murzov 1990Arkhipkin A.I., Murzov S.A. 1990. Age and growth pattern of the micronectonic squid, Abraliopsis atlantica (Enoploteuthidae). Biol. Morya. 11:19-25., Arkhipkin 1994Arkhipkin A.I. 1994. Age, growth and maturation of the squid Enoploteuthis leptura (Oegopsina: Enoploteuthidae) from the central-east Atlantic. J. Molluscan Stud. 60: 1-8. https://doi.org/10.1093/mollus/60.1.1 , 1996aArkhipkin A.I. 1996a. Age and growth of the squid Abraliopsis pfefferi (Oegopsina: Enoploteuthidae) from the Central-East Atlantic based on statolith microstructure. Sci. Mar. 60: 325-330.). However, the lifespan is longer than that of the smallest enoploteuthid squid, P. gemmata, which has a lifespan of 2.5 months (Arkhipkin 1997Arkhipkin AI. 1997. Age of the micronektonic squid Pterygioteuthis gemmata (Cephalopoda: Pyroteuthidae) from the central-east Atlantic based on statolith growth increments, J. Molluscan Stud. 63: 287-290, https://doi.org/10.1093/mollus/63.2.287 ). The short lifespan is a characteristic feature of squids of tropical waters. A six-month life span has also been described for many ommastrephids in oceanic tropical waters (Arkhipkin 1996bArkhipkin A. 1996b. Geographical variation in growth and maturation of the squid Illex coindetii (Oegopsida, Ommastrephidae) off the north-west African coast. J. Mar. Biol. Assoc UK. 76: 1091-1106. https://doi.org/10.1017/S0025315400040984 , Arkhipkin et al. 1998Arkhipkin A.I., Laptikhovsky V.V., Nigmatullin CM., et al. 1998. Growth, reproduction and feeding of the tropical squid Ornithoteuthis antillarum (Cephalopoda, Ommastrephidae) from the central-east Atlantic. Sci. Mar. 62: 273-288. https://doi.org/10.3989/scimar.1998.62n3273 ) and nearshore loliginid squids in the tropical waters (Jackson 2004Jackson G.D. 2004. Advances in defining the life histories of myopsid squid. Mar. Freshw. Res. 55: 357-365. https://doi.org/10.1071/MF03152 , Jin et al. 2019Jin Y., Li N., Chen X., et al. 2019. Comparative age and growth of Uroteuthis chinensis and Uroteuthis edulis from China Seas based on statolith. Aquac. Fish. 4: 166-172. https://doi.org/10.1016/j.aaf.2019.02.002 , Sajikumar et al. 2019aSajikumar K.K, Sasikumar G, Mohan G., et al. 2019a. Age and growth of the little Indian squid, Loliolus hardwickei (Gray, 1849) in the Arabian Sea. J. Mar. Biol. Assoc. UK. 99:1621-1625. https://doi.org/10.1017/S0025315419000560 ). Sexual dimorphism is also reflected in age and growth rates. Females of the enoploteuthid squid Abraliopsis atlantica live 1 to 1.5 months longer than males (Arkhipkin and Murzov 1990Arkhipkin A.I., Murzov S.A. 1990. Age and growth pattern of the micronectonic squid, Abraliopsis atlantica (Enoploteuthidae). Biol. Morya. 11:19-25.). The biological reasons for the sexual dimorphism in the age of A. andamanica is unclear.

The age versus DML data could be explained by a linear function for males and a power function for females. Previous studies in A. trigonura from Hawaiian waters showed a logistic model for females and a Gompertz model for males, while a linear function was reported in P. gemmata from the central-east Atlantic (Arkhipkin 1997Arkhipkin AI. 1997. Age of the micronektonic squid Pterygioteuthis gemmata (Cephalopoda: Pyroteuthidae) from the central-east Atlantic based on statolith growth increments, J. Molluscan Stud. 63: 287-290, https://doi.org/10.1093/mollus/63.2.287 ). The congener A. trigonura is characterized by exponential growth in the paralarval stage, followed by a slower growth stage, which is evident from a length of 16 mm ML onwards (Bigelow 1992Bigelow K. 1992. Age and growth in paralarvae of the mesopelagic squid Abralia trigonura based on daily growth increments in statoliths. Mar. Ecol. Prog. Ser. 82: 31-40. https://doi.org/10.3354/meps082031 ). In A. andamanica, females grew faster than males, but the lack of individuals below 15 mm DML and the limited number of samples prevented us from the constructing a growth curve.

The relative weight of the reproductive system (mean=23.6%) was the same as that of enoploteuthids from the Atlantic (Laptikhovsky 1999Laptikhovsky V.V. 1999. Fecundity and spawning in squid of families Enoploteuthidae and Ancystrocheiridae (Cephalopoda: Oegopsida). Sci. Mar. 63: 1-7. https://doi.org/10.3989/scimar.1999.63n11 ). However, the GSI was higher than in A. trigonura (GSI=9.7%) from Hawaiian waters (Young and Mangold 1994Young R.E., Mangold K. 1994. Growth and reproduction in the mesopelagic boundary squid Abralia trigonura. Mar. Biol. 119: 413-421.). The oocytes were small (mean=0.87 mm), as in all enoploteuthids studied (Laptikhovsky 1999Laptikhovsky V.V. 1999. Fecundity and spawning in squid of families Enoploteuthidae and Ancystrocheiridae (Cephalopoda: Oegopsida). Sci. Mar. 63: 1-7. https://doi.org/10.3989/scimar.1999.63n11 ). The oviducal fecundity of enoploteuthid squids reaches 400 in Abraliopsis atlantica and A. trigonura, and a maximum number of 2800 oocytes were observed in Enoploteuthis anapsis Roper, 1964 (Young and Mangold 1994Young R.E., Mangold K. 1994. Growth and reproduction in the mesopelagic boundary squid Abralia trigonura. Mar. Biol. 119: 413-421., Laptikhovsky 1999Laptikhovsky V.V. 1999. Fecundity and spawning in squid of families Enoploteuthidae and Ancystrocheiridae (Cephalopoda: Oegopsida). Sci. Mar. 63: 1-7. https://doi.org/10.3989/scimar.1999.63n11 ). The absolute fecundity of Abralia andamanica from the Arabian Sea was estimated to range from 2400 to 3200 oocytes (Amelekhina 1983Amelekhina A.M. 1983. Some aspects of the life history of the Indian ocean squid Abralia andamanica Goodrich, 1896. Taxonomy and ecology of Cephalopoda. Zoological Institute of the USSR Academy of Sciences, Leningrad.(In Russian): 108-109.). The mean oocyte number was 251 in the oviducts of Abralia verany Rüppell, 1844 from the eastern Mediterranean Sea (Salman and Laptikhovsky 2005Salman A., Laptikhovsky V. 2005. Fecundity and spawning of Abralia verany (Rüppell, 1844) (Cephalopoda: Enoploteuthidae) in the Aegean Sea. Sci. Mar. 69: 211-214. https://doi.org/10.3989/scimar.2005.69n2211 ), and 284 in A. siedleckyi from the Arabian Sea (Sajikumar et al. 2019bSajikumar K.K., Lipinski M.R., Venkatesan V., et al. 2019b. New record of Abralia (Heterabralia) siedleckyi Lipinski, 1983 (Cephalopoda: Enoploteuthidae) from south-eastern Arabian Sea with some remarks about its biology. J. Mar. Biol. Assoc. India. https://doi.org/10.6024/jmbai.2018.60.2.2060-15 ), figures which are comparable to our estimated mean of 274 oocytes. Young and Mangold (1994)Young R.E., Mangold K. 1994. Growth and reproduction in the mesopelagic boundary squid Abralia trigonura. Mar. Biol. 119: 413-421. indicated that A. trigonura is a multiple spawner, and Laptikhovsky (1999)Laptikhovsky V.V. 1999. Fecundity and spawning in squid of families Enoploteuthidae and Ancystrocheiridae (Cephalopoda: Oegopsida). Sci. Mar. 63: 1-7. https://doi.org/10.3989/scimar.1999.63n11 showed that spawning is prolonged and intermittent for enoploteuthids.

The present study provides information on the age and growth of A. andamanica, a mesopelagic squid from the tropical Arabian Sea, which reveals a short lifespan, slow growth and sexual dimorphism.