Age and growth of bigfin reef squid, Sepioteuthis lessoniana (Cephalopoda: Loliginidae), in Gulf of Mannar Marine Biosphere Reserve, Indian Ocean

Abstract

Statolith growth increments were analysed in the bigfin reef squid, Sepioteuthis lessoniana lineage B, for estimating the age and growth in the Gulf of Mannar Biosphere Reserve (GOM), southeast coast of India. The identification of S. lessoniana lineage B was determined by mitochondrial cytochrome c oxidase I gene sequence. The statolith increment age analysis indicated that the wild-captured squid population of S. lessoniana in the study area undergoes rapid growth. The age of S. lessoniana in males ranged from 61 (95 mm dorsal mantle length (DML)) to 220 d (390 mm DML), while it was 64 (98 mm DML) to 199 d (340 mm DML) in females. The average daily growth rate in males and females was 1.63 and 1.55 mm DML d⁻¹, respectively. The instantaneous growth rate varied from 0.85 (210 d) to 4.1% (110 d) for males and 0.65 (190 d) to 3.7% (110 d) for females. The age at first maturity was 114 and 120 d for males and females, respectively. Back-calculated hatching dates and the attainment of maturity in females suggested that the reproduction of S. lessoniana is year-round, with two distinct spawning peaks during July–August and February months; accordingly, the hatching dates were spread throughout the year, with the presence of two cohorts. Based on the statolith data, it can be concluded that S. lessoniana lineage B in the GOM has a potential lifespan of up to 7 months. This finding contradicts the previous growth estimates based on length-frequency data, which underestimated the true growth potential of this species.

Introduction

The bigfin reef squid Sepioteuthis lessoniana is a demersal neritic species and one of the most widely distributed loliginid squid of the Indo-West Pacific region (Jereb and Roper, 2010) and it has also been reported from northern Australia, New Zealand, central Japan, the Mediterranean Sea and eastward to the Hawaiian Islands (Lefkaditou et al., 2009; Jereb and Roper, 2010). Given its widespread distribution in the Indo-Pacific region, S. lessoniana is an important species in coastal fisheries across many countries (Jereb and Roper, 2010). Moreover, the bigfin squid is utilized for biomedical research and holds commercial value as a mariculture species (Lee et al., 1994; Walsh et al., 2002; Nabhitabhata and Ikeda, 2014).

Most squids exhibit rapid growth and have a short lifespan (Arkhipkin, 2004; Jackson, 2004). Understanding key parameters such as age and growth is essential to comprehend the critical life history patterns necessary for effective management (Jackson, 2004). Age and growth of S. lessoniana have been extensively studied using validated daily statolith increments (Jackson, 1990; Jackson et al., 1993; Balgos and Pauly, 1998). Several studies have directly documented the age and growth of S. lessoniana in captivity (Lee et al., 1994; Nabhitabhata, 1995, 1996; Forsythe et al., 2001) supporting the growth estimates derived from size-at-age information based on statolith analysis. These captive-rearing studies indicate a lifespan ranging from 115 to 333 d (SEAFDEC, 1975; Nabhitabhata, 1978, 1996; Tsuchiya, 1982; Segawa, 1987; Lee et al., 1994; Walsh et al., 2002). Statolith increment-based age and growth of the S. lessoniana were studied in the waters of Tropical Australia, Subtropical Australia, Thailand, the Arabian Sea and Philippines (Jackson, 1990; Balgos and Pauly, 1998; Jackson and Moltschaniwskyj, 2002; Chen et al., 2015; Sajikumar, 2021) that showed a short lifespan with age ranges of 124–260 d.

Recent studies based on both morphological and molecular evidence indicate that S. lessoniana may be a species complex (Okutani, 2005; Triantafillos and Adams, 2005). The three lineages (lineage A, B and C) were present in markedly different abundances (Cheng et al., 2014). Lineage C appeared to be most abundant while lineages B and A were less abundant, but co-occurred with lineage C and occasionally each other (Cheng et al., 2014). In Indian seas, both lineages B and C have been reported, with lineage B identified in the Gulf of Mannar (GOM; Cheng et al., 2014).

Sepioteuthis lessoniana accounts for around 7% of the Indian east coast cephalopod landings, all from Palk Bay and the GOM (Jereb and Roper, 2010). The asymptotic models of squid growth curves generated from length-frequency analysis of S. lessoniana continue to gain support from Sri Lanka (Charles and Sivashanthini, 2011) and Palk Bay, India (Venkatesan, 2012) and showed a lifespan up to 3.3 years, while age estimation using statoliths has been initiated recently in the Arabian Sea (WIO) suggesting a short life span (Sajikumar, 2021). Significant spatial and temporal variation occurs in the growth rates and maturity of equatorial, tropical and subtropical populations of S. lessoniana (Jereb and Roper, 2010). Understanding the specific traits within the S. lessoniana species complex in a region is crucial for developing management plans. Therefore, the present study focuses on investigating the age and growth of S. lessoniana, particularly the lineage B, by using statolith growth increments, obtained from the Gulf of Mannar Marine Biosphere Reserve, Eastern Indian Ocean (EIO), located in southeast coast of India.

Material and methods

Study area

The GOM in the Indian Ocean is located between 78°08′ E to 79°30′ E and 8°35′ N to 9°25′ N on the Southeast Coast of the Indian Peninsula. It spans a coastline of 365 km, and is bordered by the districts of Kanyakumari, Tirunelveli, Tuticorin and Ramanathapuram in the state of Tamil Nadu, India. The GOM, which encompasses 21 coral islands, is also a Marine Biosphere Reserve, recognized for its rich coastal and marine habitats such as seagrass, seaweeds, mangroves, coral reefs and estuaries, all of which support a diverse array of species (Kumaraguru et al., 2006).

Sampling

The S. lessoniana samples included in this analysis were caught using trawls and jigs operated in the GOM during the months of April, May, August–October 2019 and January–December 2020. In the laboratory, the dorsal mantle length (DML) of the squids was measured with an accuracy of ±0.1 mm and the total weight (TW) was recorded to the nearest ±0.01 g. Specimens were dissected to determine sex and maturity. A total of 782 S. lessoniana specimens were examined, with males ranging in DML from 59 to 390 mm (n = 412) and females ranging from 70 to 349 mm (n = 370). Maturity stages for each sex were categorized according to the modified scale proposed by Mangold-Wirz (1963) as I – immature, II – maturing, III – mature and IV – spawning for females and I – immature, II – maturing, III – mature and IV – spent for males.

DNA isolation and PCR reaction

The DNA isolation from the S. lessoniana samples was carried out through the conventional phenol-chloroform method and the DNA was stored at −20 °C for further use in PCR reactions (Sambrook et al., 1989). The PCR reaction was performed using a Thermo Scientific^TM master mix (Cat. No. K0171) and the primers LCO 1490 and HC02198 (Folmer et al., 1994). The PCR amplification was carried out under normal cyclic conditions of initial denaturation at 94 °C for 180 s, then cycling at 94 °C for 30 s, annealing at 56 °C for 40 s and extension at 72 °C for 40 s for 30 cycles, followed by a final extension of 72 °C for 7 min in the thermal cycler (Agilent sure cycler 8800, USA). The resulting amplified products were cleaned and sequenced at the Genurem Biosciences LLP Sequencing Facility (Applied Biosystems 3730xl DNA Analyzer) in India. The cytochrome c oxidase subunit I gene sequence was blast in the NCBI blast portal and matches with S. lessoniana lineage B. The gene sequence was submitted to NCBI and accession number (OP572033.1) was received.

Phylogenetic tree analysis

The phylogenetic tree was constructed using the COI sequences of the current study with the earlier reported individual gene alignments of different lineages (A, B and C) along with out-groups including Sepioteuthis sepioidea (AF075392) and Loligo bleekeri (AB573754) (Figure 1). The phylogenetic tree was constructed by the maximum likelihood method, Kimura 2-parameter model (Kimura, 1980) using the MEGA 11 program (Tamura et al., 2021) based on the 15 numbers of nucleotide sequences found in the NCBI GenBank database (Table 1). The percentage of trees in which the associated taxa clustered together was depicted next to the branches. A discrete Gamma distribution was used to model evolutionary rate differences among sites (5 categories ( + G, parameter = 0.2901)). Codon positions included were 1st + 2nd + 3rd + non-coding.

Figure 1. Phylogenetic tree of Sepioteuthis lessoniana in this study with the specimens representing distinct lineages. The tree was reconstructed with COI sequences (614 bp) using the Maximum Likelihood method and Kimura 2-parameter model. The lineages marked in the tree were based on the classification of Cheng et al. (2014).

Table 1. The details of GenBank accession numbers of COI gene sequences of Sepioteuthis lessoniana and its lineage used for phylogenetic tree construction in this study

Statolith collection and processing

Paired statoliths were dissected from fresh specimens, cleaned and preserved in 70% ethanol. The extraction and preparation for counting growth increments were carried out in accordance with Arkhipkin and Shcherbich (2012). The anterior section of the statoliths was mounted on a microscope slide using thermoplastic cement glue (Crystalbond^TM 509). The statolith length (SL) was measured to the nearest 1 μm, parallel to the longitudinal axis of the statolith, using a transmitted light microscope (Nikon Eclipse-80i, Japan) and image analysis software and measurements converted to mm for analysis. Statoliths were ground and polished using a 1500-grit fine-lapping film.

Estimation of growth using growth increments

To count growth increments, statolith was observed under a transmitted light microscope (Nikon Eclipse-80i) at a magnification of 600×, and total number of growth increments were determined by averaging the increment counts from three separate observations. The mean count was considered valid if the difference between the first and second counts was below 10%. If the deviation exceeded 10%, the counting process was repeated. The lateral dome of the statolith was used to determine the age, as its daily periodicity has been validated and the daily deposition of statolith increments has been authenticated as daily increments (Jackson, 1990; Jackson et al., 1993). The increments were counted from the hatching ring to the outer edge of the lateral dome, where they were most clearly distinct (Dawe et al., 1985; Villanueva, 1992). In cases where the increments on the lateral dome edges were not clearly recognizable, the number of increments for that section was extrapolated (Natsukari et al., 1988). Extrapolation involved estimating the increment counts in the indistinct area based on the widths of approximately 10 of the most recent countable increments from adjacent areas (Hoving et al., 2007). The images of the statoliths were captured with a transmitted light microscope, Nikon Eclipse-80i, or Leica DM6B with an sCMOS camera.

Daily growth rate

Previous studies have confirmed that the total number of increments in the squid statolith corresponds to the age of an individual in days (Jackson, 1990; Sajikumar, 2021). Therefore, the daily growth rate (DGR) was calculated using the equation provided by Jackson et al. (1997):

$${\rm DGR\;}( {{\rm mm\;\;}{\rm d}^{ \hbox{-} 1}{\rm \;}} ) {\rm} = \displaystyle{{{\rm DML\;}( {{\rm mm}} ) \hbox{-}{\rm Hatchling\;size\;}( {{\rm mm}} ) } \over {{\rm Age\;}( {{\rm days}} ) }}$$

where hatchling size was 5.0 mm DML (Segawa, 1987), DML = dorsal mantle length

$${\rm DGR\;}( {{\rm g\;d}{\rm \;}^{ \hbox{-} 1}} ) {\rm} = \displaystyle{{{\rm TW\;}( {\rm g} ) \hbox{-}{\rm Hatchling\;weight\;}( {\rm g} ) } \over {{\rm Age\;}( {{\rm days}} ) }}$$

where the hatchling weight was 0.06 g (Segawa, 1987), TW = total weight.

The power function yielded the best fit for the relationship between DML and DGR (mm DML d⁻¹ and g BW d⁻¹) for both sexes.

Statolith length index (SLI)

The statolith length index (SLI) was calculated using the terminology and measurements of statoliths after Clarke (1978):

$${\rm Statolith\;length\;index\;} = {\rm SL}/{\rm DML} \times 100{\rm \;}$$

where SL = statolith length (mm), DML = dorsal mantle length.

Instantaneous relative growth rate (IRG)

The instantaneous relative growth rate (IRG) was calculated in intervals of 20 d for each sex following Forsythe and Van Heukelem (1987) as:

$${\rm IRG\;} = {\rm \;}[ {{\rm ln\;S}2-{\rm ln\;S}1} ] /{\rm T}$$

where S₂ and S₁ are DML at the start and the end of each interval of time (T). Conversion of IRG to the per cent increase in DML d⁻¹ was done by multiplying IRG by 100.

Age at maturity

The mean age at which 50% of the squid attained maturity (L _m) was estimated by fitting a logistic function to the proportion of mature squid in the 10 mm size categories (Udupe, 1986).

Hatching date calculation

The hatching date of S. lessoniana was estimated by back calculation from the date of capture of the specimen using the age in days estimated from statolith daily growth increments.

Spawning date calculation

The spawning date was estimated by back calculation using the hatching date and the egg incubation period of S. lessoniana based on experimental rearing in Thailand by Nabhitabhata (1996). An average egg incubation time of 20 d was used for the calculation.

Data analysis

The data were statistically analysed by statistical package, SPSS version 20 (Carver and Nash, 2008). The difference in DGRs of male and female squids was evaluated using analysis of variance. Student t-test was carried out to assess the significant difference between males and females of different length groups at different ages. Comparisons were made at the 5% probability levels and all statements of statistical significance were based on P < 0.05

Result

Statolith microstructure

The statolith of S. lessoniana is robust and relatively long. It displays a round dorsal dome with a long, thin rostrum and broad wing (Figure 2). The nucleus is located within the central region of the statolith. The growth increments were counted from the lateral dome area, and from the hatching ring towards the statolith edge following the axis with the best visibility (Figure 3). Checks between post-nuclear zone and the peripheral zone were regularly recorded in many statoliths.

Figure 2. Dorsal (A) and ventral (B) view of the statolith of Sepioteuthis lessoniana (212 mm DML) from the Gulf of Mannar. DD, dorsal dome; LD, lateral dome; R, rostrum; W, wing (scale bar = 500 μm).

Figure 3. Light micrograph of the ground statolith of Sepioteuthis lessoniana adult (male of 225 mm DML) from the Gulf of Mannar (A). Magnified view of the area outlined by the rectangle (B) (scale bar = 500 μm)

Statolith size

In male squid, the length of the statolith (SL) ranged from 0.86 mm (juvenile, 59 mm DML) to 2.45 mm (spent adult, 350 mm DML). The SL in females ranged from 1.02 mm in juvenile (70 mm DML) to 2.43 mm in spawning (340 mm DML) squid. The relationship between DML and SL, and statolith growth index (SI) was best described by the power function. The SL to DML allometric relationship performed by the power function shows negative allometric growth for both males (b = 0.5112; R ² = 0.902) and females (b = 0.518; R ² = 0.876). The SLI against DML for males reduced from 1.51% (60 mm DML) to 0.60% (340 mm DML) and for females from 1.49% (70 mm DML) to 0.49% (252 mm DML). The SI decreased steadily with increasing DML (Figure 4A–D). The slopes of the relationships between DML and SL, and SI did not differ significantly (P > 0.05) between sexes.

Figure 4. Relationship between dorsal mantle length and statolith length of Sepioteuthis lessoniana: (A) males; (B) females; relationship between dorsal mantle length and statolith length index (C) males; (D) females from the Gulf of Mannar

Age and growth (hard part ageing)

The age of S. lessoniana males ranged from 62 (juvenile, 59 mm DML) to 220 d (spent, 390 mm DML) and in females ranged from 74 (juvenile, 88 mm DML) to 199 d (spawning, 340 mm DML) (Table 2). The most abundant age groups were 120–130 (18.2%) and 130–140 d (19.5%) of males and females, respectively (Figure 5). A comparison of age and DML indicated that the DML of males was greater than that of females above 80 d age group (Figure 6). The size of males between 100 and 160 d age was significantly (P < 0.05) larger than females.

Table 2. Age, size and growth rate of males and females of Sepioteuthis lessoniana from the Gulf of Mannar

DML, dorsal mantle length; DGR, daily growth rate; N, number of specimens.

The values given in the parenthesis are arithmetic mean ± SE.

Figure 5. Age distribution of Sepioteuthis lessoniana from the Gulf of Mannar

Figure 6. Size range of males and females in the different age groups of Sepioteuthis lessoniana from the Gulf of Mannar

Daily growth rate

The DGR (mm d⁻¹) of S. lessoniana ranged from 0.87 to 2.33 (mean = 1.63) mm DML d⁻¹ for males and 0.86 to 2.07 (mean = 1.55) mm DML d⁻¹ for females were observed. The DGR (g d⁻¹) for males ranged from 0.28 to 9.32 g (mean = 3.58 g) d⁻¹ and in females it ranged from 0.38 to 9.35 g (mean = 3.37 g) d⁻¹. In both sexes, the DGR (mm d^–1) increased with age, reaching a maximum of 121 d in males and 143 d in females. Male S. lessoniana recorded a higher DGR than females. The maximum age in S. lessoniana was 220 d in males (spent) and 199 d in females (spawning). The relationship between DML and DGR (mm d⁻¹) for both sexes, expressed as a power function, DML = 0.145 × Age^0.452(r ² = 0.74) for males and DML = 0.151 × Age^0.437(r ² = 0.72) for females. The relationship between DML and DGR (g d⁻¹) was BW = 0.0001DML^1.885 (r ² = 0.91) for males and BW = 0.0001 DML^1.9178 (r ² = 0.92) for females. The relationships between DML and DGR (mm d⁻¹) showed highly significant differences between sexes (P < 0.0001), while DML and DGR (g d⁻¹) were not significantly different between male and female squids (P > 0.05).

Instantaneous growth rate

The instantaneous growth rate (IGR) ranged from 0.85% (210 d) to 4.1% (110 d) for males and 0.65% (190 d) to 3.7% (110 d) for females (Figure 7). In both sexes, the IGR of DML changed throughout its lifespan. During the initial stage (70 d), the IGR was minimal in both males and females and after 70 d the IRG exhibited a steep increase and reached its maximum between 90 and 110 d beyond which, it decreased continuously. The IGR of males was slightly higher than those of females. The IGR decreased continuously in relation to age. It was very clearly noticed that, before the initial maturity age, both sexes showed higher rates, and as they got closer to maturity, this rate declined.

Figure 7. Instantaneous growth rate for males and females of Sepioteuthis lessoniana from the Gulf of Mannar

Maturation

Males attained maturity between 95 and 185 d (145–337 mm DML) and females between 102 and 168 d (130–280 mm DML). The size range of mature females was less than that of mature males, and the youngest mature male was younger (95 d) than the youngest mature female (102 d) (Table 2). According to the present study, the age at first maturity was 114 d for males, 120 d for females and 117 d for sexes combined (Figure 8). Squids older than 122 d for males and 136 d for females were mature (Figure 9). The oldest male was 220 d old at the spent stage, measuring 390 mm DML and weighing 2027 g. The oldest female squid was the age of 199 d in the spawning stage, measuring 340 mm DML and weighing 1450 g.

Figure 8. Age at first maturity of Sepioteuthis lessoniana from the Gulf of Mannar: males (A), females (B) and pooled sex (C)

Figure 9. Age-wise proportion of immature, maturing, mature and spent of Sepioteuthis lessoniana from the Gulf of Mannar: (A) males; (B) females; (C) pooled

Hatching and spawning

Back-calculated hatching dates and the attainment of maturity in females suggest that the reproduction of S. lessoniana is year-round, with spawning peaks during July (14%), August (11%) and February (16%) months (Figure 10). From the hatching date frequency distribution of S. lessoniana, it can be concluded that hatching occurs throughout the year with evident peaks, August (15%)–September (13%) and March (16%) (Figure 11) indicating the two major cohorts. The back-calculated hatching date in the present study suggests that S. lessoniana hatched continuously and grows up to the targeted size within 4–5 months after hatching.

Figure 10. Monthly spawning frequency distribution of Sepioteuthis lessoniana from the Gulf of Mannar

Figure 11. Monthly hatching frequency distribution of Sepioteuthis lessoniana from the Gulf of Mannar

Discussion

Lineage confirmation

Recently S. lessoniana has been discriminated using DNA markers such as partial sequences of mitochondrial cytochrome oxidase subunit I (COX1 or COI) for better stock identity, distribution and stock status (Cheng et al., 2014; Tomano et al., 2015, 2016). Cheng et al. (2014) reported that S. lessoniana contains lineages A, B and C whereas Tomano et al. (2015) classified the S. lessoniana into species 1, 2 and 3. In these classifications, it can be equated that lineage A = species 2, B = 1 and C = 3. The phylogenetic tree demonstrated that the S. lessoniana samples from this study formed a clade with other lineage B samples. Based on the earlier classifications (Cheng et al., 2014; Tomano et al., 2015), the DNA barcoding of S. lessoniana samples used in this study belonged to lineage B by Cheng et al. (2014) that equals to species 1 described by Tomano et al. (2015). The genetic difference of S. lessoniana among the different lineages showed noticeable differences among them. A similar response was noted by Cheng et al. (2014) while studying genetic divergences among the different lineages. In the present study, we adopted the nomenclature/terminology to our samples as lineage B rather than species in line with Cheng et al. (2014) whose samples represented similar sequences from our study area.

Statolith microstructure

The statolith of S. lessoniana is robust and rather large in length. The statolith pattern and form matched those identified in Australia for this species (Jackson, 1991). The nucleus was surrounded by a distinct hatching ring, and increments observed between the nucleus and the hatching ring may have been the embryonic ring, reflecting statolith growth within the egg (Jackson, 1994; Perez et al., 2006). The growth increments were most evident in the region of the lateral dome, while they were difficult to distinguish on the rostrum. The rostrums of species such as Idiosepius pygmaeus (Jackson, 1990) and Uroteuthis (Photololigo) edulis (Natsukari et al., 1988) contain plainly countable clear increments. The size of this hatching ring corresponded closely to the statolith's outer margin of the freshly hatched S. lessoniana proving that it is a hatching check. This similar feature has been reported in loliginid squids (Lipinski, 1986; Sajikumar et al., 2022) in the Loligo spp.

Statolith size

The SL in S. lessoniana from GOM is higher than 0.819–1.445 mm, reported by Sajikumar (2021), and 0.239–0.796 mm by Balgos and Pauly (1998). The allometric relationship of SL to DML indicated negative allometric growth in both males and females, a phenomenon observed in other loliginid squids, including U. duvaucelii (Sajikumar et al., 2022), U. edulis (Wang et al., 2010), L. vulgaris (Rocha and Guerra, 1999), Alloteuthis subulata, A. africana (Arkhipkin and Nekludova, 1993) and the mesopelagic squid Ancistrocherius lesueurii (Arkhipkin, 1997).

Age and growth

In the GOM region, the maximum lifespan of S. lessoniana is likely to be about 7 months. A comparison of age with DML indicates that the DML of male was larger than the female in all age groups, except those between 60 and 80 d. Bat et al. (2009) and Huang (2006) have reported that U. chinensis and U. edulis exhibit a similar pattern. The age and growth of S. lessoniana have been previously studied from tropical to sub-tropical waters. Previous research has demonstrated geographic variation in the lifespan of S. lessoniana, with a maximum age between 132 and 188 d. The maximum age of 220 d recorded for S. lessoniana in this study is greater than the ages previously reported of this species in tropical and subtropical waters [Australia – 188 d (Jackson, 1990); 187 d (Semmens and Moltschaniwskyj, 2000); Philippines – 132 d (Balgos and Pauly, 1998); Taiwan – 192 d (Chiang et al., 2020); and India – 156 d (Sajikumar, 2021)]. Chen et al. (2015) estimated the age of S. lessoniana to be 216 d from Taiwan water, which is more in line with the present study.

Individuals of S. lessoniana exhibited extremely varied growth rates, indicative of the plastic growth responses typical of squid (Jackson, 1994). In Japanese waters, S. lessoniana was estimated to live for approximately 1 year (Ueta and Jo, 1989). The rearing experiment of S. lessoniana from Japan suggests that this species can live for 11 months and reach a maximum size of 350 mm DML (Lee et al., 1994), but the largest specimen of S. lessoniana (480 mm DML) recorded by Walsh et al. (2002) in Japan water had an age of 262 d. Culture studies of S. lessoniana from different regions indicate that the maximum age of S. lessoniana ranged from 115 to 333 d (Table 3). The length-frequency analysis of S. lessoniana published in earlier studies indicates that the age of this species from Indian waters has been overestimated. Venkatesan (2012) reported 3 years for the squid of 300 mm DML and projected a lifespan of about 3.3 years. Rao (1954) similarly modelled a size of 95, 166 and 219 mm DML at the end of the first, second and third years, respectively. Likewise, Charles and Sivashanthini (2011) estimated a lower K value of 0.85 year⁻¹ for S. lessoniana demonstrating a long-life span (>3 years) for this species.

Table 3. Comparison of maximum size and age in Sepioteuthis lessoniana

DML, dorsal mantle length; M, male; F, female; – indicates sex is not mentioned

The values given in the parenthesis are culture temperature

Growth rates

The DGR in length, 1.63 mm DML d⁻¹ for males and 1.55 mm DML d⁻¹ for females is higher than the previous estimates reported for S. lessoniana. Sajikumar (2021) recorded a growth rate of 1.38 mm d⁻¹ for males in Indian waters and 1.19 mm d⁻¹ for females. According to Jackson (1990), males and females in Australian water grow at a rate of 1.42 and 1.34 mm d⁻¹, respectively. Balgos and Pauly (1998) measured a growth rate of 0.5 mm d⁻¹ in water from the Philippines. The estimated DGR in weight (3.58 g d^–1 for males and 3.37 g d⁻¹ females) in the present study is higher than Jackson and Moltschaniwskyj (2002), who reported the DGR of 2.89–3.18 g d⁻¹ from the Australian waters and 3.24 g d⁻¹ from Thailand waters. According to Jin et al. (2019), regional growth rates were notably diverse. A previous study on captive-reared S. lessoniana from Thailand revealed a rapid DGR of 1.6 mm and 3.82 g d⁻¹ (Nabhitabhata, 1996). In laboratory studies, the majority of loliginid squid species studied exhibited rapid growth rates (Turk et al., 1986; Yang et al., 1986; Villanueva, 2000; Vidal et al., 2014). The DGR increased with age in both sexes, with males having a larger DGR than females, a trait exhibited by loliginid squids (Wang et al., 2010), except for the genus Loliolus sp. (Sajikumar et al., 2019). However, the difference in the DGR between male and female S. lessoniana is lesser compared to other loliginid squids, U. edulis and U. singhalensis (Sajikumar, 2021). In both sexes, the IGR of DML changed during its lifespan. The IGR decreased progressively with age. It was very clearly noticed that, before the initial age of maturity, both sexes showed higher rates, but as they approached maturity, these rates decreased. Certain Ommastrephidae species have also been shown to exhibit a reduction in growth rates with gonadal development (Arkhipkin and Bizikov, 1991).

Maturation

The minimum age of maturity for males and females from the GOM was lower than that reported by Sajikumar (2021), where the youngest mature female was 113 d and the youngest mature male was 102 d of age from the Arabian Sea, Southwest coast of India. Similarly, the age at maturity appears to be variable under captivity, where Nabhitabhata et al. (2005) reported 60 d as age at maturity for S. lessoniana with a maximum life of 176 d in Thailand waters; Ohshima and Choe (1961) observed 90 d as age at first maturity for this species. In general, cephalopods reach maturity earlier in captive conditions than in the wild (Mangold, 1987) as the constant, relatively high temperature in captivity may accelerate maturation. Sepioteuthis lessoniana laboratory cultures have shown that water temperature affects the growth rate and size at maturity (Segawa, 1987; Forsythe et al., 2001; Ikeda et al., 2009a, 2009b; Amida et al., 2019). Such high variation in age-at-maturation of S. lessoniana suggested that there may be many intra-annual cohorts that experience various environmental conditions and grow at various rates, which results in a wide range in age at maturity (Pecl, 2001; Moreno et al., 2005).

Hatching and spawning

Different spawning peaks have been reported for S. lessoniana from different regions. In GOM, S. lessoniana spawning and hatching is year-round, with spawning peaks in the present study observed during July–August and February. Similarly, prolonged breeding was reported by Rao (1954) based on the presence of the egg capsules from January to June on weeds and other objects in the waters of the GOM and the Palk Bay. Venkatesan (2012) also reported a prolonged breeding season of S. lessoniana from December to July with a major spawning peak from January to March from Palk Bay, India.

The hatching rate depends on the biotic and abiotic history during incubation and embryonic development. For S. lessoniana, the incubation days varied between 14 and 55 d in different waters (Ohshima and Choe, 1961; SEAFDEC, 1975; Tsuchiya, 1982; Segawa, 1987). The average embryonic period is 20 d (a range of 17–23 d) at about 28 °C (Nabhitabhata, 1978, 1996; Nabhitabhata and Kbinrum, 1981; Nabhitabhata et al., 2005). The duration of the incubation period depends upon the water temperature and is longer at lower temperatures (Table 4). The hatching date frequency distribution of S. lessoniana shows hatching occurs throughout the year with evident peaks, August – September and March indicating the two major cohorts. For short-lived species like squids, year-round hatching of intra-annual cohorts, or ‘micro-cohorts’, leads to continuous recruitment (Caddy, 1991; Boyle and Boletzky, 1996).

Table 4. Comparison of incubation period and temperature in Sepioteuthis lessoniana from culture studies

Note: The spawning date was estimated by back calculation using the hatching date and the egg incubation period of S. lessoniana based on experimental rearing in Thailand by Nabhitabhata (1996). An average egg incubation time of 20 d was used for the calculation. The water temperature in the hatchery condition in the study location was 27–29℃ and the sea water was 27–31℃ at various seasons which is similar to the water temperature in the Thailand region.

Conclusion

Interpretation of the statolith growth increments of S. lessoniana estimates a maximum lifespan of 7 months for lineage B with a rapid growth rate. These specific growth traits within the S. lessoniana species complex in a region are crucial in the context of conservation and developing management plans for sustainable fisheries. Further, it was found that S. lessoniana was likely to show great plasticity in growth as determined by environmental temperature variation along its wide geographical range.