| ABSTRACTThe Argentine shortfin squid, Illex argentinus, is an important economic species, sustaining the most important cephalopod fishery in landing volume in the world, and also is the most important cephalopod resource for the Chinese Oceanic squid fishery in the southwest Atlantic Ocean. The species is characterized by a short lifespan, fast growth, and semelparity. As a consequence, the stock biomass in a given year is entirely dependent on the annual recruitment. The spawning strategy favored by the post-recruitment would largely impact the offspring’s survivorship and enable this species to achieve sustainable development under the viable marine environment. Thus, a good understanding of the spawning strategy is crucial for improving our knowledge on the dynamics of this resource. However, the spawning strategy in this species was just assumed to be alike to that of other similar species, or generally characterized as strategies for r/K species. There was little study of the spawning strategy in this species based on the histological analysis of ovary and oogenesis and reproductive investment. In this dissertation, the squids, randomly sampled from the catch of Chinese jigger fleets during the fishing season from December 2012 to March 2013 and from April to June 2014 in the high sea of southwest Atlantic Ocean, were used for the analysis. The squids were analyzed for their morphological and anatomic characteristics. The squids’ fecundity was counted, the oocyte size was measured, and tthe oocytes ovulation was analyzed. The cryostat technique was used to prepare ovarian histological sections, which were observed under microscope, and then the development of ovarian development and oogenesis were described. The tissue energy detection technique was used to measure the tissue energy of mantle, arms and reproductive system, separately, and then further to study the mode of reproductive investment. The main results and conclusions were as follows:(1) Reproductive biological characters: The female and male squid samples ranged from 158 to 344 mm and 140-289 mm in dorsal mantle length(ML), and weighed 72-814 g and 56-655 g in body weight(BW), respectively. The distributions of both dorsal mantle length and body weight in females were characterized as bimodal patterns. The first mode was from 190 mm to 230 mm in dorsal mantle length distribution and from 100 g to 200 g in body weight distribution, and the individuals were mainly from the samples collected during December 2012 and March 2013. The second mode was 260-300 mm ML and 490-640 g BW, which was dominated by the individuals collected from April to June 2014. There was no significant difference between the proportions of females and males, and the overall sex ratio(female: male) was 1.16:1, but significant differences could be found among different months. The females were in larger proportions during December 2012 and January 2013 and from April to June 2014, but decreased from February to March 2013 and attained the lowest proportion in March 2013. Furthermore, there was significant difference in the sex ratio according to dorsal mantle length group, and the ratio was biased to males when the individuals were measured less than 180 mm ML or between 250 mm ML and 260 mm ML, whereas the ratio was biased to females when dorsal mantle length was 210-230 mm ML or longer than 260 mm ML. For females, the immature individuals were found in the largest proportion with a percentage of 67.33%, and the mature and mated squids were found in the ratio of 29.77% and 2.90% respectively. Moreover, the immature females were increased from December 2012 to February 2013 but decreased from April to June 2014, matures attained their larger proportion in January 2012 and May 2014, and mated ones were only found from December 2012 to March 2013. In contrast to females, the mature individuals were found in the largest proportion with a ratio of 47.69% for males, and immature and mated individuals were in the ratio of 40.22% and 12.09% respectively. The immature males attained their largest proportion in February 2013, and decreased from April to June 2014. The proportion of the mature males increased from December 2012 to March 2013 but decreased from April to June 2014, and the mated ones attained their largest proportion in March 2013 but zero was found during April and June 2014. The gonadosomatic index was 0.06-17.80% and 0.93-15.30% for both females and males, respectively. There were significant differences in the gonadosomatic index for each sex among different months, and both females and males attained their lowest values in February 2013. The nidamental gland index was 0.015-12.66% for females, and the spermatophoric complex index was 0.089-6.31% for males, and both the indices also attained their lowest values in February 2013. These findings have again supported the hypothesis for the existence of two spawning seasons from December to February and from May to June for I. argertinus.(2) Fecundity and spawning pattern: Potential fecundity(PF), defined as total oocyte stock both in the ovary and oviducts, was determined from physiological maturing stage to spawning stage with values of 48.08-126.43 thousand oocytes, 49.96-134.77 thousand oocytes and 25.38-65.45 thousand oocytes for physiological maturting, mature and spawning individuals separately. There was no any significant difference in the PF values between physiological maturing and mature individuals. The ripe eggs in oviducts were about 4.04-24.75% of PF for mature individuals with an average value of 8.57% PF, and 3.27-30.10% of PF for spawning individuals with a mean value of 20.91%. PF was closely related to individual’s dorsal mantle length by power regression, but no any relationship to body weight. Relative fecundity(RF), calculated as the ratio of PF to body weight, was 177-572 oocytes per gram. Potential reproductive investment index(PRI), calculated as the product of RF and the weight of an individual ripe egg, was 0.083-0.285. PRI for physiological maturing individuals was slightly, but not significantly larger than that for mature individuals. The major axis length of oocytes in the ovaries varied from 0.08 mm to 1.81 mm. Oocytes were significantly different among different maturity stages, but no difference was found for each mature individual. The frequency of the major axis length of oocytes was bimodal distribution for both physiological maturing and mature individuals, with two modes of 0.45-0.75 mm and 0.95-1.35mm(or 1.15-1.45 mm for advanced matures).However, the major axis length of oocytes for spawning individuals was unimodal with a single mode of 0.45-0.65 mm, and then decreased gradually when the major axis length was longer than 0.75 mm. The major axis length of eggs in the ovdicucts was measured from 0.94 mm to 1.68 mm, and the length frequency follows unimodal distribution with the single mode of 1.15-1.35 mm. These results indicated that oocyte growth was synchronous and matured in batches, with a decreasing volume of PF and batch eggs at each ovulation after spawning.(3) Ovary development and oocytes maturation mode: The ovary development was classified into six distinct histological phases, while oocyte development was characterized by five stages, and the presence of post-ovulatory follicles was an important criterion for these phases identified. The development of ovary was monocyclic. The frequency distribution of both number and occupied areas of each stage oocyte in the ovary histological section were unimodal for each ovary histological phase, and the unimodal distribution was moving forward gradually along with the development of ovary. In ovary Phase I, ovaries contained oogonia and oocytes at Stage 1, Stage 2 and early Stage 3, where the Stage 1 and Stage 2 oocytes were in the largest number. The mean diameter distribution of oocytes was unimodal with the most frequency of 50-250 um. In ovary Phase II, the oocytes at Stage 2 was growing in great number, and the mean diameter distribution of oocytes was also unimodal but the peak being 100-250 um. In ovary Phase III, the oocytes at Stage 3 was predominant, with some oocytes at Stage 4, and the mean diameter of oocytes was mostly distributed in the range of 100-300 um. In ovary Phase IV, the oocytes at Stage 4 were growing greatly, with the most advanced oocytes at Stage 5. The mean diameter distribution of oocytes was showing three peak modes, which are 150-250 um, 300-350 um and 400-450 um. In ovary Phase V, the oogonia and oocytes at Stage 1 had been absent, and the oocytes at Stage 5 were getting into blooming. The mean diameter distribution of oocytes was mainly at the range of 450-650 um, and presented the fourth peak mode of 800-900 um. In ovary Phase VI, the oocytes at Stage 5 were gradually developing as ripe eggs, with more number of the post-ovulatory follicles appearing. The mean diameter distribution of oocytes showed a great number of oocytes at the range of 200-400 um and 650-900 um, and the ripe eggs at the range of 1100-1150 um appeared as a single small peak mode. The relationship between ovary development, defined as histological maturity index, and sea-surface temperature, chlorophyll-a, sea-surface height, capture areas and capture date(month) was investigated by applying generalized additive models(GAMs). The GAMs indicated that the ovary development was significantly affected by sea-surface temperature, chlorophyll-a and capture date. There was evidence of an opposite relationship between ovary development and sea-surface temperature, where ovary was at a higher level of development under 10 ℃. Ovary development attained its lowest level when the chlorophyll-a was about 0.50 mg/m3, and attained its highest level when the chlorophyll-a was about 0.80mg/m3. The pattern associated with month showed a peak in ovary development from March to April, but with a lowest level in February, indicated that I. argentinus would have a peak activity of spawning during February. The results indicated the squid was intermittent terminal spawner, with synchronous oocytes development in their ovaries before sexual maturity and then maturation and ovulation in batches after sexual mature. The sea-surface temperature and chlorophyll-a were the main environmental factors affecting the gonad growth.(4) Tissue energy allocation and reproductive investment. The tissue energy density was determined for the mantle, arms, ovary, nidamental gland and oviducal complex, and the ovary had the highest density of tissue energy, but the nidamentl gland had the lowest density of tissue energy. There was no significant difference in tissue energy density between different sexual maturity stages in mantle, arms or nidamental glands, separately. However, the tissue energy density was significant different each other by different sexual maturity stages in ovary, or ovducal complex, separately. Both mantle and arms attained their maximum tissue energy at maturity stage III. In the reproductive system, ovary attained its maximum tissue energy at maturity stage V, whereas the tissue energy was gradually increasing in nidamental gland and oviducal complex, with a burst increasing in oviducal complex after maturity stage IV. Under the same maturity stage, the somatic tissue energy, calculated as the sum of mantle and arms tissue energy, was always in the larger proportion, but the gonad tissue energy, calculated as the sum of ovary, nidamental gland and oviducal compex tissue energy, was in the lower percentage. However, during the onset of maturation, the proportion of somatic tissue energy was decreasing gradually; but the proportion of gonad tissue energy was increasing. The somatic tissue energy was partly remobilized for the development of gonad tissue, but the process did not impact the integrity of soma from maturity stage III to V. The energy accumulation of gonad tissue was closely related to chlorophyll-a concentration based on the(GAM) between tissue energy and environmental factors, and the gonad tissue energy accumulation attained their maximum value when the chlorophyll-a concentration was approximately 0.61mg/m3 and 0.8mg/m3, indicating that the reproductive energy investment was mainly from food intake.(5) The results above provided an evidence for the first time that the spawning strategy in Illex argentinus is intermittent terminal spawning based on the histological analysis, with monocyclic ovary development, newly oocytes stopping growth before ovarian histological phase V, reproductive investment from food intake, synchronous oocytes development and maturation, and ripe eggs batch ovulation. |
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