Flexibility in the reproductive strategies of Nototodarus gouldi, MaCoy 1888 (Cephalopoda: Ommastrephidae) from southeastern Australia
Creators: name: McGrath Steer, Belinda L.
Publisher: name: University Of Tasmania
Year: 2023

description: Phenotypic plasticity in squid reproductive traits has been identified in a small number of recent studies. Factors affecting repro-somatic investment seem to be tightly linked to ambient environmental conditions, thus variability in the reproductive strategy of squid are expected, depending on their life-span and distribution. Phenotypic variation in squid has largely been ignored, although the consequences for life-histories are considerable, as they influence squid biology from fisheries applications to general physiology. This thesis therefore examined the flexible nature of reproductive strategies in the arrow squid Nototodarus goukk sampled from the range of environmental conditions naturally encountered in Australian waters. The underlying reproductive strategy of female N goukk from Tasmanian waters indicated the relative weight of the mantle, fin and digestive gland remained unchanged during ovarian development, suggesting energy was not being diverted away from somatic growth during sexual development, and consequently neither muscle nor digestive gland was being utilised as an energy store. Mean GSI (gonado-somatic index) was low, which is characteristic of a multiple spawning strategy, and it was likely that the cost of maturation was largely being met by food intake. The presence of stretched empty oviducts was further evidence that egg production in N. gouldi was slow and steady, with ova being released in discrete batches over a period of time. Changes in the reproductive strategies of N gouldi were sex specific, and varied over both broad spatial (4 locations) and temporal scales (bi-annual), and also over finer, monthly temporal scales. Over broad spatial scales the division of energetic resources showed little evidence of gonad development occurring at the expense of the soma regardless of season, sex, location or life-time growth rate. Female strategies varied on a broad-scale between high and low latitude sites, with squid from lower latitude sites showing greater levels of gonad investment in comparison to their higher latitude counterparts. In contrast, when females were caught on a monthly basis, females caught during the cool months were larger, grew slower and had lower gonad investment and better somatic condition than females caught during the warmer months, suggesting a trade-off between gonad investment and somatic condition during summer. Males on the other hand, showed both broad spatial and fine-scale temporal changes in reproductive traits, with both low latitude and spring caught males having greater levels of gonad investment. Patterns of repro-somatic investment had implications for spawning strategies, as females with higher gonad investment appeared to release eggs simultaneously, whereas females with low gonad investment possibly spawned eggs independently of one another. Muscle fibre dynamics of N gouldi were also investigated relative to reproductive development to quantify the cellular cost of reproduction. Changes in the proportion of large and small muscle fibres, a decline in the proportional zones of mitochondriarich fibres throughout the mantle, and a decrease in the width of muscle blocks at the anterior end, suggests a decline in energy available for muscle growth occurs with maturation. However, as these cellular changes could not be identified at the whole animal level, the cost of reproduction to mantle tissue was likely to be small. In terms of life-history theory, male squid were able to rapidly respond to environmental fluctuations without compromising either their gonad or soma. Although mature females did not respond as quickly to ambient conditions, they did produced two distinct reproductive traits in response to oceanographic conditions, possibly to maximise offspring survival in both a predictable and a variable environment. It is apparent that both male and female reproductive strategies are intimately linked to the environment, however their response to environmental change are quite different, probably due to the varying energetic constraints of each sex.