Comparative life history of tropical and temperate Sepioteuthis squids in Australian waters

The population dynamics of cephalopods are principally influenced by marked phenotypic plasticity in response to environmental factors, yet we have a poor understanding of how the life history characteristics of natural populations may alter over spatial or seasonal scales. The focus of this thesis is an examination of the variation in the life history traits of two closely related cephalopod species, Sepioteurhis lessoniana and S. australis, each from two divergent latitudinal zones along the east coast of Australia. The study centred on an assessment of the growth, lifespan, patterns of energy allocation between growth and reproduction, and the timing and mode of reproduction of each species as a function of geographical location, and for temperate S. australis, season of hatching. The growth, lifespan, body size and maturation of Sepioleulhis lessoniana and S. australis from the different latitudinal zones was examined using validated statolith derived age estimations. In tropical waters, S. lessoniana grew fast initially, however the growth rate declined rapidly with age and individuals matured at small sizes and young ages. In the sub-tropical waters of Brisbane, S. lessoniana grew comparatively slower early in the life span, but growth rate did not decline as rapidly with age and individuals matured at larger sizes and older ages. In S. australis, although growth appeared faster in individuals from the sub-temperate waters of Newcastle, maturation occurred at younger ages and smaller sizes in S. australis from temperate Tasmania, despite Tasmanian squid having a longer lifespan and achieving a larger maximum size. Back-calculated dates of hatching derived from statolith age estimations revealed that growth rate of S. australis from Tasmania was strongly influenced by hatching season, with individuals hatched in warmer seasons having a final size that could surpass that of earlier hatched (and therefore older) individuals. Faster growing squid hatched under the warming temperature regimes of spring and summer generally had heavier mantles at a given length, suggesting better condition at the whole animal level compared with slow growing squid hatched during the cooling temperatures of autumn and winter. Although males and females showed similar patterns in condition, the relative levels of reproductive investment of individuals in each hatching season differed between the sexes. Males hatched in warmer seasons had a higher level of reproductive investment compared with males from cooler seasons, whereas females hatched in spring and summer had lower levels of reproductive investment relative to their autumn and winter hatched counterparts. This study assessed the potential of Sepioteuthis lessoniana and S. australis to spawn multiple batches of eggs at discrete times throughout the adult lifespan. This was achieved by histological examination of the ovarian gametogenic cycle and detailed morphological assessments of the reproductive system, in conjunction with other biological information. Sepioleulhis australis caught in Newcastle showed evidence of a high correlation between body size and quantity of mature eggs, suggesting that eggs may be accumulating to be laid in a single batch. Although maturation was also a size related process in S. lessoniana and Tasmanian S. australis, oviduct size was not correlated with body weight in mature females, which is indicative of multiple spawning. Further supporting evidence included relatively low gonadosomatic indices, the heavier weight of the ovary relative to the oviduct, and the sustained feeding activity of mature animals. In Tasmanian waters, summer caught females showed a negative correlation between egg size and egg number within the oviduct, suggesting that some individuals were producing fewer larger eggs and others many smaller eggs. Sepioteuthis lessoniana and S. australis demonstrate a high degree of plasticity in their life history processes in terms of growth, reproduction and repro-somatic investment, and such flexibility in life history strategies within populations is consistent with an opportunistic lifestyle. Although there were substantial differences in the growth and reproductive biology between Sepioteuthis lessoniana and S. australis, and across latitudinal zones, seasonally related intra-specific variation in these traits within the S. australis population from temperate Tasmania were of similar magnitude. Importantly, this research also highlights the fact that a species may employ substantial variations on a reproductive strategy within different parts of their distribution, or with small genetic differences between populations. This, combined with other findings of this study, demonstrates that caution must be applied when extrapolating life cycle characteristics of cephalopod species across large geographical scales