Embryonic development and early life history of the southern calamary, Sepioteuthis Australia Quoy &amp; Gaimard, 1832
Creators: name: Steer, Michael A
Publisher: name: University Of Tasmania
Year: 2023

description: The southern calamary, Sepioteuthis australis is a sub-annual, commercial species that exhibits little to no generation overlap. Therefore, the strength of one generation depends on the reproductive success and survivorship of offspring spawned by the previous generation. Quantitative estimates of mortality rates during embryonic developmental and the subsequent juvenile phase are virtually nonexistent, therefore it is not understood what role early life history plays in determining recruitment strength. Through extensive egg surveys and laboratory experiments this project investigated the role of fluctuating environmental temperature; the position of the embryo within an aggregated egg mass; the substrate upon which its attached; the density of the egg mass; the role of fouling organisms colonising the eggs; and the effects of maternal condition on the developing embryo. Furthermore this study investigated mortality rates in the subsequent paralarval phase by using novel collection methods and statolith measurements to explore the 'bigger is better' hypothesis. Embryo mortality rates were highly variable both spatially and temporally ranging from 2 to 25%. Dramatic increases in mortality rates were not strongly associated with natural fluctuating temperatures and there was weak evidence suggesting that fluctuating salinity was responsible. Examining embryonic development in relation to the egg mass revealed that the position of the embryo within the mass influenced hatching success. Embryos located in the centre of the mass, where egg density is greatest, developed slower and suffered higher mortality rates than those located around the periphery. This was attributed to the inability of the embryos to adequately respire and eliminated metabolic wastes and was exaggerated in large, dense egg masses that had been physically dislodged from attachment. Maternal condition also influenced embryo mortality. Using a model, multiple spawning cephalopod, conducive to laboratory manipulation (Euprymna tasmanica) revealed that maternal ration and not temperature significantly effected egg viability. Low ration females produced sub-optimal eggs where 60% of embryos failed to develop. Egg viability deteriorated over successive clutches and by the third clutch 100% of the eggs died suggesting that low ration eggs were not receiving their full complement of maternally derived resources. Embryo mortality did not exceed 60% in high ration females regardless of treatment. Hatchling size was extremely variable ranging from 4.3 to 7.3 mm (ML), with significantly larger animals hatching out in November and the smallest in February. By comparing natal statolith dimensions between recently hatched ( &lt; 13 hrs old) and adult S. australis it was possible to determine whether size selective processes were operating during the early life history. In all but one month a significant difference between the size distribution of the natal radii in hatchlings and adults was found and was due to the absence of adults with small natal radii. This indicated that smaller hatchlings were less likely to recruit, suggesting that an element of size-mediated mortality exists in S. australis populations. This study is the first to quantify early mortality rates and identify processes responsible in an inshore, multiple spawning cephalopod. Results obtained will aid in reducing some of the variability encompassed within existing stock-recruitment relationships, potentially improving predictive recruitment models and allowing fisheries managers to make more informed decisions about commercial squid fisheries.