Reproductive strategies of the southern bottletail squid, Sepiadarium austrinum

In many species, female ejaculate consumption is an important driver of both ejaculate evolution, and the mate choice strategies of both sexes. This is particularly evident in species where spermatophores are used for ejaculate transfer, with the costs of producing such secondary ejaculatory components favouring the evolution of strategic reproductive effort in males. This thesis examines the reproductive strategies of the southern bottletail squid, Sepiadarium austrinum, a small, annual species endemic to the southern waters of Australia. In particular, I focus on male reproductive investment strategies, and the implications of these adaptations on squid behaviour and life history evolution. Female bottletail squid regularly consume the spermatophores of their partners and direct the nutrients received into both somatic maintenance and egg production (Chapter 1). This behaviour, along with short-term sperm storage in females, is likely to exert strong selection on male ejaculates and the reproductive strategies of this species. In response to this ejaculate consumption, males appear to target the largest females when attempting to mate. Although all females consistently exhibit spermatophore consumption, the smallest females eat the most (Chapter 2). However, during copulation, males do not alter their level of spermatophore investment according to female size or the associated risk of spermatophore consumption. Under somatically challenging conditions (e.g. in low resource environments, or when mating frequencies are high) individuals will often trade-off somatic requirements with reproductive investment. I found male bottletail squid exhibit different ejaculatory investment strategies depending on both nutrient availability and their mating history (Chapter 3). Virgin males produce lower quality ejaculates when nutritionally stressed, most likely as a means to assist in somatic maintenance. However, previously mated males produce high quality ejaculates, even when starved. Males consistently transfer the greatest number of spermatophores in their first copulation, with significantly lowered spermatophore investment in subsequent matings (Chapter 4). Although female quality is often an important determinant for male reproductive success, it did not influence the number of spermatophores males transferred during copulation, with males incurring no apparent longevity costs to mating. The risks of sperm depletion are likely to greatly influence squid reproductive investment strategies, particularly given the prevalence of female spermatophore consumption in this species. The results of my thesis highlight how studying cephalopod reproductive behaviours can unveil important insights into how sexual selection has shaped the evolution of these diverse marine predators. Spermatophore consumption appears to play a key role in shaping the reproductive adaptations of both sexes in Sepiadarium austrinum, in both precopulatory mate choice, and postcopulatory selection outcomes