Sexual selection in the sea: female promiscuity and male sperm competition in squid

A project undertaken at the Department of Zoology, University of Melbourne, and supervised by Dr Devi Stuart-Fox

Sexual selection accounts for much of the weird and wonderful diversity of life on earth - from the iconic peacock's train to sexual cannibalism in preying mantises. The full spectrum of this diversity is represented in a single group of animals: the cephalopods (cuttlefish, squids and octopuses). Cephalopods exhibit a bewildering array of reproductive behaviours and adapatations, from complex, technicolored courtship displays to elaborate sperm storage structures, sperm removal devices and even sexual cannibalism.

We examined the benefits to females of mating with multiple males as well as the costs of mating in dumpling squid. We also examined reproductive strategies in bottletail squid. Both are small, short-lived species common in coastal waters along the coast of southeastern Australia.

We have documented the first evidence that females benefit from polyandry (mating with two or more different males) in any cephalopod. Females that mate with two different males (polyandrous females) produce clutches of eggs faster (less time between successive clutches) than females that mate with only one male. Females that mate with the same male twice lie in between (Figure 5). This is likely to give polyandrous females a reproductive advantage given that dumpling squid are short-lived and very vulnerable to predators. It is also likely to compensate for the high costs of mating, which we have found in this species.

We have also documented the first case of females consuming male spermatophores in any cephalopod. Female bottletail squid store male spermatorphores around their buccal cavity (mouth). After mating, females simply eat many of the spermatophores, resulting in wasted investment for the males. We are conducting additional experiments to discover why females have evolved this behaviour.

Collaborators: Dr Mark Norman (Museum Victoria) and Dr. Bob Wong (Monash University)

Figure 1. The Southern Dumpling Squid, Euprymna tasmanica, horizontal view. (Image by Mark Norman)

Figure 2. A mating pair of Southern Dumpling Squid. (Image by Mark Norman)

Figure 3. A Southern Dumpling Squid showing the'arm flower display'. (Image by Mark Norman)

Figure 4. Highly modified suckers on the left dorsal arm of mature males of the Southern Dumpling Squid, potentially used to displace the sperm packets of rivals. (Scanning electron micrograph by Mark Norman).

Figure 5. The average number of days between clutches laid by females that have mated with one male (monandry1), the same male twice (monandry2) and two different males (polyandry). Different letters show significantly different groups