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.

Natural history observations, morphological traits and genetic evidence of multiple paternity all suggest that sperm competition plays a central role in the reproductive strategies of cephalopods yet experimental confirmation is lacking. Our proposed study on an endemic Australian squid species will experimentally assess:

  1. strategic male sperm allocation in response to sperm competition; and
  2. evolutionary consequences of female promiscuity.

Both are crucial for understanding processes generating the remarkable diversity of reproductive strategies in this ecologically and commercially important group of marine predators.

When females are promiscuous and sperm from different males compete to fertilise eggs (sperm competition) two key factors determine male and female reproductive success:

  1. how males allocate their reproductive effort in response to sperm competition; and
  2. the effect of multiple mating on female fitness.

Comprehensive studies of these two aspects of sexual selection exist for very few systems. Our proposed research will be the first to comprehensively examine male sperm allocation strategies, as well as the consequences of promiscuity for female fitness, in any cephalopod species. Specifically we will:

  1. quantify levels of multiple paternity in both the lab and in the wild;
  2. test whether males strategically allocate sperm depending on perceived levels of sperm competition; and
  3. test whether females mate multiply to ensure the fertilisation of all their eggs or to ensure they obtain either genetically compatible or genetically superior fathers for their offspring.

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).