Yoko Iwata's website

Research Interests

・Sexual selection
・Evolution of life history
・Evolution of polymorphism
・Alternative mating tactics
・Sperm competition & Cryptic female choice
・Reproductive ecology in cephalopod

Loliginid squids provide an excellent opportunity to understand maintenance mechanisms of intra-specific phenotypic diversity!!

1 Alternative mating tactics in squid

Loliginid squids have a characteristic mating system, where males adopting one of two mating tactics to pass spermatophores on "alternative sperm storage sites" of females. Males show body-size dimorphism, associated with alternative mating tactics.

First, I investigated the effect of body size on male competition and mating behaviours by behavioural observations in captive. The body size of males correlated with the outcome of male competition and mating behaviours: large males paired with females, and small males mated with females that have already paired with another male.

Second, I investigated the fertilization success of each mating behaviour by paternity analysis using microsatellite DNA polymorphism. Such diverged mating behaviors resulted in different paternity rates among males: paired males fertilized about 90% eggs, and sneaked males fertilized the rests (Iwata et al. 2005)

Now, I am applying the study in wild. Dr Paul Shaw (Royal Holloway University of London, UK), Dr Warwick Sauer (Rhodes University, South Africa) and myself started SCUBA-based field study in Matsumae, Hokkaido, Japan from 2007.

2 Male dimorphism associated with alternative mating tactics

Male dimorphism has been thought to correlate with alternative mating behaviours. Alternative mating behaviours promote asymmetry in sperm competition, and the differences in fertilization success could promote adaptations in ejaculate characteristics in relation to each reproductive behaviour. Using allometric analysis, I demonstrated that there was clear ejaculate dimorphism in males of the squid Loligo bleekeri, a species with body-size related alternative mating behaviours.

A morphological switch point was detected in internal characters: larger individuals made discontinuously longer spermatophores than smaller individuals, although any switch point was not detected in external characters (fin length, fin width, head width, mantle width, tentacle length and hectocotylus length) except bimodal body size.

The dimorphism could be an adaptation to the characteristic alternative mating behaviours of loliginid squid, in which males pass spermatophores to alternative sperm storage sites in and on females (Iwata & Sakurai 2007).

The squid system, with clear sperm competition and constraint on it by female characteristics, should provide some general resolution about how male traits evolve under the selection force of post-copulatory male competition, cryptic female choice and sexual conflict.