Final Report Summary - SQUID-SWITCH (How do post-copulatory male-male and male-female interactions shape the evolution of mating strategies? A test using two species of squid) Females of many species mate with multiple males, and it is now widely recognised that post-copulation male competition and female choice are pervasive forces in evolution. A situation commonly associated with multiple mating by females is the adoption of alternative mating tactics by males: large 'consorts' accompany and guard females during mating and egg laying, and small 'sneakers' attempt mating without challenging the consort male for the female. For males, alternative mating tactics create differences in sperm competition risk, and often result in morphological and physiological adaptations. Theory predicts that sneaker males with high sperm competition risk put more energy into sperm production to compensate for their disadvantage in behavioural competition. However, post-copulatory sperm competition usually occurs inside the female, and so it is difficult to separate pre- and post-copulatory processes driving sexual and natural selection on evolution of male traits.Loliginid squid offer an opportunity to examine the evolution of alternative reproductive strategies among males. These squid display male size dimorphism associated with alternative mating tactics. Males generate spermatophores (packages of sperm) and place them in specific locations on the female's body. Females have sperm storage sites in two locations on their bodies (the oviduct inside the mantle cavity, and near the mouth), correlated with the mating position adopted by males: parallel mating for consorts, spermatophores placed in/on the oviduct opening; head-to-head mating for sneakers, spermatophores placed near the sperm storage organ (below the mouth). Sperm in both sites can achieve fertilisations during egg laying (eggs are extruded in a gelatinous string from the oviduct, then passed forward and held within the arm crown around the mouth). So the alternative storage sites places a constraint on post-mating sperm competition and cryptic female choice. A powerful approach can be achieved by comparing results among related species that have differing mating biology. Both the South African species Loligo reynaudii and the Japanese species Loligo bleekeri display the alternative mating tactics described and multiple paternity within egg strings. But there are differences between species in mating characteristics, that are predicted to affect fertilisation opportunities (and paternity bias within broods) of consorts and sneakers, and so affect sperm allocation strategies adopted by males. The main objective of this project was to investigate post-copulatory sexual selection by comparison of two squid species, to examine interactions of male sperm competition strategy and female strategies for sperm storage and usage.During the project new techniques were developed to assess male morphological traits, to study mating in captivity, and to study mating dynamics in the wild, for both the South African and Japanese squid species. Initial data indicate that fertilisation opportunity affects sperm investment strategy in these squid species. In L. bleekeri, there was discontinuous spermatophore dimorphism related to alternative male mating tactics. Gonado-somatic index (GSI) was correlated with squid size (mantle length), but discontinuous between consort and sneaker males. GSI was not different between male groups, but higher in smaller males within each tactic, suggesting strong intra-group sperm competition. In L. reynaudii, again spermatophore dimorphism was found, related to male tactic, but the relationship between male size and spermatophore size was continuous, and GSI was higher in smaller (sneaker) males. L. reynaudii sneakers had GSI double that of L. bleekeri sneakers, suggesting that the sperm of sneakers has higher opportunity to engage in sperm competition against consorts. These results suggest that alternative mating tactics with different fertilisation opportunity would strongly affect the sperm investment strategies among males.
