Research Projects
Mary K. Hart
Mary K. Hart
mkhart0@yahoo.com
Doctoral candidate; Department of Biology, University of Kentucky: Sex allocation and egg trading strategy in a simultaneous hermaphrodite: examining sexual conflicts in ecological context
My dissertation research focuses on how the expression of sex-related traits indicative of sexual conflict varies across an environmental gradient. Sexual conflict over mating is driven by the sexes having different optimal mating rates. This is a direct result of anisogamy – the difference in gamete size means unequal energetic investment in reproduction. The end result being that a single mating is more costly for females than for males because production of a single egg is more energetically expensive the production of the sperm that fertilizes it. This can result in adaptations in males to increase the number of matings or to be more effective in sperm competition. Often these male adaptations can be detrimental to females and females evolve adaptations to counter the detrimental effects. Simultaneous hermaphrodites provide excellent models to study sexual conflict because a balance must be struck between male and female sex roles according to available mating opportunities. This balance may shift across different environments.
My study species, chalk bass or Serranus tortugarum (Serranidae: Serraninae) is a small simultaneously hermaphroditic coral reef fish that engages in egg trading wherein partners alternate roles as male and female several times during a given spawning period, subdividing the egg clutch, in a strategy called egg parceling. Pairings are size assortative and spawning occurs daily over a 2- hour period just before sunset. Chalk bass are planktivorous and found in site-attached aggregations at a variety of densities in coral rubble habitat of the shallow lagunal reefs in the Bocas del Toro area.
Parceling of the egg clutch protects the female-role investment in egg production but may also be used to cheat if an individual can sequester more parcels than its current partner to exchange for parcels from another mate. Thus parceling strategy may indicate the intensity of intra-pair conflict over mating role. And in circumstances where the chance is good that a mating pair will break-up and re-mate with other individuals, parceling rate should be relatively high.
Another behavior called ‘streaking' is frequently used, even by paired individuals, to gain extra paternity by intruding on other spawning pairs and releasing sperm. Streaking is a form of sperm competition that occurs more frequently at high densities and is likely to affect sex allocation strategy. Both members of a pair try to avoid streakers. Streakers reduce the value of each egg clutch to male-role mating partners through reduced paternity and interfere with choice of mate. Male gonadal allocation (% testicular tissue in the total gonad) may indicate the intensity of conflict between paired individuals and streakers
Results of preliminary field work and theoretical models have generated 2 key hypotheses for empirical testing: (1) Local density will have a positive effect on # streakers per spawn and male allocation. (2) Mate availability (density) will have a positive effect on parcel number (# of parcels egg clutch is divided into in one day). But streaking may moderate this effect if pairs spend more time avoiding streakers or attempting to streak on neighboring pairs themselves.
The aim of my study is to gain insight on evolutionary responses to sexual conflict and flexibility in mating systems by testing these hypotheses using empirical studies. My study will lend insight as to how intensities of sexual conflicts vary across environments, potentially interact, and ultimately shape mating systems.