Sex Changes and Hermaphrodites in Coral Reef Fish
sexual reproduction raises many questions about the spread of genes in populations (evolution) -- why should sexual reproduction evolve in the first place? -- sexual reproduction involves allowing another individual to share "gene space" in your progeny -- once sexual reproduction evolves, why should gametes evolve into two distinct kinds (eggs and sperm)? -- why should individuals differentiate into two distinct sexes (female and male)? -- in some species of vertebrates (and many plants and invertebrates) individuals produce both eggs and sperm -- thinking about these questions often brings us back to short-term and long-term consequences of alleles
fish illustrate patterns of sexual reproduction in vertebrates -- in most species of fish, individuals remain one sex for life (like all terrestrial vertebrates) -- in some species, individuals have functioning ovaries and testes at the same time (simultaneous hermaphrodites) --in some species, individuals change sex (sequential hermaphrodites) -- in many wrasses and parrotfish, individuals are first female and later change to male (protogyny) -- in anemonefish, individuals are first male and later female (protandry)
in protogynous species, large males (transformed females) develop spectacular coloration -- referred to as terminal males (or supermales) -- Bluehead Wrasse is most thoroughly studied example -- supermales defend small territories at traditional sites at the downstream edges of coral clumps -- gravid females visit these sites and most spawn in the area defended by one male -- during a short "run" female sheds her eggs and male fertilizes them before they float away as plankton -- usually largest supermale defends the preferred spawning site smaller males that look like females hang around these mating sites or follow gravid females -- they are not recognized as rivals by the supermales and sometimes mate with a female or accompany a spawning run -- direct behavioral interactions with supermales inhibit large females from changing sex (at least in aquaria) -- experimental removal of females (followed by replacement with new females) shows that females choose a place (not a male) and then males compete to defend that place what conditions favor the evolution of sex changes? when males compete with each other for access to mates, size is often important -- males cannot win contests unless they are big -- big males can chase away small males and fertilize the eggs from many females -- individuals maximize their lifetime reproductive success (total offspring) by starting life as females and changing to males at a size at which they have higher fecundity as a male than as a female on large reefs, females do not take the risk of swimming to the downstream end of the reef and instead often mate opportunistically -- in this situation, more females mate with small males -- as predicted, on large reefs, more of the small fish are males than on small reefs
simultaneous hermaphrodites include hamlets and harlequin bass (genera Hypoplectrus and Serranus, both small species in the sea-bass family Serranidae) how should simultaneous hermaphrodites allocate resources to production of eggs and sperm? -- should they produce equal amounts of eggs and sperm, or more eggs than sperm, or more sperm than eggs? harlequin bass spawn in groups -- sperm from different individuals compete to fertilize eggs -- individuals have testes about as large as their ovaries -- why put equal resources into eggs and sperm? -- if most individuals put more resources into eggs than sperm, then it would pay for an individual to produce more sperm and alleles increasing sperm production would spread -- on the other hand if most individuals put more resources into sperm than eggs, then it would pay to produce more eggs -- the only evolutionarily stable strategy (ESS, one that alternatives cannot invade) is to allocate resources to eggs and sperm equally same argument was made by R. A Fisher (1929) to explain the evolution of 50:50 sex ratios in most animals -- the advantage of producing male (or female) offspring is frequency-dependent -- it depends on the proportion of each sex produced by other individuals in the population -- the only ESS is a 50:50 ratio of parental investment in each sex hamlets spawn in pairs -- two fish alternate in expelling a few eggs and fertilizing them -- sperm from different individuals do not compete in fertilizing eggs -- hamlets have testes much smaller than their ovaries and thus put most of their energy into eggs -- they only need a few sperm to fertilize the partners' eggs -- why do they alternate during courtship, instead of one expelling all of its eggs and then the other? thus sperm competition (competition between sperm from different individuals to fertilize eggs) determines the size of testes in relation to ovaries -- and hence allocation of energy to producing testes and ovaries
rare cases of protandry emphasize these points about the evolution of sexual strategies -- in anemonefish (also called clownfish), a male and female together defend a clump of sea anemones -- a component of the fishes' skin secretions inhibits the nematocysts on the anemone's tentacles so the anemonefish do not get stung -- they use the anemone as a refuge for protection from predators reproductive success by a monogamous pair is greatest if the larger member of the pair is female (produces the eggs) -- smaller one can produce enough sperm to fertilize the larger one's eggs (compare with the argument for hamlets above) so these fish change from male to female as they grow -- a male changes sex if he loses his mate and attracts a new mate smaller than himself
note that sex change in fish illustrates the two kinds of biological explanation . . .
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