Sperm competition and sex change: a comparative analysis across fishes.

Current theory to explain the adaptive significance of sex change over gonochorism predicts that female-first sex change could be adaptive when relative reproductive success increases at a faster rate with body size for males than for females. A faster rate of reproductive gain with body size can occur if larger males are more effective in controlling females and excluding competitors from fertilizations. The most simple consequence of this theoretical scenario, based on sexual allocation theory, is that natural breeding sex ratios are expected to be female biased in female-first sex changers, because average male fecundity will exceed that of females. A second prediction is that the intensity of sperm competition is expected to be lower in female-first sex-changing species because larger males should be able to more completely monopolize females and therefore reduce male-male competition during spawning. Relative testis size has been shown to be an indicator of the level of sperm competition, so we use this metric to examine evolutionary responses to selection from postcopulatory male-male competition. We used data from 116 comparable female-first sex-changing and nonhermaphroditic (gonochoristic) fish species to test these two predictions. In addition to cross-species analyses we also controlled for potential phylogenetic nonindependence by analyzing independent contrasts. As expected, breeding sex ratios were significantly more female biased in female-first sex-changing than nonhermaphroditic taxa. In addition, males in female-first sex changers had significantly smaller relative testis sizes that were one-fifth the size of those of nonhermaphroditic species, revealing a new evolutionary correlate of female-first sex change. These results, which are based on data from a wide range of taxa and across the same body-size range for either mode of reproduction, provide direct empirical support for current evolutionary theories regarding the benefits of female-first sex change.

Biology of extinction risk in marine fishes.

We review interactions between extrinsic threats to marine fishes and intrinsic aspects of their biology that determine how populations and species respond to those threats. Information is available on the status of less than 5% of the world's approximately 15500 marine fish species, most of which are of commercial importance. By 2001, based on data from 98 North Atlantic and northeast Pacific populations, marine fishes had declined by a median 65% in breeding biomass from known historic levels; 28 populations had declined by more than 80%. Most of these declines would be sufficient to warrant a status of threatened with extinction under international threat criteria. However, this interpretation is highly controversial, in part because of a perception that marine fishes have a suite of life history characteristics, including high fecundity and large geographical ranges, which might confer greater resilience than that shown by terrestrial vertebrates. We review 15 comparative analyses that have tested for these and other life history correlates of vulnerability in marine fishes. The empirical evidence suggests that large body size and late maturity are the best predictors of vulnerability to fishing, regardless of whether differences among taxa in fishing mortality are controlled; there is no evidence that high fecundity confers increased resilience. The evidence reviewed here is of direct relevance to the diverse criteria used at global and national levels by various bodies to assess threat status of fishes. Simple life history traits can be incorporated directly into quantitative assessment criteria, or used to modify the conclusions of quantitative assessments, or used as preliminary screening criteria for assessment of the approximately 95% of marine fish species whose status has yet to be evaluated either by conservationists or fisheries scientists.

Life-history correlates of the evolution of live bearing in fishes.

Selection for live bearing is thought to occur when the benefits of increasing offspring survival exceed the costs of reduced fecundity, mobility and the increased metabolic demands of carrying offspring throughout development. We present evidence that live bearing has evolved from egg laying 12 times in teleost (bony) fishes, bringing the total number of transitions to 21 to 22 times in all fishes, including elasmobranchs (sharks and rays). Live bearers produce larger offspring than egg layers in all of 13 independent comparisons for which data were available. However, contrary to our expectation there has not been a consistent reduction in fecundity; live bearers have fewer offspring in seven out of the 11 available comparisons. It was predicted that live bearers would have a larger body size, as this facilitates accommodation of developing offspring. This prediction was upheld in 14 out of 20 comparisons. However, this trend was driven by elasmobranchs, with large live bearers in seven out of eight comparisons. Thus, while the evolution of live bearing in elasmobranchs is correlated with predicted increases in offspring size and adult size, teleost live bearers do not have such a consistent suite of life-history correlates. This suggests that constraints or selection pressures on associated life histories may differ in live-bearing elasmobranchs and teleost fishes.

Evolutionary transitions in parental care and live bearing in vertebrates.

We provide the first review of phylogenetic transitions in parental care and live bearing for a wide variety of vertebrates. This includes new analyses of both numbers of transitions and transition probabilities. These reveal numerous transitions by shorebirds and anurans toward uniparental care by either sex. Whereas most or all of the shorebird transitions were from biparental care, nearly all of the anuran transitions have been from no care, reflecting the prevalence of each form of care in basal lineages in each group. Teleost (bony) fishes are similar to anurans in displaying numerous transitions toward uniparental contributions by each sex. Whereas cichlid fishes have often evolved from biparental care to female care, other teleosts have usually switched from no care to male care. Taxa that have evolved exclusive male care without courtship-role reversal are characterized by male territoriality and low costs of care per brood. Males may therefore benefit from care through female preference of parental ability in these species. Primates show a high frequency of transitions from female care to biparental care, reflecting the prevalence of female care in basal lineages. In the numerous taxa that display live bearing by females, including teleosts, elasmobranchs, squamate reptiles and invertebrates, we find that live bearing has always evolved from a lack of care. Although the transition counts and probabilities will undoubtedly be refined as phylogenetic information and methodologies improve, the overall biases in these taxa should help to place adaptive hypotheses for the evolution of care into a stronger setting for understanding directions of change.