Visual discernment of sexual recruits is not feasible for Acropora palmata

Abstract

In fragmenting corals, estimation of population structure, extinction risk, and recovery potential requires accurate assessment of the relative contribution of sexual versus asexual reproduction. This yields an operational tendency for field ecologists to surmise levels of sexual recruitment from visual surveys, as in a recent study by Grober-Dunsmore et al. (2006; Mar Ecol Prog Ser 321:123-132). The recent development of microsatellite markers for threatened elkhorn coral Acropora palmata allowed us to test the accuracy of such visual assessments in 2 separate populations, showing them to be highly unreliable. Therefore, for clonal species that rely heavily on fragmentation, extreme caution is required in determining levels of sexual recruitment or recovery potential in the absence of molecular genetic screening.

Discernment of sexual recruits is not critical for assessing population recovery of Acropora palmata

“…Miller et al (2007, this volume) have critiqued this approach, suggesting that demographic measures are necessary, but not sufficient to assess recovery of clonal invertebrates; they argue that some measure of genetic diversity is 'quintessential'. We agree that genetic diversity is indeed important for long-term ABSTRACT: Miller et al (2007;Mar Ecol Prog Ser 335:227-231, this volume) dispute the ability of visual surveys to distinguish between colonies of Acropora palmata formed by sexual and asexual processes. They argue that approaches that do not consider genetic diversity are not appropriate for assessing population recovery.…”

“…They argue that approaches that do not consider genetic diversity are not appropriate for assessing population recovery. Visual surveys are clearly not reliable for distinguishing colony origin; however, the significance of the findings in Miller et al (2007) for assessment of population recovery is not clear. While genetic diversity may indeed be important for population survival and species persistence, our study Mar Ecol Prog Ser 321:123-132) assessed population recovery by temporally sampling demographic attributes which are critical for ecosystem function (e.g.…”

“…Furthermore, little is known about the importance of genetic diversity of residual populations as it relates to persistence and, in this case, recovery of Acropora palmata around the island of St. John, US Virgin Islands. Miller et al (2007) raise an excellent point: visual sampling cannot reliably distinguish between recruits arising from sexual propagules and those arising from asexual propagules. It should be noted, however, that the term 'recruitment' in our study refers to the addition of new members to a population (Richmond 1997), without implication of sexual or asexual origin.…”

“…Unless a sampling scheme captures the genetic diversity across a potentially interconnected population (by fragmentation, or asexual or sexually derived propagules), genetic methods (e.g. Miller et al 2007) will overestimate the proportion of 'sexually'-derived individuals and underestimate the number of asexually-derived individuals.…”

“…In spite of this conservative approach, we still found considerable increases in colony abundance, and we suggested that recovery may be occurring based on changes in size class distributions over time. Miller et al (2007) critiqued our use of size class information to infer recovery, based on their sampling of 2 patches of Acropora palmata with similar size class distributions that differed to extremes in clonality. We agree that size frequency distribution data may be misleading, since colony size may not be directly related to age or origin, and the formation of die-backs can increase the number of individual colonies.…”