Henrich J. Bruggemann

Cryptic Diversity in Indo-Pacific Coral-Reef Fishes Revealed by DNA-Barcoding Provides New Support to the Centre-of-Overlap Hypothesis

Diversity in coral reef fishes is not evenly distributed and tends to accumulate in the Indo-Malay-Philippines Archipelago (IMPA). The comprehension of the mechanisms that initiated this pattern is in its infancy despite its importance for the conservation of coral reefs. Considering the IMPA either as an area of overlap or a cradle of marine biodiversity, the hypotheses proposed to account for this pattern rely on extant knowledge about taxonomy and species range distribution. The recent large-scale use of standard molecular data (DNA barcoding), however, has revealed the importance of taking into account cryptic diversity when assessing tropical biodiversity. We DNA barcoded 2276 specimens belonging to 668 coral reef fish species through a collaborative effort conducted concomitantly in both Indian and Pacific oceans to appraise the importance of cryptic diversity in species with an Indo-Pacific distribution range. Of the 141 species sampled on each side of the IMPA, 62 presented no spatial structure whereas 67 exhibited divergent lineages on each side of the IMPA with K2P distances ranging between 1% and 12%, and 12 presented several lineages with K2P distances ranging between 3% and 22%. Thus, from this initial pool of 141 nominal species with Indo-Pacific distribution, 79 dissolved into 165 biological units among which 162 were found in a single ocean. This result is consistent with the view that the IMPA accumulates diversity as a consequence of its geological history, its location on the junction between the two main tropical oceans and the presence of a land bridge during glacial times in the IMPA that fostered allopatric divergence and secondary contacts between the Indian and Pacific oceans.

Community assembly and diversification in Indo-Pacific coral reef fishes.

Theories of species coexistence have played a central role in ecology and evolutionary studies of the origin and maintenance of biodiversity in highly diverse communities. The concept of niche and associated theories predict that competition for available ecological space leads to a ceiling in species richness that influences further diversification patterns. By contrast, the neutral theory supports that speciation is stochastic and diversity independent. We examined the phylogenetic community structure and diversification rates in three families and 14 sites within coral reef fish communities from the Indian and Pacific oceans. Using the phylogenetic relationships among 157 species estimated with 2300 bp of mitochondrial DNA, we tested predictions in terms of species coexistence from the neutral and niche theories. At the regional scale, our findings suggest that phylogenetic community structure shifts during community assembly to a pattern of dispersion as a consequence of allopatric speciation in recent times but overall, variations in diversification rates did not relate with sea level changes. At the local scale, the phylogenetic community structure is consistent with a neutral model of community assembly since no departure from a random sorting of species was observed. The present results support a neutral model of community assembly as a consequence of the stochastic and unpredictable nature of coral reefs favoring generalist and sedentary species competing for living space rather than trophic resources. As a consequence, the observed decrease in diversification rates may be seen as the result of a limited supply of living space as expected in a finite island model.

Isolation and characterisation of 16 microsatellite loci from a widespread tropical hydrozoan, Lytocarpia brevirostris (Busk, 1852)

We isolated and characterized 16 polymorphic microsatellite loci for Lytocarpia brevirostris (Aglaopheniidae), a hydrozoan common in the tropical Indo-Pacific region. Four to 34 alleles per locus were detected at the Indo-Pacific scale. At the population level, observed (Ho) and expected (He) heterozygosities ranged across 0.100–0.625 and 0.097–0.597, respectively. Three markers showed significant deviation from Hardy–Weinberg equilibrium, all of them presenting null alleles. Linkage disequilibrium was detected in three pairs of loci among 120. These primers provide powerful tools for studying population genetic diversity and the implication of life cycle strategies on population differentiation in tropical hydrozoans. This will be valuable for the conservation of coral reefs biodiversity and the design of marine protected areas.

Seagrass beds as feeding territory for farming Stegastes spp. (Pomacentridae)

Territorial Stegastes damselfishes play key trophic roles in benthic coral reef communities (Gobler et al. 2006). They typically occupy hard substrates such as live and dead corals, but may also use foliose macroalgae for cultivation of palatable epiphytes (Ceccarelli et al. 2005). Despite their widespread distribution and high abundance (Ceccarelli et al. 2001), Stegastes damselfishes inhabiting and using tropical seagrass beds have not been reported. Stegastes spp. are the dominant territorial herbivores in shallow inner reef flats of Reunion Island (South-West Indian Ocean), where they cultivate thick filamentous turf algae (e.g., Polysiphonia) on dead branching corals (Letourneur 2000). During monthly surveys in 2014 and 2015, territories of three species of Stegastes (S. limbatus, S. nigricans and S. punctatus) were observed in monospecific seagrass beds of Syringodium isoetifolium (0.5– 1.2 m depth) (Fig. 1). Their territories always included hard substrata used as shelter (Fig. 1a, d) and they were vigorously defended against conspecific intruders (see supplementary material). Seagrass leaves within their territories showed distinct epiphytic growth (Fig. 1b, c). Furthermore, the damselfishes were observed feeding and weeding on long turf algae on seagrass leaves (Fig. 1e, f). Seagrass habitats commonly present within or close to coral reefs may provide new surfaces for epiphytic communities farmed by damselfishes, extending their habitat and food resources. These observations suggest that the territorial and farming behaviours of Stegastes spp. may strongly influence seagrass bed trophodynamics, as has been found for coral reefs (Ceccarelli et al. 2001), and may disturb seagrass productivity. Indeed, they increase epiphyte loads that are known to reduce light and nutrient availabil i ty for seagrasses (Frankovich and Fourqurean 1997), as well as they exclude intruders and particularly associated herbivores, which are able to stimulate seagrass productivity (Valentine and Heck 1999). Further study of Stegastes spp. and seagrass interactions is needed in order to better understand the functional processes taking place between coral reefs and adjacent seagrass beds. Communicated by R. Thiel