Vermetid gastropods mediate within-colony variation in coral growth to reduce rugosity

Abstract

Intraspecific variation in coral colony growth forms is common and often attributed to phenotypic plasticity. The ability of other organisms to induce variation in coral colony growth forms has received less attention, but has implications for both taxonomy and the fates of corals and associated species (e.g. fishes and invertebrates). Variation in growth forms and photochemical efficiency of massive Porites spp. in lagoons of Moorea, French Polynesia (17.48°S, 149.85°W), were quantified in 2012. The presence of a vermetid gastropod (Ceraesignum maximum) was correlated with (1) reduced rugosity of coral colonies and (2) reduced photochemical efficiency (Fv/Fm) on terminal “hummocks” (coral tissue in contact with vermetid mucus nets) relative to adjacent “interstitial” locations (tissue not in contact vermetid mucus nets). A manipulative field experiment confirmed that the relative growth rate of coral tissue was greater in interstices than hummocks when vermetids were present and similar (but with a trend for faster growth on hummocks) when vermetids were absent. Collectively, these results indicate that vermetid gastropods interact (presumably via their mucus nets) with coral colony architecture to impair photochemical efficiency, reduce growth rates of specific portions of a coral tissue, and induce a smoothed colony morphology. Given that structural complexity of coral colonies is an important determinant of “habitat quality” for many other species (fishes and invertebrates), these results suggest that the vermetid gastropod, C. maximum (with a widespread distribution and reported increases in density in some portions of its range), may have important indirect effects on many coral-associated organisms.