Functional morphology of coral shape and passive hydrodynamic self-righting in recentManicina areolata

Abstract

Thescleractinian Manicina area/ala is a common coral on Caribbean hard and soft substrata and was studied at Lee Stocking Island (Exurna Cays, Bahamas). It is not only found in reefal areas but also on bioclastic sand with seagrasses. The meandroid coralla typically grow to sizes of 2 to 10 cm and are morphologically variable. Growth form can vary from conical with round to oval cross-section to turbinate forms with few meanders and flat oval cross-section. The conical morphotype is usually attached to hard substratum, while the turbinate morphotype tends to live unattached, usually upright, in soft substratum. In infratidal areas, both attached and unattached turbinate forms were found in close vicinity, however, conical attached forms were rare. Habitats with sandy softgrounds, where free-living turbinate morphotypes were common, were influenced by strong tidal currents with concurrent danger of burial or 'disorientation' of the coralla. Therefore, strategies are necessary for the corals to remain in the upright position. Besides a high self-cleaning potential (FABRICIUS 1964), the colony shape itself leads to passive cleaning and self-righting, which is achieved by the flat-turbinate morphology of the corallurn, with a concave side and a flat to slightly convex opposite side, under high current speeds. The concave side, and particularly the median lobes formed by many meandroid coralla, are the critical morphological factors. Particularly the grooves formed inbetween the lobes channel currents in a way that scour underneath the coral and drag produced by the lobes act together to allow passive self-righting.