How are coral populations structured by light? Marine light regimes and the distribution of Madracis

Abstract

We studied the relation of light with coral colony morphology and coral distribution over a 5 to 50 m reef slope. We describe the characteristics of 6 species of the genus Madracis in relation to the light (photosynthetically active radiation (PAR): 400 to 700 nm) colonies receive at a small spatial scale (1 to 10 cm). Species differ in their response to light but we could distinguish 3 strategies: (1) Species strive for maximum light capture and colonies occur above a threshold light value. This limits the distribution of such species toward greater depths. These species are found in shallow ( 30 m) of the reef slope. Colony morphological variation is also unrelated to variation in the amount of light the colonies receive. (3) Species prefer low light habitats. Such species are found over the entire reef slope and they show a bimodal light preference. One part of the population (1st mode) prefers the maximum amount of light available at a particular depth. The other part of the population (2nd mode) prefers cryptic habitats with low light levels (5 to 60 µmol photons m -2 s -1 ). In these species, morphological variation in colony shape relates to light availability. Structural complexity of the reef surface within the same depth causes such vari- ation in local light availability. This variation ranges between the maximum value at a depth pre- dicted by physical laws and the minimal values encountered in cryptic habitats. The large variation in available light occurring over spatial scales as small as 10 cm distorts the direct relation between depth and light. Depth and light are only correlated as ecological factors for species that strive for maximum light capture (Strategy 1 or 2). A large depth distribution does not imply adaptation to a large light range when species have a strict preference for cryptic light environments. For such species (Strategy 3) depth is not a proxy for light. The different light strategies of species are related to morphological variation and species exploration of the reef habitat.