Mesophotic Coral Ecosystems

(a)

(b)

Figure 2.9. (a) In shallow waters, the Caribbean coral Montastraea cavernosa exhibits a boulder-like morphology, shown at 5 m (photo John Reed), and (b) in mesophotic waters, a flattened morphology, shown at 75 m (photo Mike Echevarria).

2.2.1. Living in the shade

In shallow water, adaptation to high light irradiance dominates coral photophysiology (e.g. photo-protective proteins, antioxidant enzyme capacity and self-shading morphologies; Falkowski and Raven 2007). However, because light attenuates exponentially with increasing depth, photosynthetic organisms eventually become light-limited (Kirk 1994). Corals (and algae) transplanted to lower light regimes often increase photosynthetic pigment concentrations per unit area to maximize utilization of ambient light. While potentially advantageous at intermediate depths, this form of shade adaptation becomes self-limiting with increasing depth, as the incremental gain in photosynthetic production per unit pigment diminishes (Falkowski et al. 1990, Stambler and Dubinsky 2007). Therefore at lower mesophotic depths, zooxanthellate corals employ multiple adaptation and

Corals existing in the low-light environment of themesophotic zone, like the plants in the understory of a rainforest, can have specialized morphology and physiological traits (Kuhlmann 1983, Kahng et al. 2014) that enable capture and efficient use of as much light as possible. For example, in shallow water, the Caribbean coral Montastraea cavernosa normally has a boulder-like shape (Figure 2.9a), while at mesophotic depths, it exhibits a flattened phenotype, which enhances light capture (Figure 2.9b; Lesser et al. 2010). Moreover, deep (> 50 m) mesophotic corals can have unique zooxanthellae clades that are adapted to low light and not found in shallower depths (Lesser et al. 2010, Bongaerts et al. 2011a, 2013b, Nir et al. 2011, Pochon et al. 2015).

MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS? 15