Factors Contributing to the Regional Decline of Montastraea annularis (complex)

Over the last 15 years the massive framework coral, Montastraea annularis (complex) has experienced a rapid decline in abundance, size and condition, and on many reefs in the western Atlantic these species are no longer the dominant corals. Surveys conducted in Puerto Rico, the Cayman Islands, Bonaire, St. Kitts and Nevis, and the Bahamas show a similar die-off and replacement by other corals, aggressive invertebrates and macroalgae, although the timing of these events is variable. Widespread colony mortality has been triggered by mass bleaching events, with coral diseases emerging after corals began to recover from bleaching. Outbreaks of yellow band disease and white plague remain the primary threat affecting these species, although other diseases, fish predation, competition with algae, and overgrowth and bioerosion by sponges are contributing to further losses. M. annularis (complex) colonies have sustained the higher levels of partial and whole colony mortality than all other species. Agaricia spp., Porites spp. and other brooding species, as well as certain broadcast spawners, exhibit successful recruitment and colonization of reef substrates and exposed skeletal surfaces of M. annularis. In contrast, few recruits of M. annularis have been documented and formerly large colonies that have survived now consist of small tissue isolates that continue to shrink in size. While much attention has focused on the decline of Caribbean acroporids, the loss of M. annularis (complex) has more significant implications as these are much longer lived, slower growing and less able to recolonize a reef through sexual recruitment or fragmentation.

Introduction

Montastraea annularis (species complex) are the most important framework corals on western Atlantic reefs. The three species in this complex (M. annularis, M. faveolata and M. franksi) form large, long-lived corals that have been dominant for millennia (Jackson 1992). They play a critical role in reef construction and community ecology (Goreau 1959; Knowlton 1992). Until recently, they were thought to be better able to survive periods of adverse conditions than most other coral species
(Johnson et al.1995).

Montastraea annularis (complex) exhibit characteristics of k-selected species, including slow growth rates, late reproductive maturity, development of large colonies with low rates of whole and partial
colony mortality, and moderate regeneration capabilities (Bak and Engel 1979; Meesters et al. 1996). Colonies can be described as ‘bet-hedgers’
because they live for centuries, require many years before first reproduction, and exhibit some of the lowest larval recruitment rates reported for western Atlantic reef-building corals (Szmant-Froelich 1985).

The high densities of large colonies seen throughout the western Atlantic suggest that, at least until recently, these species were- less susceptible to the stressors that contributed to the demise of Acropora spp. and other Caribbean corals (Bythell et al. 1993). These corals are extremely robust and resistant to the effects of all but the most severe hurricanes (Woodley et al. 1981; Bythell et al. 2000). Damaged colonies also exhibit high rates of healing and survival (Bak and Engel 1979), which may be
related to their large body size (Bythell et al. 1993; Meesters et al. 1996; Bak and Meesters 1998).

Since 1995, M. annularis (complex) have exhibited a conspicuous trend of decline due to disease, bleaching, predation, and increased  competition by other benthic organisms (Bruckner and Bruckner 2003, 2006a,b; Miller et al. 2006; Edmunds and Elahi 2007; Bruckner and Hill 2009; Rogers et al. 2009). These corals are susceptible to at least 5 major diseases (Weil 2004), and they often show signs of multiple infections simultaneously (Bruckner and Bruckner 2006a). They have also been severely impacted by recent (1995, 1998, 2005, 2009) mass
bleaching events. This manuscript examines the population structure of these corals within 5 countries through use of the Atlantic and Gulf Rapid Reef Assessment (AGRRA) protocol. Factors contributing to mortality and implications of losses are discussed…