Echinometra sea urchins on Caribbean coral reefs: Diel and lunar cycles of movement and feeding, densities, and morphology

Highlights

• Diel patterns of Echinometra foraging reversed over the lunar cycle.

• The foraging range of E. viridis was 0.3–5% that of Diadema antillarum.

• High densities of E. viridis occurred at 25% of sites in the Greater Caribbean.

• Most areas have low densities of E. viridis, restricting its herbivory potential.

• Morphological traits affecting vulnerability to predation varied across habitat types.

Abstract

Two species of Echinometra are found on tropical western Atlantic reefs. E. viridis and E. lucunter. Both are grazers and bioeroders, and on some disturbed reefs E. viridis has become the dominant herbivore, particularly since the disease-caused decline of the much larger echinoid Diadema antillarum. Thus, it is critical to understand factors affecting herbivory by Echinometra, including foraging behaviors and distances, diel and lunar cycles of foraging, and densities of Echinometra, particularly E. viridis, across habitats and locations.

Twenty-four hour tracking studies undertaken in the San Blas Islands of Panama, conducted at different times in the lunar cycle, revealed that on new moons both species of Echinometra primarily moved and fed at night, but the pattern reversed on full moons. These data suggest that vulnerability to predators and/or aggression by damselfishes during the night on full moons may be a significant selective force affecting diel foraging behavior. Daily distances moved (minimum estimate) averaged 19 cm for E. viridis and 11 cm for E. lucunter. The average foraging range for E. viridis was estimated to be 0.3–5% the size of that of D. antillarum, while per capita daily algal consumption by E. viridis is 20% that of D. antillarum.These results suggest more intense, localized grazing by E. viridis compared to D. antillarum.

A review of published densities of E. viridis show that 50% of studied sites (location/habitat combination) have densities less than 5 m⁻², a level unlikely to produce a strong herbivory effect. Densities greater than 15 m⁻² were found at 25% of sites, including the San Blas Islands, Panama patch reefs in this study, which averaged 26 m⁻². Where high densities are combined with low predator numbers due to fishing, E. viridis can have strong direct effects on algal assemblages, with potential indirect effects on corals and other benthic invertebrates. However, available published data indicate E. viridis has strong regional variation in abundance, as yet unexplained, that limits its potential Caribbean-wide role as a reef herbivore.

Morphological traits of E. viridis that are likely related to vulnerability to predation varied across habitats. A newly-described melanic color morph comprised 4–16% of the population, with significant variation in frequency across habitat types. Body sizes (test diameter) also varied among habitats, with larger individuals much more common in habitats with fewer shelter sites. Similarly, in habitats offering less protection from predators, spine lengths were longer relative to test diameter. Morphological traits that reduce predator detection or vulnerability to predation, as well as the refuges afforded by habitat, will ultimately affect both the number of E. viridis on a particular reef and their ability to forage in the presence of predators.

Introduction

Sea urchins are abundant, ecologically important components of coral reef communities. On Caribbean reefs, Diadema antillarum was the dominant echinoid grazer, foraging over large areas, controlling algal abundance and allowing corals to recruit and survive (Carpenter, 1981; Sammarco, 1982a, Sammarco, 1982b). However, the catastrophic decline of D. antillarum due to disease in 1983/4 (Lessios et al., 1984a, Lessios et al., 1984b), with limited recovery since (Lessios, 2016), has focused attention on other Caribbean reef sea urchins. The resulting research over the past two decades has increasingly recognized the important ecological role of Echinometra (Aronson et al., 2002a, Aronson et al., 2002b). On overfished reefs, and with the ongoing rarity of Diadema antillarum, E. viridis has become the dominant herbivore on some reefs, capable of top-down control of algal abundance and assemblage composition, and influencing coral (and other benthic animals) recruitment, growth and survivorship (Kuempel and Altieri, 2017). Similarly, E. lucunter can be an abundant grazer in shallow reef habitats and both species of Echinometra contribute to bioerosion of reefs (Griffin et al., 2003; Brown-Saracino et al., 2007; Perry et al., 2014).

These community effects of sea urchin grazers, however, are determined by foraging behaviors and population density, which in turn are influenced by vulnerability to predation at various times of day and lunar month and in various habitats. The activity patterns, distribution, densities, and ecological importance of Diadema antillarum have been well studied (Ogden et al., 1973; Sammarco, 1982a, Sammarco, 1982b; Carpenter, 1984), as have the foraging and boring behavior of E. lucunter in the rocky intertidal and shallow reef flats (McPherson, 1969; Abbott et al., 1974; Lewis and Storey, 1984). In contrast, these aspects of the biology and ecology of E. viridis have received less attention, are likely influenced by interactions with predators (Dunn et al., 2017, Dunn et al., 2018), and can be critically important in determining the relative dominance of algae versus corals in the benthic community (Aronson et al., 2005; Sangil and Guzman, 2016a, Sangil and Guzman, 2016b).

In this paper, I focus on three sets of questions regarding the ecology and biology of E. viridis, with additional comparative data on its sister species, E. lucunter for the first of these. (1) What are the foraging ranges of these two species and how does foraging activity vary over diel and lunar cycles? Activity cycles may be related to selection by predators or competitors (Hobson, 1972) and understanding foraging range illuminates potential spatial patterns in the effects of grazing on algal prey. (2) What are the densities of E. viridis, and how do they vary across habitats and regions? Ultimately, the direct and indirect community effects of E. viridis grazing is, at least in part, a function of population density; here I report densities from habitats in the San Blas Islands, Panama, as well as data collated from studies throughout the greater Caribbean. (3) How do morphological features of E. viridis, likely related to natural selection by predators and affecting the sea urchins ability to survive and graze, vary across habitat types? These morphological traits include coloration, body size, and the allometry of spine length relative to body size.