Caribbean sponge species typical of coral reefs are generally inhibited from living in seagrass meadows by their vulnerability to predation by the large seastar Oreaster reticulatus. Oreaster reticulatus are generally confined to seagrass meadows, where the typical sponge inhabitants are well defended against them (11 of 14 seagrass and rubble bed sponge species were rejected in experiments, versus only 3 of 20 coral reef sponge species rejected; Wulff 1995). Thus, typical coral reef sponges are inhibited from extending their habitat distributions into seagrass meadows by their vulnerability to O. reticulatus, which can quickly eliminate them if they are washed into seagrass meadows in a storm. In a seagrass meadow in Belize, Wulff (2008) found several sponge species living in clusters, often associated with small patches of hard substrata that frequently also include the scleractinian coral Porites furcata. In many cases, the sponge species Lissodendoryx colombiensis was overgrown by all the others. This sponge was the only sponge species in the studied seagrass meadow that is not typically a member of the seagrass associated fauna, its typical habitat being patch reef and lagoonal environments in water less than 6 meters deep and growing on sand and coral rubble, on dead lateral parts of massive corals, and between branches of ramose and foliose corals (Lissodendoryx colombiensis can also be found on mangrove prop roots at sites where mangroves are closely associated with reefs, e.g., in Bocas del Toro, Panama, and in the Pelican Cays, Belize). Overgrowth of portions of L. colombiensis individuals by unpalatable sponges may be the most effective way for this sponge to avoid complete elimination by starfish feeding. Wulff (2008) found that O. reticulatus avoided portions of L. colombiensis individuals that were covered or surrounded by unpalatable sponges. Thus, these associations among sponge species appear to actually help the overgrown species in extending its habitat distribution into "enemy territory". Wulff (2008) argues that mutually beneficial interactions such as this may result in higher species diversity in some ecological communities. (Wulff 2008)
Oreaster reticulatus feeds on sponges by everting its stomach onto a sponge and digesting the tissue, leaving behind the sponge skeleton. In a study by Wulff (1995) on the feeding habits of this species in Panama, 54.2% of the 1549 starfish examined from February 1987 to June 1990 at eight sites were feeding, and 61.4% of these were feeding on sponges, representing 51 species. Feeding on sponges was disproportionately heavily in comparison to their abundance, which was only 9.7% of available prey. In feeding choice experiments, 736 pieces of 34 species of common sponges from a variety of shallow-water habitats were offered to O. reticulatus in individual underwater cages. Sixteen of 20 species that normally grow only on the reefs were eaten, but only 1 of 14 species that live in the seagrass meadows and rubble flats surrounding the reefs was eaten. Oreaster reticulatus lives in the seagrass meadows and rubble flats, and avoid the reefs, and the acceptable reef sponges are thus generally inaccessible until a storm fragments and transports them into starfish habitat. After Hurricane Joan washed fragments of reef sponges into a seagrass meadow in October 1988, starfish consumed the edible species. When the seagrass meadow was experimentally seeded with tagged reef sponge fragments in June 1994, O. reticulatus consumed edible species and accumulated in the area seeded. Reef sponges that were living in a seagrass meadow, from which O. reticulatus had been absent for at least 4 yr (from 1978 to 1982), were eliminated when the starfish migrated into the area, and the sponges were unable to recolonize (as of June 1994). Oreaster reticulatus feeding and habitat preferences appear to restrict distributions of many Caribbean reef sponge species to habitats without O. reticulatus and may have exerted significant selective pressure on defenses of those sponges that live in O. reticulatus habitats. (Wulff 1995)
Like many other echinoderms, Oreaster reticultaus may form dense, moving feeding aggregations. Scheibling (1980b) studied aggregations of Oreaster reticulatus in a large sand patch within an offshore seagrass bed (Syringodium filiforme) off St. Croix (U.S. Virgin Islands). These aggregations were characterized by protracted fronts of high density (2 to 7 individuals/m2), which continued to move during the entire 10-week study. Fronts mlgrated across the sand patch at approximately 7 meters/day and were refracted and deflected at the grassbed border. Intensive microphagous (feeding on tiny particles) feeding effected a 2-fold turnover of surface sediments in the area covered by a front during 24 hours, resulting in a marked decrease in chlorophyll concentration. Scheibling suggested that an intrinsic increase in gregariousness, possibly associated with reproductive state, may precipitate front formation as aggregations of 0. reticulatus become localized. Particulate organic matter in the surface sediments is rapidly removed by intense feeding activity within aggregations, resulting in large-scale patchiness of the organic content of the substratum. As aggregations migrate across the patch, the movement of individuals along the leading border is slowed as they advance into nutrient-rich sediments. Trailing individuals, foraging upon disturbed sediments, move farther and spend less time at feeding sites, eventually overtaking those in the lead to form a high-density front. The dissolution of fronts may occur by a reverse mechanism: aggregations become more diffuse, resulting in a decrease in the large-scale heterogeneity of the organic content of the substratum and an increase in the uniformity of rates of movement of foraging individuals. Scheibling suggested that formation of dense aggregations may be atypical for Oreaster reticulatus, occurring only when there is a dense population in a confined habitat. (Scheibling 1980b)
Oreaster reticulatus is widely distributed on both sides of the Atlantic, from North Carolina (U.S.A.) to as far south as Brazil and the Cape Verde Islands in West Africa (Scheibling 1980a; Hendler et al. 1995, cited in Guzman and Guevara 2002). Throughout the tropical Caribbean, O. reticulatus inhabits calm, shallow waters (Scheibling 1980a). Studies from St. Croix (Virgin Islands), the Grenadines, and Panama indicate that the species is found more abundantly on coarse, calcareous sandy bottoms that are isolated or surrounded by seagrasses such as Turtle Grass (Thalassia testudinum), Shoal Grass (Halodule wrightii), and Manatee Grass (Syringodium filiforme) and calcareous macroalgae such as Halimeda incrassata and Penicillus capitatus. Aggregations of reproductive individuals have a greater tendency to occur in dispersed sand patches and not in beds of dense Turtle Grass, whereas juveniles tend to be found in very dense meadows of Turtle Grass and Manatee Grass, where there is greater protection from predators. Juveniles, especially, are also found in soft sand and mud substrates that are typical of mangroves, lagoons, and some shallow reef environments. (Scheibling 1980a; Guzman and Guevara 2002 and references therein)
Scheibling and Metaxas (2010) found that coastal mangroves and fringing coral reefs, along with luxuriant seagrass beds, are important habitats for recruitment (production of the next generation) of O. reticulatus. They note that throughout the Caribbean (and the tropics worldwide) these habitats are threatened by shoreline alteration, pollution, destructive and unsustainable fishing practices, and, for coral reefs especially, the impacts of ocean warming and acidification
Oreaster reticulatus is an omnivore that feeds on a great variety of epiphytic microorganisms such as filamentous algae, diatoms, and small detritus particles. The number of microorganisms it consumes generally depends on their availability and its ability to capture them. It also feeds on sea urchins; sea cucumber juveniles; meiofauna such as polychaete worms, copepods, ostracods, and crab larvae; and sponge tissue. (Guzman and Guevara 2002 and references therein)
The Cushion Sea Star (Oreaster reticulatus) is large (up to 50 cm wide) and thick. Juveniles are green; adults are yellow, orange, and/or tan, with a network of contrasting tubercles (Kaplan 1988).
Scheibling and Metaxas (2010) suggest that Oreaster reticulatus appears to be thriving in Central America relative to other parts of the Caribbean.
Oreaster reticulatus is widely distributed on both sides of the Atlantic, from North Carolina to as far south as Brazil and the Cape Verde Islands in West Africa (Scheibling 1980a; Hendler et al. 1995, cited in Guzman and Guevara 2002).
Throughout the tropical Caribbean, O. reticulatus inhabits calm, shallow waters (Scheibling 1980a). Studies from St. Croix (Virgin Islands), the Grenadines, and Panama indicate that the species is found more abundantly on coarse, calcareous sandy bottoms that are isolated or surrounded by seagrasses such as Thalassia testudinum, Halodule wrightii, and Syringodium filiforme, and calcareous macroalgae such as Halimeda incrassata and Penicillus capitatus. Aggregations of reproductive individuals have a greater tendency to occur in dispersed sand patches and not in beds of dense T. testudinum, whereas juveniles tend to be found in very dense meadows of T. testudinum and Syringodium filiforme, where there is greater protection from predators. Juveniles, especially, are also found in soft sand and mud substrates that are typical of mangroves, lagoons, and some shallow reef environments. (Scheibling 1980a; Guzman and Guevara 2002 and references therein)
In Belize, mangroves and fringing reefs were found to be important habitats for juvenile Oreaster reticulatus. Mangrove and fringing reefs, like dense seagrass beds, may provide a protective environment for recruits that undergo an ontogenic shift to adult habitats, such as sand flats, as they mature reproductively (Scheibling and Metaxas 2010).
Metaxas et al. (2008) studied larval development and behavior of Oreaster reticulatus. This sea star free-spawns large, negatively buoyant (slowly sinking) eggs. At average current velocities in natural habitats, the authors found that these eggs could disperse at least 20 meters before sinking to the bottom, which increases the probability of fertilization in this sparsely-distributed species. The planktotrophic larvae completed development within 23 days at 23 degrees C. In laboratory experiments, larvae settled mainly on the undersides of pebbles encrusted with coralline algae in both light and dark treatments, indicating a preference for cryptic microhabitats. Competent (i.e., ready to transform) larvae spent approximately half their time exploring the substratum by moving in straight or circular paths at an average speed of 1.14 cm per minute, stopping periodically to probe the surface and attach with a preoral or brachiolar arm. Once attached, larvae rotated around the adhesive disk for 20 to 180 seconds, sequentially attaching and detaching one or two brachiolar arms. Final attachment and metamorphosis occurred within 24 hours of settlement. Based on speed and duration of movement, displacement distance could be up to 7 meters for a searching larva under no flow. Recently metamorphosed juveniles moved at an average speed of 0.074 cm per minute over sand and coralline-encrusted pebbles, sequentially attaching and detaching tube feet in the direction of travel. Overturned juveniles were capable of righting themselves in 16 to 28 seconds. Post-settlement movement could enable settlers to locate cryptic microhabitats, particularly in seagrass beds, and thereby reduce the probability of being overturned or consumed.
Scheibling and Metaxas (2010) studied Oreaster reticulatus in mangrove, fringing reef, seagrass, and sand habitats at sand cayes off the southern coast of Belize. Estimated density and biomass ranged from 1.7 to 18.3 individuals per 100 m2 and 0.9 to 5.5 kg per 100 m2 among six sites. Dispersion was random, except at a fringing reef where sea stars were aggregated (at the sampling scale of 1.5 m2). Mean individual size (radius, R) ranged from 9.7 to 13.7 cm among sites. Populations in both mangrove and reef habitats were mainly juveniles (R < 12 cm: 83% and 91%, respectively), with characteristic cryptic coloration. Juveniles occurred primarily along mangrove banks and were closely associated with coral colonies in fringing reefs. The proportion of juveniles also was high on a sand flat (54%), although juvenile color morphs were not observed there. Scheibling and Metaxas (2010) suggest that these findings, when considered together with results from previous work in Panama (Guzman and Guevara 2002), indicate that levels of O. reticulatus recruitment (production of a new generation) in Central America may be higher than in other parts of the Caribbean.
In field experiments, fertilization rates for Oreaster reticulatus were similar to those that have been observed for other seastars. Recorded rates were as high as 20% at 32 meters downstream, suggesting that the low population density of O. reticulatus may not be as limiting as suspected with respect to successful fertilization and zygote production. Because O. reticulatus feeds on sediments with low organic content, individuals must sample large areas to obtain sufficient resources. The spatial distribution of a population of this sea star probably reflects a trade-off between a density that minimizes competition during feeding and one that maximizes the probability of fertilization (Metaxas et al. 2002 and references therein).
Scheibling and Metaxas (2010) found that coastal mangroves and fringing coral reefs, along with luxuriant seagrass beds, are important habitats for recruitment (production of the next generation) of Oreaster reticulatus. They note that throughout the Caribbean (and the tropics worldwide) these habitats are threatened by shoreline alteration, pollution, destructive and unsustainable fishing practices, and, for coral reefs especially, the impacts of ocean warming and acidification.
Oreaster reticulatus is an omnivore that feeds on a great variety of epiphytic microorganisms such as filamentous algae, diatoms, and small detritus particles. The number of microorganisms it consumes generally depends on their availability and its ability to capture them. It also feeds on sea urchins such as Meoma ventricosa, Tripneustes ventricosus, and Echinometra viridis; holothuroid (sea cucumber) juveniles such as Holothuria mexicana; meiofauna such as polychaete worms, copepods, ostracods, and crab larvae; and sponge tissue. (Guzman and Guevara 2002 and references therein)
Oreaster reticulatus is a microphagous (feeding on tiny particles) grazer of sand and seagrass substrata in the Caribbean (Scheibling 1980b and references therein). In a study of Oreaster reticulatus in Belize, Scheibling and Metaxas (2010) found that most individuals had their stomachs everted on sediments or detrital matter, but sponge and ascidian prey also were consumed in a mangrove habitat.
