BULLETIN OF MARINE SCIENCE, 32(2): 504-510, 1982

FEEDING HABITS OF RETICULATUS (ECHINODERMATA: ASTEROIDEA)

R. E. Scheibling

ABSTRACT In situ feeding observations among eight populations in various grassbed and sand-bottom habitats in the Grenadines and off St. Croix. U.S. Virgin Islands indicate that is primarily an omnivorous. microphagous substratum grazer. It extrudes its extensive cardiac stomach upon seagrass. sand, and/or algal substrates and ingests associated micro-organisms and particulate detritus. O. reticulatus also functions as an opportunistic predator and scavenger of any available slow-moving, sessile or moribund macrofauna and is occasionally cannibalistic. The feeding habits of O. reticulatus may be exemplary of tropical, shallow-water asteroids in general.

Oreaster reticulatus (Linnaeus) is an abundant and conspicuous inhabitant of seagrass and sand bottoms in the Caribbean (Scheibling, 1980a,b). However, published information on its feeding biology is limited. A few incidental obser- vations and circumstantial evidence have led to divergent conclusions: O. reti- culatus typically digests microscopic organic material upon the substratum but occasionally feeds on (Thomas, 1960); it preys upon the urchin (Kier and Grant, 1965); and, based on anatomical specialization of the gut, it functions as a suspension feeder (Anderson, 1978). Anderson (1978) has expressed the need that "studies soon be made on the feeding biology of Oreaster reticulatus, to supplement incidental observations, and to determine the validity of conclusions that can now be only inferred from considerations of the anatomy of the digestive system." This study surveys the feeding habits of Oreaster reticulatus in situ in several populations from various habitats in the Carribbean. Microphagous substratum grazing and macrophagous predation and scavenging are examined as alternate feeding modes in relation to anatomical specializations of the digestive tract and food availability.

MATERIALS AND METHODS

In situ observations of feeding in Oreaster reticulatus were conducted among populations in grassbed habitats (Hillsborough Bay and L'Esterre Bay, Carriacou Island, and Chatham Bay, Union Island, Grenadines) and in sand-bottom habitats (Watering Bay and Manchioneal Bay, Carriacou Island, and Buck Island Channel, Coakley Bay and Horseshoe Patch, St. Croix, U.S. Virgin Islands) by overturning individuals and noting the type of substratum or macroscopic prey encompassed by the broadly extended stomach. O. reticu/atus were sampled systematically in transect surveys (Schei- bling, 1980a,b) or as encountered in unidirectional traverses of a study site. Ingested particulate organic matter was collected from the stomachs of individual Oreaster reti- cu/atus feeding upon sand substrata in Buck Island Channel and Horseshoe Patch. The asteroids were overturned rapidly and the nozzle of a 50-ml syringe was inserted into the closing mouth. (Disturbance evokes retraction of the stomach and closure of the mouth and ambulacral grooves within 3-5 sec.) With the syringe clamped by the mouth, a 10-ml sample of seawater, entrapped within the lobes of the cardiac stomach during retraction, was withdrawn. Ten samples (different individuals) were pooled for each study site and centrifuged for 15 min in a Sorvol high-speed cen- trifuge; the particulate residue was examined microscopically. The density of sea urchin prey of Oreaster reticu/atus was measured concurrently with that of the asteroid. In Chatham Bay, Tripneustes ventricosus were enumerated in 32, 25-m2 quadrats placed at 25-m intervals along 8, IOQ-m long transects (Scheibling, 1980a). In Horseshoe Patch, Meoma ventricosa were enumerated in 28, 10-m diam. circular census stations (Scheibling, I980b). In

504 SCHE1BLlNG: FEEDING HABITS OF OREASTER RETICULATUS 505

Hillsborough Bay, a 16-m2 wire mesh enclosure was set in the Ha/odu/e wrightii zone (Scheibling, 1980a) and stocked with 20 O. reticu/atus and 20 T. ventricosus. The predation of T. ventricosus by O. reticu/atus in the enclosure was monitored for 3 days. Asteroids that escaped and urchins that were preyed upon were replaced daily.

RESULTS The summary of 10,328 in situ observations of feeding in Oreaster reticulatus from three populations in grassbed habitats and five populations in sand-bottom habitats is given in Table I. Feeding is divided into two basic types: (I) substratum grazing, in which the cardiac stomach is everted upon the substratum and func- tions as a microphagous feeding organ, and (2) predation and scavenging, in which the stomach envelops macrofaunal prey or carrion. Substratum grazing is sub- divided according to the predominant substratum type (e.g., sand, seagrass, al- gae). Microphagous substratum grazing is the primary feeding type of O. reti- culatus in nature, accounting for 99-100% of all feeding observations in 7 of the 8 populations studied and 90% in the remaining one (Chatham Bay). Substratum Grazing.-In grassbed habitats, Oreaster reticulatus fed prefer- entially upon Halodule wrightii (Table 1 and Scheibling, 1980a). Grassblades were clustered beneath the disc by the tube feet and enveloped by the everted cardiac stomach. Degradation of the blades was not discernible, indicating that O. reticulatus was probably digesting epiphytic micro-organisms, filamentous , and particulate detritus associated with H. wrightii and not the seagrass per se. Dense mats of filamentous green algae (Chaetomorpha, Enteromorpha) were intensively grazed by O. reticulatus during periodic blooms (2-3 mo inter- vals). These mats covered from 50-75% of the bottom and harbored abundant micro-organisms and detritus. O. reticulatus fed upon bare carbonate sand in pockets amid the beds of Halodule wrightii, in an inshore sandy zone (Hillsbor- ough Bay and L'Esterre Bay), and in patches of sparse Thalassia testudinum (Chatham Bay). Infrequently, O. reticulatus everted its stomach upon clusters of dead, detached blades of T. tesludinum ("Other Substratum Grazing" in Table I) that had collected in the sand pockets, probably to digest films of bacteria and other micro-organisms on the surface of the litter. In sand-bottom habitats, Oreasler reticulatus fed almost exclusively upon the bare sand substratum (Table 1). The carbonate surface sediments were raked by the tube feet (in the same manner as grassblades) to form a low mound beneath the disc upon which the cardiac stomach was everted (Scheibling, 1980d). Stomachs of feeding individuals in Buck Island Channel and Horseshoe Patch contained an array of particulate organic matter including: numerous dia- toms; dinoflagellates, foraminifera and other protozoans; filaments of green and bluegreen algae; meiofaunal crustaceans (e.g., harpactacoid copepods, ostra- cods, crab larvae), polychaetes, nematodes; and bacterio-detrital aggregates. O. reticulatus was infrequently observed with its stomach everted upon seagrass (Syringodiumfiliforme) or red macroalgae (Lophocladia trichoclados, Wrangelia biscuspidata, Laurencia corallopsis, Wurdemania miniata) which occurred spo- radically in sand-bottom habitats. The red algae were discolored (orange or red) where in contact with the stomach. In Watering Bay, O. reticulatus was infre- quently observed with its stomach everted upon coral rubble ("Other Substratum Grazing" in Table 1) apparently digesting organic films and/or encrusting coralline algae on the rubble surface.

Predation and Scavenging.-Oreaster reticulatus infrequently preyed upon ses- sile or slow-moving macrofauna. In the grassbed populations, the sea urchin 506 BULLETIN OF MARINE SCIENCE. VOL. 32. NO.2. 1982

Table l. Tbe feeding habits of Oreaster reticulatus in populations from (a) grass bed and (b) sand- bottom habitats: percentage frequencies of substratum grazing on various substratum types, and of predation/scavenging

Percentage of Feeding Observations

Pre- Number of Substratum Grazing dation! Feeding Scaveng- Populations Observations Sand Grass Algae Other Total ing

(a) Grassbed Habitats Hillsborough Bay 3,346 7.7 74.4 ]7.2 0.1 99.4 0.6 L'Esterre Bay 374 ]5.4 74.5 9.3 0.4 99.6 0.4 Chatham Bay 611 19.8 48.5 21.9 0.] 90.3 9.7 (b) Sand-Bottom Habitats Buck I. Channel 1,833 98.4 0.3 1.2 0 99.9 0.1 Coakley Bay 2,355 95.1 0 4.9 0 100.0 0 Horseshoe Patch 1,162 95.2 2.2 1.9 0 99.3 0.7 Watering Bay 568 %.2 0 0 2.6 98.8 1.2 Manchioneal Bay 79 100.0 0 0 0 100.0 0

Tripneustes ventricosus was a predominant prey item. Predation of T. ventricosus by O. reticulatus was rare in Hillsborough Bay and L'Esterre Bay (7 out of a total of 3,720 feeding observations) due to the paucity of the urchin on beds of Halodule wrightii inhabited by the asteroid. When offered T. ventricosus in an enclosure in the H. wrightii zone, 10-20 O. reticulatus consumed an average of six urchins/day (range: 4-9) over a 3-day period. Predation of T. ventricosus was more frequent in Chatham Bay (43 out of 611 feeding observations) and increased with urchin density within various substratum types (Table 2). The density of O. reticulatus, however, was greatest in beds of H. wrightii where T. ventricosus was least abundant. Oreaster reticulatus attacked Tripneustes ventricosus within a radius of 10-30 cm. The tip of the leading ray was curled upwards, and the terminal tentacles were extended and active as the asteroid moved towards its potential prey. Con- tact by the ray tip evoked vigorous spine activity, projection of globiferous ped- icellaria and a flight response in T. ventricosus. The pedicellaria affixed them- selves to the epidermis and tube feet of the ray tip, eliciting its momentary retraction, but pursuit followed. Chases lasting 5-10 min were observed; in each instance the faster moving urchin escaped O. reticulatus. T. ventricosus occa- sionally initiated flight prior to contact by an attacking asteroid. When feeding upon Tripneustes ventricosus, Oreaster reticulatus mounted and overlaid the urchin, enveloping the test with the fully everted cardiac stomach. Once the epidermis was digested, the stomach was inserted into the test through the periproct to digest internal tissue and gut contents of the urchin. Upon com- pletion of digestion, little more than the intact test, jaw apparatus (Aristotle's lantern) and detached spines remained. In Horseshoe Patch, Oreaster reticulatus co-occurred with the spatangoid ur- chin Meoma ventricosa which formed dense aggregations (50-200 urchins/100 m2). Despite its availability, M. ventricosa represented only 4 out of the 1,162 systematic feeding observations (Table 1). Instances of feeding upon M. ventri- cosa were exceedingly conspicuous due to the large size of the urchin, and an additional 17 incidental observations were recorded during other in situ studies. In most cases, O. reticulatus scavenged M. ventricosa that had been drilled by cas sid gastropods (Cassis tuberosa, C. madagascariensis), occasionally while the gastropods were still feeding on the soft internal tissues. SCHEIBLING: FEEDING HABITS OF OREASTER RETICULATUS 507

Table 2. The relationship between the incidence of predation of Tripneustes ventricosus by Oreaster reticulatus and the densities of predator and prey on various substratum types in Chatham Bay based on transect samples

Substratum Type

Halodule Thalassia wrighlii testudinum Sand

Incidence of Predation (% of N feeding observations) 1.2 (N = 86) 22.7 (N = 22) 25.0 (N = 8) Density of Tripneustes ventricosus (urchins/IOO m2) 7.1 69.3 45.1 Density of Oreaster reticulatus (sea stars/IOO m2) 5.6 1.5 0.6

Oreaster reticulatus attacked Meoma ventricosa by draping its leading ray over the urchin which moved slowly away, waving its spines and emitting a greenish fluid. After an extended chase (up to 2 m in one case), O. reticulatus mounted the urchin and everted its cardiac stomach. O. reticulatus digested only the epi- dermis of M. ventricosa, leaving patches of denuded test and scattered spines. The stomach was not inserted into the buccal or anal openings. Diadematid urchins Diadema antillarum, Astropyga magnifica and Centroste- phanus rubricingulus occasionally were observed among populations of Oreaster reticulatus in both grassbed and sand-bottom habitats, but were not preyed upon. Sponges were the second most frequent macrofaunal prey of Oreaster reticu- latus. Desmapsamma anchorata was the predominant prey ; others in- cluded: Dysidea etheria, D. fragiLis, Haliclona rubens, H. viridis and Neofibu- laria noLitangere (identified by Dr. H. Reiswig). The sponges generally were viable fragments which probably had been dislodged by wave or current action and swept into the habitat of O. reticulatus. Undigested fragments or whole individuals were rarely observed among populations of O. reticulatus in- dicating that sponges were readily consumed by asteroids. When feeding upon sponges, the extent of cardiac stomach eversion was proportional to the size of the fragment. The sponge tissue was discolored and disintegrated where in contact with the stomach; upon completion of digestion only the refractory network of spongin fibers remained. Other macrofaunal prey of Oreaster reticulatus included a holothurian (prob- ably a juvenile Holothuria mexicana), a sea hare (Aplysia dactylomela), small dead fish (jacks) or fish parts (apparently relinquished by fisherman or piscivorous birds), crab remains (Calappa sp.), and a colonial ascidian (unidentified species). Cannibalism was observed three times in Hillsborough Bay and in Horseshoe Patch. The cardiac stomach was everted over the aboral surface of the victim, digesting the epidermis and, in more advanced stages, the body wall of the disc and internal organs. A few asteroids at these sites were observed with epidermal scar patches on their aboral disc, characteristic of cannibalistic encounters.

DISCUSSION Oreaster reticulatus is primarily, and in many areas almost exclusively, an omnivorous, microphagous substratum grazer feeding upon diverse micro-organ- isms and particulate detritus associated with sand, seagrass, and/or algal sub- 508 BULLETIN OF MARINE SCIENCE, VOL. 32, NO.2, 1982 strates. It also demonstrates a limited capacity for herbivory and has various enzymes capable of hydrolyzing plant carbohydrates (Scheibling, 1980c). O. reticulatus readily digested filamentous green alga (Chaetomorpha crassa) in the laboratory (Scheibling, 1981a) which suggests that intensive grazing by asteroids contributed to the gradual degradation of green algal mats in grassbed habitats. O. reticulatus may also utilize fine-structured plumose red algae (Lophocladia trichoclados, Wrangelia bicuspidata) directly as food while other more robust macroalgae (Laurencia corallopsis, Wurdemania miniata) are more refractory to digestion and, like seagrasses, are probably exploited primarily as a source of adherent particulate organic matter (Scheibling, 1979). Anderson (1978) stated that "The unusually large, extensively eversible cardiac stomach of Oreaster reticulatus seems primarily adapted for handling large pieces of food outside the body " However, macrophagous feeding occurs rarely in nature and is generally of incidental value to the nutritional economy of O. reticulatus (although it may assume increased importance in local situations where suitable macrofaunal prey are readily available). In view of the primary importance of microphagy, a more important adaptive function of the extensive stomach would be to maximize the area of contact between this f1agellary-mucus feeding organ (Anderson, 1978; pers. obs.) and the substratum. The accumulation of the substratum beneath the disc by the tube feet prior to stomach eversion may also serve to increase the stomach-substratum interface (Scheibling, 1980d). Anderson (1978) also proposed that Oreaster reticulatus was a filter feeder based on the presence of specialized structures in the digestive system, including elaborate Tiedemann's pouches, strongly flagellated, folded structures of the py- loric caeca, and unusually large intestinal caeca, which he interprets as adapta- tions for enhancing the circulation of water through the gut. Anderson cites un- published observations of J. Halpern that O. reticulatus filter-feeds during strong tidal currents, often using sponges (Spheciospongia vesparia) "as a purchase so as to be able to outstretch one, two, or even three arms." I have never observed O. reticulatus to uplift one or more rays into the water column in the character- istic posture of filter-feeding asteroids, even when draped over sponge prey. Also contrary to Halpern's observations, I observed a marked decrease in micropha- gous feeding during strong tidal currents, when O. reticulatus retracted its stom- ach, and closed its mouth and ambulacra (Scheibling, 1979). The specialized features of the digestive tract of Oreaster reticulatus described by Anderson can be interpreted as adaptations for microphagous substratum feed- ing rather than for suspension feeding. O. reticulatus probably utilizes strong f1agellary currents and specialized pumping organs such as Tiedemann's pouches and the intestinal caeca to inhale particle-laden interstitial water from sediments, clusters of grassblades, or algal networks. The asteroid is maximally inflated while substratum feeding and periodically expels water, occasionally with some force, through the anus. "Anal fountains" have been reported for other microph- agous asteroids such as Porania pulvillus (Gemmill, 1915) and Patiria miniata (Araki, 1964). Gemmill (1915) attributed this phenomenon in P. pulvillus to the contraction of the muscular intestinal caeca. These organs are sometimes greatly distended with watery fluid in dissected specimens of O. reticulatus (Tennant and Keiller, 1911; pers. obs.). Dissolved as well as particulate organic matter may be acquired during sub- stratum feeding. Nutrient solutions including milk and filtered, aqueous extracts of sponge, algal and urchin tissue homogenates were observed to stimulate car- diac stomach eversion of Oreaster reticulatus in laboratory aquaria. Araki (1964) and Ferguson (1969) also employed dissolved nutrients to induce feeding activity SCHEIBLlNG: FEEDING HABITS OF OREASTER RETlCULATUS 509 in Patiria miniata and Echinaster echinophorous respectively, and experimen- tally demonstrated the uptake of these nutrients by internal digestive organs. The incidence of macrophagous feeding in Oreaster reticulatus is determined by the availability or capturability of suitable prey. In Hillsborough Bay, Trip- neustes ventricosus was abundant in dense beds of Thalassia testudinum bor- dering the asteroid population, where the urchin enjoyed a spatial refuge from predation due to the limited mobility of O. reticulatus on this substratum (Schei- bling, 1980a). Urchins that were experimentally displaced into the adjacent Halodule wrightii zone were rapidly consumed by O. reticulatus. In Chatham Bay, T. ventricosus and O. reticulatus co-occurred, resulting in a much greater incidence of predation. Macrophagous feeding probably was an important nutri- tional supplement for O. reticulatus in Chatham Bay (Scheibling, 1979), and may have accounted for the asteroid's greater size (Scheibling, 1980a) and repro- ductive capacity (Scheibling, 1981b) there relative to Hillsborough Bay. Kier and Grant (1965) concluded that Oreaster reticulatus frequently fed upon Meoma ventricosa based on one such observation and circumstantial evidence of bare, spineless patches on the tests of urchins. Chesher (1969) never observed O. reticulatus feeding upon the urchin and indicated that the injuries described by Kier and Grant (1965) could have been caused by infections. In Horseshoe Patch, predation of M. ventricosa by O. reticulatus rarely occurred despite the co-occurrence of large numbers of the two . The urchin's large size, dense covering of minute spines and the pigmented secretion emitted when dis- turbed may have a deterrent effect. Chesher (1969) reports that the secretion "was repellent to small fish and killed small fish and crustaceans in confined conditions." Infrequent feeding upon M. ventricosa, with digestion limited to the thin epidermal layer of the urchin, was probably not a significant source of nu- trition to the population of O. reticulatus of Horseshoe Patch. Cannibalism in Oreaster reticulatus is rare in nature, but occurred in associ- ation with food limitation and in laboratory studies (Scheibling, 1979). Cannibal- ism has been recorded for other asteroid species both in nature (Hancock, 1958; Dayton et aI., 1977) and in the laboratory (Hancock, 1974). The feeding habits of Oreaster reticulatus are similar to those of other oreas- terids (Thomassin, 1976) and may be typical of tropical asteroids in general. Of 28 species of asteroids (16 genera and 6 families) from the tropical Pacific (Ya- maguchi, 1975) and Indian Ocean (Thomassin, 1976) whose feeding habits have been noted, 27 are substratum grazers of particulate organic matter and benthic algae (the one other species was only observed scavenging). Of these 27 species, 6 also scavenged dead and 9 also preyed upon sessile macrofauna (corals, sponges, ascidians). The predominance of omnivorous, substratum grazing among tropical asteroids is attributed to the relatively low availability of macrofaunal prey resources. Similarly, microphagous deposit feeding and opportunistic scav- enging characterize the feeding habits of polar (Pearse, 1965; Dayton et at., 1974) and deep sea (Madsen, 1961; Carey, 1972) asteroid species. In contrast, temperate shallow-water asteroids are typically predaceous (Feder and Christensen, 1966; Mauzey et aI., 1968) in accordance with their co-occurrence with abundant macro- faunal prey such as bivalves, barnacles, urchins and ascidians.

ACKNOWLEDGMENTS

This paper is based on research conducted in partial fulfillment of the doctoral requirements of the Marine Sciences Centre, McGill University, Canada. I am grateful to Dr. J. M. Lawrence, Dr. C. M. Lalli and Dr. H. M. Reiswig for critically reviewing the manuscript and to Dr. R. F. Dill for the usc of research facilities at the West Indies Laboratory in St. Croix. I am indebted to my wife Anna for her invaluable assistance in the field. Funding was provided by a McConnell Memorial Fellowship, 510 BULLETIN OF MARINE SCIENCE. VOL. 32. NO.2. 1982 a David Stewart Memorial Fellowship and a Graduate Faculty Summer Research Fellowship from McGill University and a Postgraduate Scholarship and Operating Grant (#5248 to Dr. C. M. Lalli) from the National Research Council of Canada.

LITERATURE CITED

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DATE ACCEPTED: July 3, 1980.

ADDRESS: Marine Sciences Centre, McGill University, Montrea/, Quebec, Canada; PRESENT AD- DRESS: Fisheries Research Division, P.O. Box 297, Wellington, New Zealand.