NOTES BULLETIN OF MARINE SCIENCE, 32(2): 624-629, 1982 HABITAT UTILIZATION AND BIOTURBATION BY OREASTER RETICULATUS (ASTEROIDEA) AND MEOMA VENTRICOSA (ECHINOIDEA) IN A SUBTIDAL SAND PATCH R. E. Scheibling Deposit-feeding echinoderms, particularly holothurians and spatangoid echi- noids, are major components of many marine soft-bottom communities. Exten- sive reworking of sediments by these animals may have a pronounced effect on the physical and biological characteristics of their environment (Chesher, 1969; Rhoads and Young, 1971; Bakus, 1973; Myers, 1977a; b; Webb et al., 1977). The asteroid Oreaster reticulatus (Linnaeus) and the spantangoid Meoma ven- tricosa (Lamarck) are large-bodied deposit-feeders which co-occur on subtidal sand-bottom habitats in the Caribbean. This study describes the spatial and tem- poral patterns of distribution and abundance of the two species and their rates of bioturbation in order to gain insight into their biological interaction and their effect on the sand-bottom community. MATERIALS AND METHODS Oreaster reticulatus and Meoma ventricosa were studied in Horseshoe Patch, a large (30,500 m2) fine-grained sand patch situated amid dense seagrass beds (Syringodiumfiliforme) at a depth of 10-13 m off St. Croix, U.S, Virgin Islands (Scheibling, I980b). Each species was censused monthly at 28 circular stations (10 m diam.) uniformly distributed throughout the patch (January to June and Oc- tober, 1977) and at seven stations located -20 m into the surrounding grassbed at points around the periphery of the patch (April to June and October). The volume of sediment ingested by Oreaster reticulatus was estimated as the average feeding rate multiplied by the volume of sediment ingested per feeding session. The average feeding rate was estimated by multiplying the feeding rate for continuous foraging-feeding activity in Horseshoe Patch (21.6 feeding sessions individual-1 d-1, Scheibling, 1981b) by the average percentage frequency of foraging-feeding activity (i.e., movement between feeding sites or feeding with stomach everted) per day. The latter was estimated by averaging the percentage of foraging-feeding asteroids during random samples at 3- to 4-h intervals over the day. The volume of sediment ingested was estimated by the volume (Vm) of an oblate hemispheroidal mound of sediment that was enveloped by the cardiac stomach: 2 Vm = 2/3rm hm where rm and hm are the radius and the height of the mound respectively (Scheibling, 1979); rm and hm were measured for a random sample of 75 feeding mounds in Horseshoe Patch. The volume of sediment ingested by Meoma ventricosa per day was extrapolated from the average rate of ingestion (25 cm" sediment individual-1 h-1) given by Chesher (1969). The rate of bioturbation was estimated by multiplying the average test width by the average rate of locomotion (7 cm h-'; Chesher, 1969) for Meoma ventricosa, and by multiplying the average feeding rate by the average area of a feeding site (SIS cm2; Scheibling, I980a) for Oreaster reticulatus. The rates of sediment ingestion and bioturbation by populations of O. reticulatus and M. ventricosa in Horseshoe Patch were calculated by multiplying rates for individuals by the number of individuals in each population. RESULTS The population of Oreaster reticulatus ranged throughout Horseshoe Patch while the population of Meoma ventricosa remained in the northern arm through- 624 NOTES 62.5 DENSITY (individuals 100 m2) II 6-10 ~ 41-50 () 11-15 ' () 16-20 () 51-75 ct 21'2' ()26'30 ct'· Hi-lOa () 31-40 IO 150 Cl " MAR 16 '-'AN 7 FEB 14 o o APR 16 MAV 16 JUN 19 OCT 17 Figure I. Distribution and density of Oreasler relicu/alus (solid circles) and Meoma venlricosa (open circles) at Horseshoe Patch in January through June and October 1977. Where densities of O. relicli/allis and M. venlricosa at a station are similar or equal, semicircles are used for graphical clarity. Stations where densities are <6 individuals 100 m-2 are blank. out the study period (Fig. 1). The average density of O. reticulatus was 14.1 (SD = ±1.7, 7 censuses) individuals 100 m-2 and the average population size was 4,310 individuals. The average density of M. ventricosa was 21.3 (±6.0) individ- uals 100 m-2 within the area of the northern arm occupied by the population and 7.1 (±2.5) individuals 100 m-2 within the entire sand patch. The average popu- lation size of M. ventricosa was 2,166 individuals. Local densities of O. reticu- latus and M. ventricosa were inversely related in Horseshoe Patch (Fig. 2). Both species were rare to absent (0-5 individuals 100 m-2) in large expanses of heavily grazed substratum in the wake of large aggregations of either species. M. ven- tricosa also inhabited the grassbeds surrounding Horseshoe Patch, particularly around the northwestern border (Fig. 1). O. reticulatus was rare to absent in these grassbeds. An aggregation of Oreaster reticulatus moving west encountered a dense ag- gregation of Meoma ventricosa in the central region of the northern arm of Horse- shoe Patch on 10 September 1977. To monitor this interaction, a sampling grid (20 x 40 m) was delineated by graduated (meters) nylon lines and asteroids and echinoids were counted daily in 25-m2 blocks. The relatively stationary echinoid aggregation was gradually circumvented by asteroids moving along its southern border, as indicated by a southwesterly shift in asteroid density and by move- ments of marked individuals in the study grid (Fig. 3). By September 14, a dense aggregation of O. reticulatus had formed immediately south of the echinoid ag- gregation. O. reticulatus rarely occurred in the midst of the echinoid aggregation where densities up to 4.3 individuals m-2 were recorded in 25-m2 blocks. Oreaster reticulatus raked the top 1-2 mm of sediment into mounds that it 626 BULLETIN OF MARINE SCIENCE, VOL. 32. NO.2. 1982 20 en ,. ,. 'f' ,. :::::l 'f' 'f' 'f' 'f' ,. .-c( ...J :::::l () 15 .•..••. N .-W a: 'e 0 a: 0 w ~ en.- -(/) c( = 10 W :::J a: '0 0 > '0 LL l::: 0 .•...• >- 5 en-.- z w 0 0-9 10-19 20-29 30-39 40-49 50-150 DENSITY OF MEOMA VENTR/COSA (i n d iv id u a /s 1 0 0 m-2) Figure 2. Mean densities of Oreaster reticulatus at stations with varying densities of Meoma ven- tricosa. Horizontal lines at top of graph join sets of means which are not significantly heterogeneous (Mann-Whitney U test, P < 0.0]). enveloped with its eversible cardiac stomach (Scheibling, 1980c). The average volume of sediment in these feeding mounds was 39.6 cm3• The average frequency of foraging-feeding activity was 93.8% over 24 h and the average feeding rate was 1 1 20.3 feeding sessions individual- d- • Meoma ventricosa ingested sediment as it ploughed through the top 1-3 cm, forming furrows which averaged 12.7 cm in width (=average width of test, N = 36). Trails of sediment egested by echinoids were continuous, suggesting continuous feeding. The estimated rates of sand ingestion for O. reticulatus and M. ventricosa were 804 and 600 cm3 sediment individual-1 d-1 respectively, or 3.47 and 1.30 m3 sediment population-1 d-1 re- spectively. The estimated rates of bioturbation by O. reticulatus and M. ventri- cosa were 1.04 and 0.21 m2 individual-1 d-1 respectively, or 4,505.9 and 461.4 m2 population-1 d-1 respectively. The combined activity of both species would com- pletely disturb an area equal to that of Horseshoe Patch in 6.1 d. DISCUSSION Restriction of the population of Meoma ventricosa to the northern arm of Horseshoe Patch may be due to microhabitat differences despite the apparent NOTES 627 SEPT 11 SEPT 14 o , • • • 000 0: • .. I • • • o ,~ 000 0:, • • . , , • • • \@ 0 0 .0 ooG/··.' , • • @---0/C!) • o o/@I • • • • • SEPT 12 • ••••• • • • ••••••• • • • 2 • • DENSITY (Individuals 25 ni ) • • .2. reticulatus M. ventrlcOsB SEPT 13 • 1-3 0 5-15 (!) • 4-6 0 16-25 0 00 , • • 7-9 0 26-35 :0, 0 0 ¢ • • • • • , ,, 10-12 36-45 ~ 0 .. • • >45 0 9" • • • • >12 , , . @ -"@ CI • • • • • 0 Figure 3. Numbers of Oreaster reticu/atus (closed circles) and Meoma ventricosa (open circles) in 5-m-square grid blocks over a 4·d period. Dashed line delineates approximate boundary of aggregation of M. ventricosa. Arrows represent movements over 24 h (September 11-12) of individuals that were marked as described by Scheibling (I980b). Grid extended 10 m south (towards bottom of page) on 14 September. homogeneity of the sand patch with respect to sediment grain size, composition, and organic content (Scheibling, 1980b). Depth was greater in the northern arm (12-13 m) which consequently was less affected by infrequent periods of heavy swell than the southern arm (10-11 m). M. ventricosa typically occurred at the substratum surface at all times in Horseshoe Patch. Less than 10% (N = 440) of the population was partially buried (2-5 cm) in the sediment. In contrast, M. ventricosa has previously been found buried beneath as much as 10 cm of sedi- ment (Kier and Grant, 1965; Chesher, 1969). Chesher (1969) observed that soml~ populations of M. ventricosa emerged on the surface only at night, and suggested that this was due to low nighttime O2 concentrations in subsurface sediments. Feeding at the surface of Horseshoe Patch enabled M. ventricosa to exploit microalgal-detrital surface films with relatively little energy expenditure, but probably increased the echinoid's vulnerability to predation by gastropods (Schei, bling, 1981a). Both Oreaster reticulatus and Meoma ventricosa formed large, feeding aggre- gations which migrated across Horseshoe Patch. Aggregation of O. reticulatus was maximal during July through September, 1977, when asteroids formed high- density (2-7 individuals m-2) fronts (Scheibling, 1980a).