Asteroidea; Echinodermata)

BULLETIN OF MARINE SCIENCE. 33(3): 703-712.1983 SOME BIOLOGICAL CONTROLS ON THE DISTRIBUTION OF SHALLOW WATER SEA STARS (ASTEROIDEA; ECHINODERMATA) Daniel B. Blake ABSTRACT Tropical shallow water sea star faunas, especially those of the Indo-West Pacific, are dom- inated by the order Valvatida. Among sea stars, valvatidans have the best-developed antipredatory devices. Vermeij (1978) found high to low latitudinal increases in antipredatory structures in various invertebrate groups (e.g., gastropods). The valvatidan occurrences suggest the presence of controls in sea stars similar to those affecting other groups. The Valvatida includes few genera that prey on active, solitary invertebrates, but such habits are common in other orders, and in cooler waters. Protective structures appear to restrict predatory abilities. The importance of sea stars as predators on solitary organisms declines in tropical latitudes, yet sea stars have evolved only limited basic structural variation since their appearance in the Ordovician. Phylogenetic constraints in adaptability appear strong in sea stars because of their evolutionary failure to maintain predatory life habits in shallow tropical waters. Families of sea stars are not uniformly distributed in the world's oceans but change in abundance relative to one another both with latitude and depth (Hyman, 1955). Considering shallow water genera only, this relationship is striking even at the ordinal level (Table 1). In tropical regions, the Valvatida provides a dis- proportionately large percentage of the genera compared to its representation in both the total fauna, and the relatively well-known North Pacific fauna. In the Indo- West Pacific, 77% of the species are valvatidans (Clark and Rowe, 1971). In Micronesia alone, 83% of the genera and 86% of the species reported by Yamaguchi (1975) are valvatidans. Two non-valvatidan genera, Luidia and As- tropecten, account for most non-valvatidan Indo-West Pacific species. If species counts for these two are dismissed, the Valvatida provides nearly 95% of the Indo- West Pacific shallow water sea star species reported by Clark and Rowe (1971). These relationships call for explanation. Vermeij (1978) described a complex suite of relationships between invertebrate morphology and biogeography in his survey of world-wide organism distributions. In general, this author found that as environmental conditions become less lim- iting to life processes (i.e., in a gradient of decreasing physiological stress) toward the tropics, the coevolution of well-armed predators and heavily armoured prey becomes relatively more important and diversity increases. In stressful environ- ments, biological interactions are limited in variety and intensity by physiological restrictions. Among tropical areas, coevolutionary structures seem best developed in the Indo-West Pacific, a region not only oflesser stress, but one that is a large area of relative climatic and geologic stability. The heavily armoured valvatidans appear to reflect these relationships because they become more important relative to other sea stars toward shallow tropical waters. Vermeij's (1978) discussion stressed gradients in distribution: Table I and the specimens illustrated in Figure 1 show gradients are also to be found among the sea stars. For ease of consideration, the sea star body can be divided into two regions: (1) the oral area and ambulacral furrow, and (2) the extra-ambulacral areas. Of the two, the furrow region is more readily considered because it is here that potentially conflicting demands of different protective strategies and the need for 703 704 BULLETIN OF MARINE SCIENCE, VOL. 33, NO.3. 1983 Table I. Numbers of sea star genera in selected shallow waters. For each order, the reported number of genera is provided first followed by the percentage of the total for that region. Where possible, selected depths follow the 20-m limit of Clark and Rowe (1971); a second 200-m limit is included where available. In a series of papers (Blake, 1979; 1980; 1981), I have argued for the transfer of six families from the Spinulosida (sensu Spencer and Wright, 1966) to the Valvatida; these changes are incorporated in the table. In addition, the Echinasteridae (Spinulosida) is separated from the Solasteri- dae and allied families (Velatida). A small number of changes in generic assignment are incorporated as well NOIO- Spinulo- Paxillosida myota Valvatida sida Velatida Forcipulatida Total Total· 34; 12% 8; 3% 134; 46% 23; 3% 9; gOAl 82; 28% 290 Indo-West Pacifict 20 m 3; 5% 0 52; 88% 2; 3% I; 2% 1;2% 59 Tropical Eastern Atlantid 20 m 2; 18% 0 6; 55% 1;9% 0 2; 18% II 200 m 3; 23% 0 7; 54% I; 8% 0 2; 15% 13 Tropical Western Atlantic§ 20 m 2; 29% 0 3; 43% I; 14% 0 I; 14% 7 200 m 2; 17% 0 8; 67% 1; 8% 0 1;8% 12 North Pacificll 20 m 3; 14% 0 5; 23% I; 5% 2; 9% II; 50% 22 200 m 5; 17% 0 6; 20% 2; 7% 3; 10% 14; 47% 30 South Africa# Cold-temp. Southern Af. 100 m 2; 14% 0 7; 50% I; 7% I; 7% 3; 21% 14 Warm-temp. South Af. 100 m 2; 13% 0 9; 56% I; 6% 2; 13% 2; 13% 16 Sources; • Spencer and Wright, 1966; t Clark and Rowe, 1971; ~ Madsen, 1950; § Downey. 1973; 11 Fisher, 1930; includes Cold Temperate North Pacific and adjacent Arctic: Warm Temperate Northwest and Northeast Pacific; the cool and warm areas dilfcr relatively little in percentages; # Clark and Courtman-SlOck, 1976. protection from both physical damage and biological attack seem more readily evaluated. Taxa mentioned in the text are of the following ordinal affinities: (1) Paxillosida: Astropecten, Craspidaster, Luidia; Porcellanasteridae (2) Valvatida: Acanthaster, Anthenea, Archaster, Choriaster, Culcita, Jconaster, Linckia. Nectria, Greaster. Pentagonaster; Asterinidae, Goniasteridae, Ophidiasteridae, Oreasteridae (3) Spi- nulosida: Echinaster, Henricia, Metrodira; Echinasteridae (4) Forcipulatida: As- terias, Astrostole; Asteriidae. In sea stars, the radial water canal and tube feet lie below the arched ambulacral ossicles in the more or less open ambulacral furrow, a position exposed to potential attack (Fig, 2A-B). The organs of the disc are potentially vulnerable as well because the open ambulacral furrows converge at the mouth region (Fig. IA, B). Published research has emphasized sea stars as predators, whereas papers evaluating them as prey generally consider occurrences in which the entire animal is taken. Never- theless, sea stars can be victims of smaller predators that attack exposed soft furrow tissues. Viviani (1978) found a fish, Doidixodon laevifrons, will nibble on the podia of sea stars attacking them even as they attempt to retreat. Russell (1966) found the tube feet and the arms (as well as certain internal organs) of Anthenea sp. to be "palatable" to four fish genera. Wickler and Seibt (1970), and Bruce (1971), both described predation on various sea star genera by the decapod crustacean Hymenoeera pieta. Predation method involves some attack on the tube feet and penetration of the skeletal armour. Glynn (1976) has shown that the shrimp Alpheus lottini and the crab Trapezia.ferruginea successfully defend their commensal coral Pocil!opora by snapping at the tube feet of Acanthaster. Among the six sea star orders, two, the Valvatida and Spinulosida, have ex- tensive protective structures for the furrow and oral areas. Protective structures are of three types: (I) stout ossicIes ofthe primary skeletal system (adambulacrals and orals), (2) encrusting ossicles (Fig. IC-H), and (3) thickened dermal tissue (Fig. iC-H). All three structure types are common in the Valvatida, but perhaps BLAKE: CONTROLS ON SEA STAR DISTRIBUTION 705 a stout primary skeleton is most important, whereas spinulosidans emphasize spines and dermal tissue, although sturdy orals can effectively cover the mouth region (Fig. 1C). The orders are varied in dominant dietary preferences (Sloan, 1980) but members generally are not predators on active, solitary organisms. Relatively well-armoured ambulacral furrows should protect a sea star from smaller predators, but furrow armour seemingly provides little help against physical damage because the substrate, against which the furrows are appressed, provides direct protection. In contrast, a larger, more open furrow might indirectly protect against physical damage because the greater available furrow area allows more space for tube feet for clinging to the substrate. Surge, or moving boulders strong enough to break the hold of the sea star would be likely to cause serious damage to the body beyond the ambulacra. Feeding on active, solitary prey (e.g., molluscs, other echinoderms) is most important in three other orders, the Forcipulatida (Asteriidae), Yelatida (Sola- steridae) and Paxillosida (Astropecten and Luidia). Representatives of all these taxa typically have broader, more open ambulacral furrows (Figs. lA, B; 2A). The more open the furrow, the larger and more numerous the food-manipulating tube feet can be; the more open the oral area and the more loosely articulated the abactinal skeleton the more effective the sea star can be with larger, more active prey (Blake, 1982, PI. 22). Tube feet are of course used in feeding by the small particle feeders as well, for example, Scheibling (1980). Specific furrow protective devices present in the Yalvatida and Spinulosida are readily cited. In most orders, the adambulacrals are curved ossicles, strongly overlapping distally and linked by more or less elongate muscle pads (Fig. 3A). The system would appear to provide as much flexibility and speed of response as is possible in an organization such as that of sea stars. In most valvatidans, however, the adambulacrals are stout with the side faces quite closely abutting (Fig. 3B), a system seemingly providing greater body strength. (Observation of living valvatidan specimens, including ophidiasterids, Oreaster and even the cush- ion star Cu/cita does reveal a much greater body flexibility than might be guessed from the inspection of dried museum material.) In the oreasterids, the adambulacrals are relatively thickened in the actinal/abactinal (i.e., lower/upper) direction, providing a strong, thickened skeleton.

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