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 environments, 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. Other features of the adambulacrals and encrusting ossicles also provide protection. In certain primitive goniasterids, the furrow margin of the adambulacral is angular, with a prominent ridge protruding into the furrow (Fig. 20). The structure provides a guide for the tube foot, and to some extent, fills the space between subsequent tube feet, affording protection. Judging from specimens in preservatives, tube feet in these species typically cannot be entirely withdrawn into the furrow, above the actinal surface. In most valvatidans, the adradial or furrow surface of the adambulacral is broadly curved (Fig. 2E). Based on sequences in preserved specimens (Fig. IG) of the oreasterid A nthenea acuta (and related species), during contraction and closure of the furrow the spines along the furrow edge can first be rotated adradially over the partially retracted tube feet. As the furrow is pulled more closely together, the furrow spines are directed ventrally and the flattish surfaces of the furrow edge of opposite adambulacrals come together, thus closing the furrow. The furrow spines along with those of the adambulacral surface behind the furrow are also drawn tightly together thus increasing their protective value. The outer, actinal face of the adambulacrals of many valvatidans (e.g., Aster- inidae, Oreasteridae) are notched so that the adradial end of the face is recessed relative to the remainder of the surface (Fig. 2B, arrow). This arrangement allows 706 BULLETIN OF MARINE SCIENCE, VOL. 33, NO.3, 1983

Figure 1. A, Psi/aster cassiope, Paxillosida, approx, X2; B, Lophaster furcifer, Velatida, approx. XI; actinal views; furrows, oral areas are only partially protected by slender spines and orals; large tube feet remain exposed. C, Echinaster purpureus, Spinulosida, approx. X2, actinal view; furrows protected by clasped spines that are covered by dense dermal tissue. D, Celerina heffernani. Ophidiasteridae, Valvatida, approx. X2, actinal view; closely spaced body ossicles, granules imbedded in dermal tissue, clasped spines all protect furrow, oral area. E. F, J, Pentagonaster duebeni. Goniasteridae, Va]vatida; E, F, actinal, abactinal views, approx. X2; closely abutting ossicles, dense pavement of polygonal spines protects furrows, oral areas; granules along edges of abactinals form protective cover over BLAKE: CONTROLS ON SEA STAR DISTRIBUTION 707 the spines on this part of the surface (along the furrow edge) to be longer and thus more effective in covering the tube feet when directed adradially, yet keeps them from extending beyond the general level of the spine armature when directed toward the substrate. In the latter orientation, furrow margin spines combine with those of the remainder of the surface to form a protective surface of generally uniform elevation, or only slightly protruding spines nearer to the furrow. Furrow spines in many sea stars are circular in cross-section, but those ofthe valvatidans typically are polygonal, thus further enhancing a tight fit (Fig. 1E). In certain species, spines can be interlocked over the furrow-like clasped fingers (Fig. 1D). In many taxa, tough dermal webbing is present between the spines, at least near spine bases, thus improving the ability to cover the furrow (Fig. IG). Other protective features are present within the furrow. In taxa with relatively open furrows, the angle between the axis of the ambulacral ossicle and the plane of articulation between the ambulacral and adambulacral is relatively low, thus contributing to an open furrow arrangement (Fig. 2A). In the valvatidans, this angle typically is relatively high, yielding a deep, narrow furrow (Fig. 2B). As the furrow is pulled together in many valvatidans, the adambulacrals are inclined into the furrows, so that the plane of ambulacralladambulacral articulation is inclined downward as one moves abradially (Fig. 2B). This orientation serves to create a closed, semi-internal channel for the tube feet and radial canal. The modification is carried a step further in Anthenea acuta (Fig. 2C) and some related taxa. Here, the abactinal adradial corner ofthe adambulacral is truncated, so that the adambulacral is permanently inclined toward the furrow, but the plane of ambulacralladambulacral articulation remains horizontal. This arrangement does not permit as wide an opening of the furrow as in conventional orientations, given a constant ambulacral morphology; the ambulacrals in Anthenea acuta are of typical oreasterid form. The protective adaptations extend to the oral areas. The oral ossicles bear spines, and in many taxa they appear to be useful in guiding or forcing food and perhaps returning the stomach into the body cavity (Fig. I). These enlarged ossicles are not used for mastication, however, because their skeletal structure is the same as that of the surrounding skeleton (unlike the teeth of echinoid lanterns). Further, orals and their spines lack signs of abrasive wear as would be expected of chewing structures. The cuneate shape of the ten oral ossicles of many valvatidans (e.g., Anthenea acuta. Fig. II) provides a tightly-fitted shield over the oral area, pro- tecting the organs of the disc. The small opening at the center of the circle (Fig. 1I) is blocked in life by oral spines which, in many species, are polygonal and webbed, providing a tight closure when drawn together (Fig. 1G, I).

retracted papulae; J, actinal pedicellariae could provide protection from smaller predators, approx. xl O. G, H, A nthenea acUla. Pseudoreaster obtusangulatus. Oreastcridae, Valvatida, approx. X2, actinal views; body furrows protected by granules imbedded in firm dermal layer, spines, and closely abutting, stout body ossicles. J, Anthenea aCUla. Oreasteridae, Valvatida, approx. X2, internal (abactinal) view of orals; ossicles form a tight protective shield over internal organs; compare with figures G, H; K, Craspidaster hesperus. Paxillosida, approx. X1112, actinal view; ossicles arranged in a closely abutting, protective pattern convergent on arrangements seen in the Valvatida. L, Nectria ocellata. Ophidias- teridae, Valvatida, approx. XI12, abactinal view; abactinal surface is expanded, separating stout paxilliform abactinals and exposing papularium to currents; upon contraction, the abactinals form a stout protective surface. Following Vermeij's (1978, Table 1.1) biogeographic classification, P. cassiope is a Tropical Eastern Atlantic species found in "rather deep water" (Madsen, 1950); L.furciferis primarily Arctic; P. duebeni and N. ocellata are primarily cold-temperate Australian; the remainder are shallow Indo-West Pacific. 708 BULLETIN OF MARINE SCIENCE, VOL. 33, NO, 3. 1983

Figure 2. (Upper) Ambulacral orientation in (A), the predator Luidia (Paxillosida), and (B), a small- particle feeding ophidiasterid (Valvatida). Marked in dark lines, the angle between the plane of adam- bulacrallambulacral articulation and the ossicle axis is small in Luidia, yielding a broad, open furrow, but large in the ophidiasterid, yielding a narrow furrow and a semi-enclosed channel for the tube feet. In (B), the adambulacrals are shown notched (arrow) for the elongate furrow spines; the notch is best developed in other families, e,g., the Asterinidae, The water vascular system is shown in dashed lines. In Anthenea acWa (C) (Valvatida), the abaetinalladradial eorner of the adambulacral is truncated (arrow) so that the adambulacral is permanently rotated into the furrow, aiding furrow closure. Ability to close the furrow also can be enhanced by furrow margin shape; adambulacrals in the primitive (?) goniasterid Pseudarchaster (0) have an angular furrow margin that serves to guide the tube foot but prevents the sides of the furrow from being drawn together; adambulacrals of the oreasterid Nidorellia (E) are broadly rounded, and bear spines (not illustrated) so that the margins can be drawn closely together. a, adambulacrals; m, marginals, Figure 3. (Lower) The adambulacrals are closely spaced and therefore solidly arranged in the valvatidan Proloreaster (A) but more widely spaced and forming a much less solid structure in the paxillosidan Luidia (B). Lateral view, looking toward the furrow; adambulacrals (a) below, ambulacrals shown truncated above; distal right.

The other order with effective furrow cover is the Spinulosida. The relative lack of success measured in generic diversity of this order is striking; two genera, Echinaster and Hem'icia, are represented in the world's oceans by large numbers of species. To a great extent, Henricia depends on spines for protection; dermal BLAKE: CONTROLS ON SEA STAR DISTRIBUTION 709

tissue is not greatly thickened, nor is the skeleton particularly stout. This genus is generally lacking in shallow tropical environments. Dermal tissue is heavier in many Echinaster species (Fig. IC) and it replaces Henricia in tropical settings. In some of these species (e.g., Echinasler purpureus) spines along the furrow are covered by dense tissue, and the spines can be interlocked over the furrow in the finger-clasping orientation. The Indo-West Pacific genus Metrodira (transferred to the Echinasteridae; Blake, 1980) perhaps is the most stoutly armoured spi- nulosidan genus, and probably for this reason its affinities were not recognized for many years. Melrodira has a very prominent adradial process on the adambulacral. This process bears numerous short, stout spines that tightly interlock about the tube feet. In spite of the flesh and spines, perhaps the general lack of stout armour has left the spinulosidans more vulnerable and ultimately less diverse generically. In the extra-ambulacral body, a massive skeleton commonly imbedded in heavy dermal tissue can provide protection both from predation and the physical en- vironment. Yamaguchi (1975) noted that sea stars that live in fully exposed habitats in Guam tend to be heavily armoured (e.g., Culcita, Choriaster, Linckia) whereas the young and soft-bodied species are cryptic, perhaps reflecting "heavy predation pressure, presumably by fish." Town (1980), however, suggested armour provides physical protection. He noted specimens of the forcipulate Aslroslole scrabra from intertidal environments had thicker skeletons and were more frequently damaged than their offshore counterparts. He noted that offshore indi- viduals are less liable to damage from rolling boulders. Sea stars can use structures other than a thickened skeleton to provide protection from predation, including, for example, the long sharp spines and pedicellariae that are found on such sea stars as Acanthasler and many asteriids. Aldrich (1976) observed that ASleriasjorbesi readily surrendered an arm to the attacking decapod Libinia emarginata, the relatively weak skeleton providing a semi-protective au- totomy. Interpretation of the extra-ambulacral skeleton thus can be difficult because a thickened skeleton can have more than one function, and there is more than one mechanism that can be used to provide protection. Whether to protect from biological attack or physical damage (or perhaps both), many warm-water valvatidans possess a stout skeletal frame constructed of thick, sturdy ossicles. In the Oreasteridae and Ophidiasteridae, two of the most important warm-water families, the body surface typically is covered by a heavy dermal tissue that usually contains tightly packed granules (Fig. ID, G, H), and, in many oreasterids, stout immobile spines mounted on the primary ossicles. Protection for soft tissues might be most complete in certain goniasterids. Pen- tagonaster duebeni (Fig. IE, F) for example has the furrow armature typical of the valvatidans. The remainder of the body is plated by large ossicles that provide a closely-fitted pavement. The distal marginals commonly are enlarged, providing a heavily armoured arm tip. Pedicellariae (Fig. 11) are numerous and widely distributed on both the upper and lower surfaces. They are generally two, but can be three-valved, small (valve length approximately 0.2 mm) and equipped with denticles at their tips. Pedicellariae are mounted in a pit in the primary ossicle that accommodates the full length of the opened valves. When fully spread, the valves thus lie below the general body surface, away from water flow and contact with foreign objects. The small size of the pedicellariae suggests use on small objects, but they are not spaced closely enough to provide a ready mechanism to pass food. The pedicellariae therefore appear to be defensive structures most effective against smaller organisms. 710 BULLETIN OF MARINE SCIENCE, VOL. 33, NO.3, 1983

Perhaps most striking, however, is the protection provided the papulae (Fig. IF). These structures occur in narrow grooves between larger primary abactinals. Much ofthe time during life, the abactinal surface apparently is expanded upward (A. M. Clark, personal communication) in this and many other goniasterids. Arching the surface will open the grooves between the abactinals and aid exposure of the papulae. Contraction of the abactinal surface, however, will not only close the papulae-bearing grooves, but actually cover them with small granules that occur in single rows on the edges of the primary abactinals. The shallow water Indo- West Pacific goniasterid Iconaster longimanus has a similar arrangement of papulae and protective ossicles. Neetria oeellata. another goniasterid, has very large paxilliform abactinals covered by closely spaced granules (Fig. IL). When the abactinal surface is expanded, large papularia are exposed, but when con- tracted, the expanded crowns of the paxillae form a stout pavement over the soft tissues. By entirely covering the papulae, covering plates seem to provide good protection from nibbling, whereas simple retraction to the body surface should adequately protect these structures from most potential physical damage. None of the adaptations discussed seem so complex as to be unlikely to develop in more than one lineage. For example, if food particle size is small enough, or the habitat does not demand the clinging ability of numerous large tube feet, then convergence could be likely. Certain deep-water porcellanasterids protect oral areas largely with soft tissues, whereas some astropectinids employ spines and some broadening of the oral frame. A striking example of protective armour in the astropectinids is provided by Craspidaster hesperus (Fig. 1K), the only shallow water astropectinid other than Astropecten, itself recorded from the Indo-West Pacific by Clark and Rowe (1971). Superficially, Craspidaster is one of the most goniasterid-like of the astropectinids, with relatively stout, closely-fitted ossicles. Spinelets around the edges of the extra-ambulacral ossicles bend outward to interlock with those of their neighbors to form covers over the channels between ossicles. Furrow spines extend over the tube feet, and a dense field of spines fits tightly about the orals. The species clearly is an astropectinid, but one quite strongly convergent on goniasterid morphologies. Similarly, Archaster is a shallow burrowing valvatidan that occurs in many shallow water sandy Indo-West Pacific environments such as are frequently occupied by Luidia and Astropeeten. Ar- chaster seems to have succeeded by combining the protective features of the Valvatida with the overall shape, marginal form and behavior of Astropecten. Important apparent exceptions to these arguments on the need for protection in shallow tropical waters are the occurrences of Astropecten and Luidia. Both genera are members of the Paxillosida (Blake, 1982) and they are represented by many species in shallow warm seas (Clark and Rowe, 1971). Both genera have broad, open ambulacral furrows and they are predators on active solitary forms. What are the reasons for their success in a life mode generally not occupied by tropical sea stars? According to Vermeij (1978), coevolution of predators and prey has led to the development of antipredatory structures in invertebrates, but different habitats within a region provide unequal stimuli for the evolution of these interactions. Predation-related architectural responses should be relatively clear on rocky bottoms, where competition for space is intense, but less obvious on soft bottoms where more space is available. On softer substrates, large, active species can avoid interactions to some extent by temporarily burying themselves in the sediment. Both Luidia and Astropecten live on and in unconsolidated sediment and both are active, hunting during certain periods, then retreating below the sediment BLAKE: CONTROLS ON SEA STAR DISTRIBUTION 711

interface to digest prey. While active, they should be less readily caught by the smaller predators likely to attack the furrows, and while buried during rest and digestion periods they are protected by a cover of sediment. Water flow over the body surface appears important to many sea stars (Gislen, 1924) including Luidia and ASlropeclen, although the functions of the currents have not been fully elu- cidated. Ossicles in these genera are aligned in distinct rows that are separated by deep, linear channels. While the animals are buried, the channels, with their many spinelets, permit maintenance of water flow without sediment interference. The evolutionary potential of sea stars seems limited in certain important ways. Throughout their history, sea stars have been very conservative in structural arrangement. Among modern representatives, major deviations from typical con- struction might include only the fusion of the oral ring in brisingids, development of the nidamental chamber in pterasterids, and perhaps development ofa spherical form in the sphaerasterids. None of these adaptations is significant in shallow tropical taxa. Among fossil sea stars, the only known major development after the Ordovician appearance of the class (and an important one) was a change in ambulacralladambulacral structure that took place at the end of the Paleozoic. None of these developments appears to require as much body reorientation as the regular/irregular transformation in echinoids, or the variation among benthic and pelagic holothuroids. Sea stars use a broad range of food materials, and different categories of feeding (e.g., scavenging, suspension feeding, predation) are to be found in single species; in these ways, sea stars are a highly versatile and successful group. As noted above, however, asteroid predators on active solitary prey, with their broad and exposed ambulacral channels, are minor in warm shallow seas. Further, it has been my impression, and that of others (A. M. Clark, A. R. G. Price, personal communication) that sea stars tend to be relatively uncommon compared to echinoids and holothuroids in many tropical marine environments. Sea stars thus have been of limited constructional flexibility, and they show a latitudinal decline in feeding diversity and perhaps abundance. These patterns suggest the presence of strong phylogenetic constraints that limit the adaptive potential of the class. In summary, distribution of families and orders of sea stars is not uniform in the shallower parts of the world's oceans, rather there is an increase in the relative significance of the heavily-armoured taxa toward lower latitudes. In discussions of molluscs and certain other invertebrates, Vermeij (1978) argued that coevolution of heavy armour in prey species with well-armed predators is important under physically less stressful conditions, whereas under more stressful conditions, physiological interactions restrict biological interactions. Distribution of armour in sea stars suggests these organisms might be responding in a manner similar to that Vermeij reported in other invertebrates. Very few tropical shallow water sea stars are predators on active solitary invertebrates (e.g., molluscs, other echinoderms) although such habits are common in cooler waters. Sea star predators consist almost entirely of species with relatively open ambulacral furrows and oral areas, rather than of those with heavy armour. The apparent need for armour in the tropics may have precluded sea stars from predatory habits in these areas. In spite of a long geologic history, sea stars have been very conservative in basic structure. This conservative structure and an inability to maintain predatory habits in the tropics suggest the presence of strong and enduring phylogenetic constraints in the adaptability of sea stars. 712 BULLETINOFMARINESCIENCE,VOL.33,NO.3, 1983

ACKNOWLEDGMENTS

The writer is grateful to A. M. Clark, D. R. Kolata and A. R. G. Price for manuscript review and useful discussions. Research at the U.S. National Museum of Natural History and the British Museum was supported by a grant from the National Science Foundation, Dr. H. and R. Lee of Lee Phar- maceuticals provided partial funding for travel expenses.

LITERATURE CiTED

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DATEACCEPTED: November 29, 1982.

ADDRESS: Department of Geology. University of Illinois. 245 Natural History Bldg., 1301 W. Green St., Urbana. Illinois 61801-2999.