BULLETIN OF MARINE SCIENCE. 32(3): 736-744, 1982 CORAL REEF PAPER AN ASSOCIATION OF A POLYCHAETE, BRANCHIOSYLLIS EXILIS WITH AN OPHIUROID, OPHIOCOMA ECHINATA, IN PANAMA Gordon Hendler and David L. Meyer ABSTRACT On the Caribbean coast of Panama Branchiosyl/is exi/is (Gravier) is associated with Ophio- coma echinata (Lamarck). They have not been found in symbiosis in other localities although both species are widespread. B. exifis was not collected on the ophiuroid species sympatric with O. echinata, but we speculate that it may associate with other hosts elsewhere in its range. Larger ophiuroids are more often infested than smaller specimens. In the majority of infestations a single polychaete is found near the mouth or on the arm on the ventral surface of the ophiuroid. The consistent occurrence of one polychaete per host may be an indication of aggression between individuals of B. exi/is. The polychaete freely enters and leaves the mouth of O. echinata and may steal food captured by the ophiuroid. The incidence of infestation varies at different collecting sites and seasonally at a single locality. It is possible that fluctuations in the incidence of infestation are related to seasonal stress and mortality of B. exilis, or are effected through changes in the density of the host population. Some of the large and relatively sedentary species of the Ophiocomidae act as hosts for associated invertebrates. For example, Indo-Pacific species of Ophio- coma have as associates a tric1ad, pycnogonid, crab, and several species of poly- chaetes (Sloan et aI., 1979). Ophiocoma echinata from Cura<rao harbors an ex- ternal copepod associate (Stock et aI., 1963). Hyman (1955) supposed that because of their agility and relatively small size ophiuroids might be less likely than other echinoderms to have polychaete associates. However, there have been several studies describing biological interactions between the two taxa (Ward, ]933; Davenport, 1953; Devaney, 1967; Gibbs, 1969). Our observations of an association between Branchiosyllis exilis and Ophio- coma echinata derive from a long-term, quantitative study and thus add a new dimension to the previously anecdotal accounts of polychaete-ophiuroid relation- ships. Since O. echinata is one of the most common Caribbean shallow-water macroinvertebrates this association might offer a useful tool for future investi- gations of marine symbiosis. METHODS This report is based on surveys carried out on the mainland of Panama and in the San BIas Ar- chipelago (Panama) from 1973 to 1980. Data on the freql,lency of infestation were gathered while collecting ophiuroids for a study of reproductive cycles (1972-1974), and occasional surveys thereafter provided additional information. Samples were collected by overturning coral rubble and coral slabs in shallow areas (1-3 m deep) of the sublittoral and the reef flat. This meant that only adult O. echinata were examined, since the smallest O. echinata occur in less accessible, cryptic habitats from the shallow reef flat to the deeper fore-reef zones, often in the interstices of colonies of calcareous algae and branching corals (unpub. obs.), Much of the research reported below was carried out at the Galeta Marine Laboratory (Smithsonian Tropical Research Institute) and additional observations on the incidence of the Ophiocoma-Branchiosy/lis association were made by the authors and their col- leagues at other sites in Panama, the Caribbean, and the Eastern Pacific. 736 HENDLER AND MEYER: POLYCHAETE·OPHIUROID ASSOCIATION 737 15 w ...~ 10 - NOT INFESTED • ""w u a: w C>. 1D 13 16 19 21 25 28 SIZE [mm DISC DIAMETER] Figure 1. (Left) Branchiasyllis exi/is. about 2 cm long, on the ventral surface of the arm of Ophia- coma echinata. Galeta Reef Flat, Caribbean, Panama. Figure 2. (Right) The size-frequency distribution of Ophiacama echinafa infested with Branchia- sy//is exi/is, superimposed on the distribution of uninfested O. echinata. Data from all samples of measured O. echinata inspected for B. exi/is in Panama are combined. N = 915. RESULTS The syllid polychaetes identified as Branchiosyllis exi/is found on Ophiocoma echinata are usually about 1.5 to 2 cm long, with overall black pigmentation (Fig. 1). The segments of the polychaete each have a thin white border such that on close examination the polychaete displays light striations. This pattern of pig- mentation makes it difficult to distinguish specimens of B. exi/is against the black, brown, and white body of O. echinata. The effectiveness of the polychaete's camouflage is heightened by its usual location on the black and white banded arms of the ophiuroid between colonnades of black and white striped spines and rapidly moving white tube feet (Fig. 1). Adult B. exi/is were found exclusively on O. echinata although 10 to 15 species of large ophiuroids were routinely collected in habitats occupied by the infested O. echinata populations. In fact, two of the common sympatric ophiuroids were congeners of O. echinata, namely O. wendti Muller and Troschel similar in size and color to O. echinata, and the smaller green-brown species, O. pumila Liitken. Surveys at other localities to determine the incidence of Branchiosyllis infestation showed that 118 O. wendti and 10 O. pumila collected were free of B. exilis while 39.1% of 838 O. echinata examined carried B. exi/is (Table 1). Over 100 additional specimens of O. wendti and O. pumila were inspected for another study and B. exi/is was never seen on either species. Additionally, hundreds of Ophiocoma aethiops Lutken and Ophiocoma alexandri Lyman were collected on the Pacific coast of Panama (at the study sites described in Hendler, 1979). Polychaetes were not found on those species, although B. exilis was re- ported from the region (Pauchald, 1977). B. exilis was not collected on O. echinata from other sites in the Caribbean. Samples of O. echinata from Cura~ao, Nassau, and Virgin Gorda (small because the species was uncommon at the sampling localities) showed no infestation by 738 BULLETIN OF MARINE SCIENCE. VOL. 32, NO.3, 1982 Table I. The numbers of three sympatric Ophiocoma species infested (with) or not infested (without) by the polychaete, Branchiosyllis exilis, at different collecting sites along the Caribbean coast of Panama O. echinata O. wendt; O. pumila Locality (Panama) Date Without With Without With Without With Galeta 24 X 1977 175 106 37 0 4 0 Galeta 21 IX 1978 170 93 45 0 5 0 Galeta 5 X 1980 27 23 5 0 I 0 Punta Guapa 29 V 1976 18 13 5 0 Largo Remo I IX 1974 69 78 II 0 Largo Remo 23 VIII 1977 36 2 5 0 Maria Chiquita 3 IV 1976 14 II 4 0 Achutupo, San BIas 25 VII 1977 I 1 4 0 Korbiski, San Bias 25 IX 1980 0 I 2 0 B. exi/is, More intensive surveys in Florida and along the Belize Barrier Reef (conducted from the U.S. National Museum Carrie Bow Cay Laboratory) and personal communications with Ms. Elizabeth Sides and Mr. Richard Bray who have carefully studied large numbers of O. echinata and other ophiuroids from Jamaica and Barbados indicate that B. exilis is not found on O. echinata on those islands (Table 2). B. exilis occurs on the arms or disc of its host. Unless disturbed, the polychaete moves along the ventral arm plates (that pave the oral surface of the ophiuroid arm) between alternating rows of arm spines and tube feet (Fig. 1). The poly- chaetes are also found partially or wholly within the oral cavity of the ophiuroid and they presumably enter the host's stomach. On occasions when the location of the polychaete could be found immediately upon collecting an ophiuroid, B. exilis occurred more frequently on the arms of the ophiuroid than on the disc (Table 3). The polychaetes generally occupied the thick, proximal portion of the ophiuroid arm, on the ventral (oral) surface. Polychaetes on the disc of the ophiu- roid were usually on the radial area at the base of the arm and the oral frame, rather than on the interradial or dorsal (aboral) areas of the disc. About 22% of the polychaetes surveyed were observed in the oral area, usually with a portion of their body hidden within the mouth of the ophiuroid (Table 3). On Ophiocoma, B. exi/is was difficult to secure with forceps but could be removed by vigorously shaking the ophiuroid in seawater. When disturbed (prod- ded with forceps) the polychaetes retreated between the arm spines, moved across the dorsal (aboral) surface of the ophiuroid arm, or moved onto the disc and possibly inside one of the host's bursae. Interestingly, when pursued the polychaetes did not take refuge in the oral cavity of the host even though they were capable of moving in and out of the ophiuroid's mouth. To find the relationship between the size of the host and the incidence of infestation, a series of O. echinata were inspected for B. exilis and measured. The superimposed size-frequency plots of infested and non-infested ophiuroids (Fig. 2) show a relatively greater incidence of infestation in the larger ophiuroid size classes. As expected, a regression of infestation (expressed as the percentage of infestation for each size class in Fig. 2) related to host size (Fig. 3) is significant (P < 0.05). The coefficient of determination (r2) indicates that 61% of the varia- tion in the percentage of infestation is a function of host size. Most infested ophiuroids carried but a single individual of B. exi/is. In a sample of 552 O. echinata, 8% of the ophiuroids hosted 2 polychaetes and only 3 spec- HENDLER AND MEYER: POLYCHAETE-OPHIUROID ASSOCIATION 739 Table 2. Incidence of infestation with Branchiosyllis exilis and relative abundance of Ophiocoma echinata (expressed as a percentage of the number of all species of ophiuroids collected) at all sites sampled for B.