A Coral-Eating Barnacle

Many rock barnacles form close associations with other organisms, yet none are known to have become wholly parasitic. In a study of balanids inhabiting corals, we encountered a species-Pyrgoma monticulariae Gray, 1831-that depends on the coral for both habitat and food. In achieving this relationship it has gained control over certain metabolic activities of the coral, including calcification, proliferation of coenenchyme, and nematocyst discharge. While balanids became associated with corals 25 million years ago, evidence suggests that this wholly parasitic relationship has developed within the last 10 million years. CERTAIN ROCK BARNACLES live in association with corals (Hiro, 1938; Nilsson-Cantell, 1938). Whereas some species gradually become engulfed and eventually overgrown, others are specifically modified for this habitat by having adapted morphologically and physiologically to the growth pattern of the coral. A critical problem in the association is maintenance by the barnacle of free communication with the surrounding water in order to carryon normal physiological processes-gathering food, mating, releasing larvae, and voiding wastes. In generalized or primitive coral barnacles, an opening to the exterior is maintained by cirri, which mechanically clean away obstructions. But, as in all rock barnacles, six pairs of bristlebearing cirri are extended to form a net employed in setose filter feeding. Such forms can be considered semiparasitic in that they depend on the coral for habitat but not for food. In a revisionary study of the coral-inhabiting balanomorph barnacles Pyrgoma and Creusia, we encountered specimens of the little known Indo-Pacific species, Pyrgoma monticulariae Gray (1831). The shell of this species has been described in detail (Hiro, 1935, 1938). Its distinguishing characteristics are an extraordinarily irregular outline and, in proportion to the size of the shell, an extremely small aperture (Fig. lA). In the specimens available from Mauritius 1 San Diego Natural History Museum, Balboa Park, San Diego, California 92112. 2 Scripps Institution of Oceanography, University of California at San Diego, La Jolla, California 92037. Manuscript received July 16, 1968. in the Indian Ocean, a layer of tissue was found growing over the entire exterior of the shell, apparently sealing the barnacle off from the external milieu. Squashed preparations of this tissue revealed numerous nematocysts, indicating that the tissue is an extension of the coral coenenchyme. Dissection revealed P. monticulariae has but one pair of biramous cirri, and these are unusually formed (cf. Figs. 2C and F). The slender rami are clearly no longer adapted for filter feeding, but rather appear to be antenniform sensory structures. The remaining five pairs of cirri are recognizable as stubs or vestigial limbs supporting few or no setae. The antenniform pair of cirri is interpreted as being the fourth, not only because of its sequential occurrence, but because in balanids the first three pairs are ordinarily closely associated with the mouth, and hence would not be likely candidates for sensory structures. Of the three posterior pairs of cirri, two are vestigial, but they are posterior to the origin of the intromittant organ, which has migrated to a more anterior position. So organized, the mouth parts, the single pair of cirri, and the intromittant organ are concentrated in the vicinity of the minute aperture (cf. Figs. IE and D). The mouth field, as compared with other species in this group, is also unusual; it is produced into a remarkably long, slender cone. The mandibles have relatively simple, straight, sawlike cutting edges instead of a few strong teeth, and produced molariform lower angles (cf. Figs. 2A and D). The first maxillae are