BULLETIN OF MARINE SCIENCE, 47(3): 680-695, 1990 INTERTIDAL BIOEROSION BY THE CHITON ACANTHOPLEURA GRANULATA; SAN SALVADOR, BAHAMAS Kenneth A. Rasmussen and Eben W Frankenberg ABSTRACT Rates of daily fecal pellet production and intertidal bioerosion by the chiton, Acanthop/eura granu/ata Gmelin, 1791 were measured along the shore of Pigeon Creek, an interior marine tidal creek on San Salvador Island. Chitons grazed upon pitted, biokarstic Pleistocene lime- stones which form outcrops within the intertidal zone. The low-energy, tide-dominated char- acter of the waterway fosters excellent preservation of delicate chiton fecal pellets, and allowed a more accurate, in situ measurement of bioerosion by directly counting their daily accu- mulation. Chitons were abundant (5.5 chitons· m-2), with adults and juveniles present in a 2.8: I ratio along the exposed rocky shoreline. An average daily production rate of67.2 pellets' chiton-I was calculated from 43 individuals monitored for 4 days. Pellet counts were highly variable among days and among individuals. Organically bound, 3-4 mm long fecal pellets were 94.3% CaCO) by weight, and composed of a variety of constituent grain types and textures. Individual pellets from adults and juveniles contained about 2.1 and 0.7 mg CaCO), respec- tively. Assuming that the mass of carbonate deposited as fecal pellets is equal to that eroded from the coast, an annual carbonate erosion rate of 41.5 g·yr-'·chiton-' results. Combined with the 1.82 g·cm-) average density of the pelletal grainstone/packstone which they graze, a volumetric erosion rate of 22.8 cm)'yr-"chiton-' results. Using local population density, an overall bioplanation rate of 0.12 mm ·yr-I exists across the rocky intertidal zone due to the activities of A. granu/ata alone. Intertidal exposure hardens otherwise easily disaggregated chiton pellets, and may enhance their geologic preservation potential. If recognized in the fossil record, preserved chiton pellets may indicate proximity to low-energy, tide-dominated rocky shores, along which this process is most common. Chitons (Mollusca, Polyplacophora) are common grazers of intertidal epilithic and endolithic algae along rocky carbonate coastlines throughout the world (Neu- mann, 1968; Glynn, 1970; 1973; Schneider and Torunski, 1983). Using a mag- netite-enriched radula (Lowenstam, 1962), they rasp limestone surfaces during their nightly grazing activities, and in doing so erode their substrate. Despite a wealth ofinformation on bioerosion in a variety of settings (Otter, 1937; Ginsburg, 1953; Neumann, 1968; Warme, 1975; Hutchings, 1986 for reviews), the role of chitons in the process of coastal limestone bioerosion remains uncertain. The ongoing nature of epilithic grazing suggests that chitons may contribute substan- tially to long-term coastal retreat, perhaps approaching the importance of endo lith- ic biotas such as sponges or bivalves (Neumann, 1966; Riitzler, 1975; Warme, 1975), which presumably bore more episodically for shelter. Chitons may have modified marine hard substrata to some degree since their first appearance in the Late Cambrian (Smith, 1960). Earliest trace fossil evidence for bioerosion by chi tons or gastropods exists in distinctive grazing scars (Rad- ulichnus) etched upon Upper Jurassic bivalves (Voigt, 1977). The erosive potential of modem chitons in combination with other intertidal biotas has gained wide- spread recognition, though the actual amount of material which they alone remove from rocky shorelines remains poorly quantified (Trudgill, 1983; Donn and Board- man, 1988). This is primarily because chitons often live along high-energy coasts, where it is difficult to monitor grazing and defecation activities in situ. Moreover, high-energy shorelines are often eroded by wave-borne projectiles through me- chanical "bombardment" (McLean, 1964). Such factors can severely influence 680 RASMUSSEN AND FRANKENBERG: INTERTIDAL BIOEROSION BY A. GRANULATA 681 intertidal community structure, as well as complicate the calculation ofa particular organism's contribution to substrate loss (Shanks and Wright, 1986). Typical rates of carbonate shoreline retreat (planation) range from 0.1-4.0 mm· yel (Hodgkin, 1964, and Trudgill, 1983, in Australia; Schneider and Torunski, 1983, in the northern Adriatic; Taylor and Way, 1976, and Trudgill, 1976, on Aldabra Atoll; Donn and Boardman, 1988, in the Bahamas). As discussed by Neumann (1968) and Spencer (1985), worldwide rates for overall coastal planation I appear to cluster around 1 mm·yr- • With reference to the chiton component, l Taylor and Way (1976) maintained that 3.4% (0.017 mm ·ye ) of the coastal planation at Aldabra Atoll is due to grazing by the small (-3.6 cm long) species Acanthopleura brevispinosa. Indirect data of Trudgill (1983) suggest that popu- lations of the larger species A. gemmata (5.7-6.9 cm long) account for up to 34% l (0.7 mm ·ye ) of the extensive planation measured at One Tree Island, Australia. Donn and Boardman (1988) measured 2.2 mm ·yr-I overall planation at the l windward coast of Andros Island, of which they estimated 9.6% (0.21 mm·ye ) was due to the common Caribbean chiton A. granulata. Their estimate was based on the unpublished laboratory grazing experiments of McLean (1964) in Barbados, who estimated that a mid-sized (-4.0 cm long) A. granulata individual can erode limestone at a rate of 21.9 g·yr-I. Glynn (1973) has cited his own unpublished data from Puerto Rico, which indicated a comparable bioplanation rate for A. granulata populations of 0.18 mm·yr-I. He did not, however, state the method by which this value was determined. All the estimates of chiton bioplanation cited above are highly dependent upon local rock density and chiton population density (both of which vary greatly), and commonly suffer from the need to extrapolate laboratory results to the field. For these reasons, the most valuable unit quantifying chiton bioerosion may be mass of CaC03 eroded per individual per unit time, measured from a large population under natural field conditions. Table 1 summarizes the results of past. attempts to determine a per capita bioerosion rate for chitons. The only direct field measurements listed are those of Taylor and Way (1976) for A. brevispinosa. They calculated an erosion rate of I only 3.3 g·yr- .chiton-I on the basis of three individuals monitored for 3 days. The remaining studies relied upon few chitons, removed from their natural habitat, and permitted to graze on small pieces of substrate in aquaria. Laboratory mea- surements of bioerosion, limited to A. granulata alone, range from 17.9-54.0 I g' yr- .chiton-I. Given the paucity of rigorous field measurement of this important process, and the three-fold range of laboratory estimates for a single species, we undertook an in situ study of substrate removal, fecal pellet production, and sediment contribution by the common intertidal chiton A. granulata on San Salvador Island, Bahamas. STUDY AREA Pigeon Creek is a shallow, low-energy, interior tidal creek located on the subtropical Bahamian island of San Salvador. Our study was situated on the southern shore of the south branch of that waterway, approximately I krn from its mouth at Snow Bay. Daily tidal exchange at the site (range = 0.4 m) is sufficient to maintain normal, subtropical marine salinity, biota, and calcareous sediment (Mitchell, 1986). A. granulata are common along outcroppings of Pleistocene pelletal grainstone/ packstone which form the modem rocky intertidal shoreline (Fig. 1). No other chiton species was detected in the study area. Areas inhabited by chitons are interrupted by dense stands of red mangrove (Rhizophora mangle), which line substantial portions of Pigeon Creek. Limestone outcrops in the study area are either vertical, in the form of intertidal notches and nips, or subhorizontal, in the form of narrow sloping terraces (Fig. IA). Muddy calcareous sand onlaps the rock in the low-intertidal zone. The shoreline is comprised mostly of coherent limestone outcroppings, with scattered areas of broken, discontinuous rock and rubble. During the day, many chitons nestle 682 BULLETIN OF MARINE SCIENCE, VOL. 47, NO.3, 1990 Table I. Summary of previous bioerosion rate estimates made for common intertidal chitons in- habiting rocky carbonate coastlines. Only one of the reported rates (d) was measured in situ; others may be influenced by metabolic stress, indirect calculation methods, and/or grazing range limitations Erosion rate (g·yr-!. Reference/species Locale/substrate Method chiton-') McLean (l964)/A. granulata Barbados/beachrock Lab (pellet wt.) 17.9" 21.9b Glynn (1973; unpubl.)/ Puerto Rico/coral rubble Lab (pellet wt.) 54.0c A. granulata Taylor and Way (1976)/ Aldabra Atoll/calcarenite Field (pellet wt.) 3.3d A. brevispinosa (gut cont.) 8.4c Hoskin et al. (1986)/A. granulata Little Bahama Bank/eolianite Lab (pellet wt.) 26.JC • Lab Test A (N = 9 chi tons, t = 24 h), defecation following unrestricted grazing of, and removal from host rock. b Lab Test C (N = I chiton, t = 24 h), defec.1lion during restricted-range grazing of host rock. o Lab results of Glynn (1973, p. 285) corrected for 94.3% Caco, content. Defecation during restricted-range grazing of host rock (N - 10-12 chitons, t - 7-10 days; Glynn, pcrs. comm.). • Pellet weights (N = 3 chitons, t = 3 days), defecation during unrestricted grazing of host rock in field. e Indirect approximation using gut mass content (N = 108 chitons) and mean % gut~contentvoided/day (N = 3, t = 3 days) following unrestricted grazing of host rock in field. r Pellet weights (N = 12 chi tons, t = 2l days), defecation during restricted-range grazing of host rock in lab. within small depressions termed "homing pits" (Mook, 1983), which are most common on high- intertidal rock surfaces (Fig. IB). Many individuals cling beneath protected ledges, or to the inner sides of grazing "trackways." These areas form smoothed gutters on the otherwise pitted and irregular coastal biokarst.