BULLETIN OF MARINE SCIENCE. 30(3): 604...(j12. 1980 NATURAL HISTORY AND POPULATION FLUCTUATION OF THE GASTROPOD ANACHIS AVARA (SAY) IN A TROPICAL SEAGRASS HABITAT, MIAMI, FLORIDA Edward B. Hatfield ABSTRACT Seasonal fluctuation in size structure of a population of Anachis (II'am at Bear Cut, Miami, Florida occurred similarly each year from fall 1970 through August 1975. This fluctuation was primarily the result of seasonal recruitment of high numbers of juveniles into the pop- ulation from planktonic veligers. Periodic differences in mortality could have been due to predation by mobile species recurrently present at Bear Cut. The results of temperature and salinity tolerance tests suggest that A. avara was not under physiological stress at Bear Cut over the duration of this work. Shoaling of the Thalassia flat towards the end of this study probably contributed to the drastic decrease in abundance of Anachis (II'am from December 1973 through June 1975. Fluctuation in size structure and abundance of many gastropod populations is caused largely by high seasonal recruitment. These fluctuations are also affected by varying individual growth and population mortality rates (e.g. Moore, 1937; Phillips, 1969; Franz, 1971; Branch, 1974). This study analyzes the causes of fluctuation in size structure and abundance of a population of Anachis avara (=Costoanachis avara Radwin, 1977) from a turtle grass, Thalassia testudinum (Konig), habitat at Bear Cut, Miami, Florida. Only Kolipinski (\964) and Bor- kowski (1971, 1974) have reported on gastropod populations in South Florida, and no known reports have been published on the population ecology for any species of the family Columbellidae to which A. avara belongs. HABITAT The habitat at Bear Cut is a subtidal T. testudinum flat bordered mostly by bare sand but with a fringe of Syringodiumfiliforma (Ki.itzing) along its intertidal edge (Hatfield, 1977). Seagrasses are particularly extensive along the coast of South Florida (Phillips, 1960; Moore, 1963; Roessler and Beardsley, 1974). Voss and Voss (\955), Tabb and Manning (\961), Tabb, Dubrow and Manning (1962), O'Gower and Wacassey (1967), Voss et al. (1969) and Brook (\978) have surveyed the fauna of South Florida turtle grass beds and provided preliminary information on the biology and ecology of a few species. The sediment surface within the Thalassia bed at Bear Cut is primarily a fine- grained quartz and carbonate sand (Wanless, pers. com.), with shelled inverte- brate remains, and seagrass and macroalgal detritus. The number of Thalassia blades per m2 ranged from 1,450 to 4,190 and averaged 3,030 from January to June 1974. Thorhaug (1974) considers over 2,000 blades per m2 to be a dense bed of turtle grass. Humm (\964), Ballentine and Humm (\975), Meyers et al. (\965) and Jones (1968) have described many of the algae and fungi which attach to the grass. In addition, tunicates, sponges, ectoprocts, hydroids, polychaetes, proto- zoa and certainly other groups of invertebrates attach to the blades. Seawater temperatures at Bear Cut from 1970 through 1975 ranged from 19.4°C in February 1970 to 31.7°C during July 1971. Six year monthly means were lowest in February and highest in the summer. Salinities ranged from 26.8%0 in June 604 HATFIELD: POPULATION FLUCTUATION OF A TROPICAL SEAGRASS GASTROPOD 605 1972 to 39.6%0 in May 1971 and averaged 34.1%0. Higher mean salinities during the spring and in September were due to high temperatures and consequent evap- oration. Salinity decreases in June and July and in October were due to rainy seasons. Because of the strong vertical mixing and flushing of the water at Bear Cut, temperatures and salinities (Moore, 1970, and later appendices) from the Rosenstiel School of Marine and Atmospheric Sciences's Laboratory dock are probably typical for Bear Cut. NATURAL HISTORY Anachis avara is reported along the United States east coast from the Gulf of Maine to Lower Matecumbe Key, Florida (Scheltema, 1969; Radwin, 1977). It inhabits primarily seagrass fIats and oyster bars in shallow subtidal waters, and in Biscayne Bay, Florida, is also found in intertidal sea walls. A. avara is epi- benthic and in Bear Cut crawls up grass blades and along the sediment surface. Shells of live Anachis are covered primarily with encrusting algae, but protozoa, hydroids, and polychaetes also occur. The smallest live snail found was 1.61 mm long with juveniles settling between 1.00 and 1.50 mm in length. Settlement size was estimated from measurements of the protoconch on juvenile shells from Bear Cut and knowing that Scheltema (1969) observed one 0.8 mm individual following metamorphosis in the laboratory. Adults at Bear Cut reached a mean terminal size of 10.50 mm, and ranged from 8.00 mm to 13.29 mm in length. Columbellids are dioecious and deposit their eggs in capsules (Marcus and Marcus, 1962). The capsules of A. avara resemble miniature volcanic cones stuck to an interconnecting basal rubbery mat (Scheltema, 1969). At Bear Cut, females deposit egg capsules on grass blades from October through May. Larvae develop into veligers within the capsules (Scheltema, 1969) and hatch into the plankton. Hatfield (1979) reported on the feeding of A. avara. This species is capable of grazing epibiota from Thalassia blades and of obtaining nutrition from organic matter in the sediment. In the laboratory, A. avara ate fresh remains of grass shrimp, bivalves, crabs, hermit crabs, and fish as well as its own eggs. Attempts to feed carrion to Anachis in the field failed, as swimming crabs (Callinectes spp.) quickly grasped the cracked clams. Anachis avara is preyed on by snails, crabs, spiny lobsters and probably by fish. I have observed predation on A. avara in the laboratory, and collected drilled and cracked shells from the field. I have also seen a small species of swimming crab, Callinectes ornatus (Ordway), which is abundant at Bear Cut, consume juvenile Anachis. Large Callinectes ornatus may be able to eat adult A. avara, in the way juvenile and adult Callinectes sapidus (Rathbun) and Pan- ulirus argus (Latreille) do. Small tulip shells, Fasciolaria tulipa (Linne) prey on Anachis. but larger tulips appear to prefer larger prey such as Melongena corona (Gmelin). Randall (1967) referred to several gastropods, including Anachis and other columbellids, being found in the gut of several species of fish. Low (1973) found these fish to be common at Bear Cut. Targett (1978) found A. avara in the gut of 19% of the pufferftsh, Sphoeroides testudineus, collected near Bear Cut. The results of laboratory temperature and salinity tolerance tests suggest that A. avara is not under physiological stress in its Bear Cut habitat. Moore and Gray (1968, 1969) developed an apparatus for laboratory determination of the upper lethal temperatures of various marine invertebrates. This equipment allows gradual temperature increases of 1°C an hour. Therefore, during a several hour experiment, snails left in the apparatus the longest are exposed to the highest temperatures. Details of the method are described by Albertson (1973). In an 606 BULLETIN OF MARINE SCIENCE, VOL, 30, NO.3, 1980 experiment begun at 27°C, Albertson found 100% mortality of A. avara at 41°C. Albertson's test was run in July 1970, when the ambient seawater temperature was 27.4°C. I began an experiment at 20°C and found 100% mortality at 40°C in January 1971, when the ambient field temperature was 19.7°C. Both of these upper lethal temperatures are substantially higher than the highest temperature of 31.7°C recorded at the Laboratory dock from 1970 through 1975. Even if temperatures on the grass flat were higher during low spring tides, it is improbable that, as a single factor, high temperature is a significant direct cause of mortality of A. avara from the population studied. Tolerance of A. avara to salinities from 0%0 to 60%0 was tested by placing individuals in a series of glass jars with salinity increasing in increments of 5%0. Separate tests were run for 12 and 40 h. Snails were placed into jars directly from ambient conditions of 35%0. One hundred percent survived 12 h in 5%0 water, and 80% survived that period in fresh water. No snails died in 12 h at 50%0. All snails survived 40 h at a low salinity of 20%0 and a high of 40%0. The tolerated limits were therefore narrowed considerably with the longer period of exposure. Field salinities were well within the tolerances of 12-h exposures and it is improbable that salinity stress causes severe mortality to A. avara at Bear Cut. Synergistic effects and overall well-being (Moore, 1966) as measured by sublethal effects were not looked at during this study. SIZE STRUCTURE Methods Analysis of population size structure from September 1970 through August 1975 was made from samples collected with a push net (Strawn, 1954). Usually four pushes of approximately 28 m were taken on a day of sampling, the total distance varying from 84 to 200 m. Sufficient numbers were collected to estimate the relative number of individuals in each size class down to the size of the smallest individuals retained by the I mm mesh net. These snails were juveniles approximately 2.50 mm long and 1.25 mm wide. Results Figure 1 shows a typical year's monthly length frequencies from the push net samples of the population for 1973. Juveniles settle from November through June and make up a high percentage of the total population during the spring months. High mortality rates for these small snails and growth (maturation) into adult size decrease the percent juveniles in the population throughout most of the year. By late fall, most of each year's newly settled individuals are indistinguishable from previous years' adults, and the proportion of juveniles in the population is greatly reduced.