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. The following year's recruits begin to settle in November and the per- cent juveniles increases again for another yearly cycle.

ABUNDANCE Methods

Quarterly quantitative samples were taken with a suction sampler described by Brook (1975). The technique consisted of placing a mesh-walled metal frame on the sediment surface and vacuuming material from the enclosed area into a I mm mesh collecting bag. Extensions of the frame's four corner posts down into the sediment stabilized the position of the frame and prevented animals from crawling in or out of the 1 mm mesh "cage" during sampling. Usually samples were collected from a 5 x 5 grid of 250.125 m2 plots, or a total area of 3.125 m2• The portion of the grass flat from which samples were taken is 150 m long, an average of 17 m wide, and 2,550 m2 in area. The area usually sampled was therefore approximately 0.1% of the total habitat. HATFIELD: POPULATION FLUCTUATION OF A TROPICAL SEAGRASS GASTROPOD 607

~ 0 ~ N 0 p S E ~ A R J C ~ J E N M T A ~ M ~5[~ F 70 , ,J 1.5 4.5 7.5 1(i5 13.5 LENGTH (MM)

Figure I. Percent per size group of Anachis avara at Bear Cut for each month of 1973.

Results from the grid sampling (Table 1) were the same as those from 25 randomly located plots (Hatfield, 1977). Counts and measurements were made of all the Anachis {Ivara collected by the suction sampler. Quantitative data for only the adult snails were used, however, as it is possible that the smallest snails were pushed through the 1 mm mesh collecting bag. The abundance of juveniles was calculated from the percentage of juveniles in the population and the abundance of adults. Total abundances were the sums of the adults and juveniles.

Results Detailed abundance data from Hatfield (t 977) are summarized in Table 2. Adult density of 97 per m2 on June 3, 1974, 98 per m2 on October 14, 1974, and 91 per

Table I. Abundance, standard error, and 95% confidence interval of adult Anachis QI'ara from 0.125 mt sample plots

Number Mean Number Date Plots Adults SE 95% C.1.

Dec. 13, 1973' 5 37.4 4.50 8.82 Feb. 21, 1974 10 19.2 1.64 3.21 Mar. II, 1974 25 20.5 0.74 1.45 Jun. 03, 1974 25 12.1 1.12 2.20 Oct. 14, 1974 25 12.2 0.95 1.86 Jan. 16, 1975 25 11.4 0.79 1.55 Mar. 24, 1975 25 7.2 0.73 1.43 Jun. II, 1975 25 1.9 0.40 0.78

• Data for plot size of 0.250 m'. 608 BULLETIN OF MARINE SCIENCE, VOL. 30, NO.3, 1980

600

500

400 200 I ~ 300 .~ ~ ~" 200 "j ::j" ~ii1 ~ ~o 100 " . ~ a DJFMAtr.tJJ ASONOJFMAMJ OJ FMAMJ J ASOHOJ FMAMJ 1973 1974 /974 1910 197.:1 1974 1914 1915 MONTH S MONTHS

Figure 2, (Left) Abundance per m' of adult Anachis avara at Bear Cut from December 1973 through June 1975, Figure 3. (Right) Abundance per m' of the total population of AnC/chis ((l'am at Bear Cut from December 1973 through June 1975. m2 on January 16, 1975, remained stable during that period (Fig. 2). Mortality of older adults was balanced by an influx of "new" adults from the growth and maturation of juveniles. Substantial changes in the number of individuals in the total population (Fig. 3) therefore reflect juvenile mortality.

MORTALITY Methods

Mortality rates can be calculated from changes in the number of individuals per m' in the population if it can be shown that these changes are not caused by migration or recruitment. In the case of A. a vc/ra , for the population as delineated in Figure 4, this can be done for adults from November through June and for the total population from July through October. The plot of adult abundance (Fig, 2) suggested exponential rates of decrease during periods when mortality occurred. Therefore natural logarithms (In N) of abundances (N) per m' were regressed on time (I) in days to calculate instantaneous mortality rates (z), Fitted numbers of adults from February 21, 1974, through January 16, ]975, were then calculated by equation (I):

In N = 4.99 - 0.00]7t, (1) where 4.99 is the natural log of abundance at the beginning of the period, and 0.0017 is the instan- taneous rate of mortality. Fitted adult abundances from March 24, 1975, through June II, 1975, were calculated by equation (2):

In N = 4.0] - 0.0169t. (2)

Table 2, Abundance of adults and juveniles, and total number of individuals per m' of AnC/chis al'ara at Bear Cut, Miami, Florida

Tolal Number Date Number of Adolts Number of Juveniles of Individuals

Dec. 13, 1973 150 80 230 Feb. 21,1974 153 290 443 Mar. II, 1974 164 388 552 Jun. 03, 1974 97 235 332 Oct. 14, 1974 98 39 137 Jan. 16, 1975 9] 91 182 Mar. 24, 1975 57 120 177 Jun. I], 1975 15 56 71 HATFIELD: POPULATION FLUCTUATION OF A TROPICAL SEAGRASS GASTROPOD 609

DISTINGUISHABLE MIXED OISTIN-

GUI SH - 0·020

ABLE

0,015

OLD OLD OLD a "'>- ADULTS ADULTS AND NEW ADULTS NEW o

ADULTS '" AND I--- ~ ()OO!5 FUTURE ::; NEW ;; ~ETTLING JUVE NILES SETTLING NEW ~ O-ODO JUVEN· JUVENILES JUVENILES " ILES ·0,005 JFMAMJJASOND DJFMAMJoJ ASONDJFMAMJ 19731974 1974 1975 MONTHS MONTHS

Figure 4. (Left) Adult and juvenile composition of the Am/chis OV{IY(! population at Bear Cut. Figure 5. (Right) Daily mortality rates for the Allochis OllaY(! population at Bear Cut from December 1973 through June 1975.

Total numbers per m~ were computed using equation (3):

In N = 6.03 - 0.00761. (3)

Results Daily mortality rates from December 1973 through June 25, 1975, are shown in Figure 5. The absence of mortality observed during late December 1973 and January and early February 1974 did not recur during the same period a year later. Substantial loss of individuals did take place during the spring of both 1974 and 1975. In 1975, this mortality was in addition to mortality throughout the winter and early spring, and the combination of these almost eliminated the pop- ulation. Calculated daily adult mortality rates fluctuated from 0.00 to 0.017 indi- viduals per m2•

DISCUSSION In the case of Am/chis avara. settlement of numerous juveniles during the reproductive season caused large seasonal changes in population size structure. Juveniles comprised up to 89% of the A. avara population during April 1973. In addition, a 91% decrease in numbers of adults occurred during the IV! years, from December 1973 through June 1975, over which abundance was measured. These two factors together show that large fluctuations occur within the Anachis population at Bear Cut. As over half the snails collected during this study were returned, sampling "predation" was not thought to be a significant cause of mortality. I suggest that the observed high periodic mortality rates were due to predation by species such as spiny lobsters (Panularis argus) which move around the shal- low coastal waters and feed on Anachis, among other prey, while at Bear Cut. The rate of this suggested predation would depend on the number of predators feeding, the length of time they remain on the flat and on the relative abundance and availability of prey species. Differences in densities of spiny lobsters, partic- ularly juveniles, by season and location around Biscayne Bay (Eldred, Futch and Ingle, 1972), are common for species of this type (Houde, pers. com.). Eldred, Futch and Ingle (1972) found juvenile lobsters in Biscayne Bay grass beds throughout the year, and peak numbers during the spring and fall. They also suggested that it is possible that larger lobsters feed over the grass beds at night. Without having made actual counts, I observed relatively high spring abundances 610 BULLETIN OF MARINE SCIENCE, VOL. 30, NO.3, 1980 of juvenile spiny lobsters at Bear Cut. I have also seen juvenile lobsters feed on juvenile A. avara in the laboratory, and larger lobsters can eat Anachis of any size. Temporary use of particular grass flats frees outside predators from depen- dence on an abundance of food at particular locations. This decreases the prob- ability of local prey availability controlling predator densities which is the more usual predator-prey relationship. Several aspects of the life history of A. avara (Hatfield, 1977) are consistent with the supposition that heavy unpredictable periodic predation causes varying mortality rates of this species at Bear Cut. Anachis grows to a small terminal length (mean of 10.50 mm) and matures quickly at an age of 6 to 7 months. Females probably deposit more than one set of eggs during a long reproductive season (Hatfield, 1977). The estimated longevity of Anachis at Bear Cut is less than 2 years. Short life requires early maturation and reproduction (Murphy, 1968) for continued abundance. It is favored by termination of individual growth, leaving more energy for reproduction. Local populations of species with plank- tonic larvae can be replenished from a reproductive pool of mature females throughout a broader area. A possible factor contributing to the drastic local decline of A. avara at Bear Cut was shoaling of the habitat from 1973 through 1975. By early summer, 1976, the west end of the flat was completely covered by bare sediment. Protection from predation was reduced with snails crawling up sparsely distributed grass blades. Habitat space and the epibiotic food supply from the Thalassia blades were decreased. Previous to the shoaling, food availability was thought to be high for A. avara at Bear Cut, even though species of other taxonomic groups probably competed for food (Hatfield, 1979). Other grazing gastropods such as Turbo cas- taneus and Astraea americana, abundant in other grass flats in the Miami area, were nearly absent at Bear Cut. Competition with other gastropod grazers may significantly affect the overall distribution and abundance of Anachis in the gen- eral southeast Florida area. No other physical conditions were known to control the abundance of Anachis at Bear Cut even though this might be predicted for a species in a habitat at an end of its geographical distribution. Temperatures and salinities were within the critical limits although Anachis could have been affected physiologically at sub- lethal levels.

CONCLUSIONS A large fluctuation in size structure of the Anachis avara population at Bear Cut takes place each year due to a high percentage of juveniles entering the population from January through June. Results of laboratory temperature and salinity tolerance tests suggest that An- achis avara is well within critical limits of these factors at Bear Cut. Food and space are also not thought to be limiting in the Thalassia habitat. An extreme decrease in abundance occurred during the 18 months over which quantitative samples were taken. This is thought to be largely the result of shoal- ing of the habitat during this period. Predation is suspected of being the cause of periodically observed high mortality rates.

ACKNOWLEDGMENTS

I thank the numerous persons who assisted in the collections of field samples. Dr. H. B. Moore, K. Gray and H. Albertson made available equipment for testing tolerance to high temperatures. I am especially grateful to Dr. H. B. Moore for his advice during all aspects of this work and to Drs. E. HATFIELD: POPULATION FLUCTUATION OF A TROPICAL SEAGRASS GASTROPOD 611

Houde, M. Roessler and D. Moore for technical assistance and lengthy discussion. Portions of the work were supported by an AEC grant, R. Bader, Principal Investigator, Sigma Xi, and the Bader Memorial Fund, University of Miami. This is a contribution from the Rosenstiel School of Marine and Atmospheric Science, University of Miami, 4600 Rickenbacker Causeway, Miami, Florida 33149.

LITERATURE CITED

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DATE ACCEPTED: February 23, 1979.

ADDRESS: Jackson Estuarine Laboratory, University of New Hampshire, RFD Adams Point, Dur- ham, New Hampshire 03824. PRESENT ADDRESS: Freedom, New Hampshire 03836.