A CONTRIBUTION TO THE ECOLOGY OF CYCLINELLA TENUIS (: ) 1

PETER B. WRIGHT AND HILARY B. MOORE University of Miami, Rosenstiel School of Marine and Atmospheric Sciences

ABSTRACT Cyclinella tenuis is a minor constituent of the infauna of two sublittoral muddy areas in Biscayne Bay, Florida. It appears to breed almost through- out the year, but with three main periods. As a result, size-frequency analyses have not yielded trustworthy data for growth. Success of spawning and subsequent density of population have been tentatively associated with summer salinities that were unusually high in the first two years of study and low in the second two. Variations in the population of C. tenuis tended to parallel those of two other lamellibranchs, elegans and Tellina martinicensis, while showing no relation to that of T. alternata.

INTRODUCTION The Atlantic Cyclinella, C. tenuis, is a member of the and occurs as far north as North Carolina (Johnson, 1934). Its study in the present programme originated in the survey of the infauna of the soft bottom of Biscayne Bay described by McNulty et al. (1962a, 1962b). Thereafter, bulk samples were taken at approximately monthly intervals for study of individual species. The survey work was concentrated at two stations, A and B. The former of these became impoverished, and the present work includes a new station, C (described by Moore & Lopez, 1966). Towards the end of the studies, B also became impoverished and only C was sampled. The study was initiated under grant No. WP-00573 from the Public Health Service, Division of Water Supply and Pollution Control, and later from the Federal Water Pollution Control Administration, and was con- tinued under grant No. WP-01433 from the latter agency. We are indebted to both agencies for their support and encouragement and to many col- leagues for help with collecting and analysis and for access to their records. These include in particular L. T. Davies, R. H. Gore, N. N. Lopez, and R. C. Work. Studies of various other species in the communities have al- ready appeared (Moore & Lopez, 1966, 1969; Fraser, 1967), or are in press or in preparation.

MATERIAL AND METHODS Whenever possible, samples were collected monthly, fiirst at station B, later at Band C, and towards the end at C only. Station A was abandoned

1Contribution No. 1227 from the University of Miami, Rosenstiel School of Marine and Atmospheric Sciences. 794 Bulletin of Marine Science [20(3) because of impoverished fauna, and this was associated with marked coarsening of the sediment. Station B in turn was abandoned because of impoverishment, and here also the sediment appeared to have become coarser. Unfortunately, while other species were found in sufficient num- bers on area C, Cyclinella proved to be scarce there. Generally, 30 bucket- dredge hauls were made at each station. In earlier work it was estimated that 6.24 buckets were equivalent to one square meter. This is only an approximate figure, but it may be used for averaging the results of a large series of hauls. It is sufficiently good for estimating the standing crop. The dredge contents were washed on the boat through sieves of 6.3- and 3.2-mm mesh diameter, and everything retained was taken to the laboratory for examination. An aliquot of one tenth-usually the first three buckets-was washed through a 1.6-mm-mesh sieve and the retained ma- terial taken back. Small individuals may be presumed to live closer to the surface than larger ones, and so be unlikely to be missed by a dredge which captured larger individuals. The sieves were unlikely to miss any but the very smallest. The periods when no small individuals were found may therefore be safely interpreted as an absence of spatfall. All shells were measured in the laboratory, and the larger individuals were then opened. A smear was made from a small fragment of each gonad, and this was microscopically examined unstained. The remainder of the tissues was then removed from the shell, dried for 24 hours at 105°C, and weighed. Data for temperature and salinity were collected whenever a station was visited. These were, of course, strongly influenced by the varying state of the tide, so are of value mainly for showing the average differences between the different stations. Furthermore, such readings were taken only about once a month. Data are available for the laboratory dock for high and low water every day, and afford a much more valid picture of seasonal changes and of differences between one year and another. A reminder is perhaps desirable that no data for water samples adequately describe the environ- ment of the themselves, as they are buffered from conditions in the overlying water to a varying extent. For part of the time we measured temperatures in the mud and in the overlying water and found differences of as much as three degrees.

SPAWNING Cyclinella has clearly visible gonads in the form of a white- or cream- colored mass covering most of the base of the foot. Sex and gonadal con- dition are readily seen on an unstained smear. Females were considered to be ripening when some of the ova had reached full size, and ripe when the nuclear membrane had disappeared. The ripe ova are noteworthy in being pear shaped, with a lobe at one end. 1970] Wright & Moore: Ecology of Cyclinella tenuis 795

37

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0 JAN I FEB I MAR I APR I MAY I JUN I JUL MONTH

FIGURE 1. Upper curve, mean salinity for four years for Biscayne Bay (labora- tory dock); lower curve, seasonal variation in percentage of ripe female in- dividuals of Cyclinella.

Studies of tissue weights were carried out on all samples, and at first it appeared that these would be valuable in indicating the spawning period. However, with all the data in hand, no clear seasonal pattern was apparent. From examination of the smears, it appeared that there were three spawn- ing periods in about January-May, June-September, and October-December (Fig. 1). In the first, and most clear-cut, of these periods, there was a marked decrease in the percentage of ripe females, accompanied by a cor- responding increase in the percentage of ripening individuals. The intervals between the first and second and between the second and third periods corresponded with periods of considerably lowered salinity in the summer rainy season. The winter interval corresponded with the time of lowest temperatures. These relationships are pointed out here, but deductions 796 Bulletin of Marine Science [20(3)

mm 1964 25 ~ V o "10 50 20 , I ,

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FIGURE 2. Size-frequency diagrams for Cyclinella at area B. 1970] Wright & Moore: Ecology of Cyclinella tenuis 797

FIRST SECOND THIRD FOURTH QUARTER QUARTER QUARTER QUARTER

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FIGURE 3. Size-frequency analyses of Cyclinella at area B, with the three spawning periods combined and grouped into four quarters. with regard to the causes of the spawning cycle would be justified only after further years of study.

SIZE FREQUENCY The size-frequency diagrams (Fig. 2) have not yielded a sufficiently consistent picture for estimation of growth rates. They have proved valu- able, though, in confirming the three breeding periods of the year. The data were divided into three groups corresponding to the three spawning periods. Each group was then divided into four quarters, according to the dates when the samples were taken. Finally all first quarter samplings were combined, all second quarter, and so on. Comparison of the four quarters (Fig. 3) shows a heavy settlement of spat in the first and second quarters, and its disappearance in the third and fourth, thus providing confirmations for the division of the year in terms of gonadal conditions.

POPULATION DENSITY For Biscayne Bay as a whole, McNulty et al. (1962a) recorded a density of Cyclinella of 0.94 per square meter in the finest sediments (particles less than 0.2 mm in diameter), 0.12 per square meter in the second finest (0.2-0.4 mm diameter), and none in the coarser sediments. For area B, in the period 1957-1958, they found a mean density of 2.33 per square meter and a mean biomass of 29.2 mg. In the four years of the present study, the numbers in areas Band C 798 Bulletin of Marine Science [20(3)

. '. . Cyclinello . . . B . . '...... ~ ------Dosinio B

T. mortinicensis B

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C> Z >- a: « > t\I •.•..E vi o Z

1964 1965 1966 1967 1968

FIGURE 4. Running means of numbers per square meter for Cyclinella tenuis, Dosinia elegans, Tellina martinicensis, and Tellina alternata, area B above, area C below. Scales vary to allow a common maximum.

averaged 10.48 and 5.63 per square meter, respectively. The corresponding mean lengths were 8.80 and 13.96 mm. Tissue weight is more or less linearly related to the cube of the length, so for biomass estimates it is more appropriate to use the cube root of the mean of the cubes of the lengths. This gives values of 11.68 and 15.97 mm. From the means of all data on tissue weights, these values are found to correspond to weights of 21.56 and 161.17 mg for dried tissue, so the biomasses on areas Band 1970] Wright & Moore: Ecology of Cyclinella tenuis 799 C were 226 and 907 mg. These are considerably higher than McNulty et al. found. Area B suffered a drastic reduction in population during the period of the study (Fig. 4); the number per square meter dropped from 16.61 in the first two years to 3.59 in the second two. On area C, fluctuations were more erratic, but here also the population decreased, the first two years averaging 7.30 and the second two 3.67.

DISCUSSION Cyclinella is one of our less adequately sampled species, but the in- formation we have been able to obtain about it is presented because of the importance of the Veneridae, of which it is a member, in soft sediments throughout the world. One of the striking points about its ecology is its long- and short-term fluctuations in population. We are fortunate in having comparable studies of three other species on the same grounds. An account of Dosinia elegans is being prepared by Moore & Lopez, an account of T ellina martinicensis has been published by Penzias (1969), and one of T. alternata by Moore & Lopez is in press. Graphs of the populational changes of all four, drawn to comparable scales, and for both grounds, are shown in Figure 4. D. elegans and T. martinicensis agree with Cyclinella in showing a very marked drop in population on area B in the second 2 years. T. alternata, on the other hand, fluctuated widely, but averaged about the same density in the two periods. At station C, we again find a similarity of pattern between Cyclinella, D. elegans, and T. martinicensis, with T. alternata following a different pattern. In the first three species there was a pronounced peak in the summer-autumn period in both of the first 2 years. In all three there was also a marked drop in density in the second 2 years, with some recovery in T. martinicensis towards the end of the second period. Sampling of Cyclinella and Dosinia was terminated too soon to see whether they too in- creased again. T. alternata appeared to follow a longer cycle, and with no great difference in the mean density in the first and second halves. The similarity in the pattern of populational changes in three species suggests a common reaction to changes in the environment. The decrease in population at area A accompanied a coarsening of the sediment. That at area B during 4 years of observation probably did the same. There was no apparent coarsening at C, although there was some decrease in popula- tion there also. Furthermore, McNulty et al. found an even lower popula- tion of Cyclinella at area B than we did, despite the fact that it contained, at that time, a fine deposit. Another possible explanation is changes in salinity. Normally there is a rainy season in the summer, with resulting lower salinities in Biscayne Bay. In the summers of the first 2 years of the present study, the salinity 800 Bulletin of Marine Science [20(3)

38 1965

37

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Z \ ....J 34 \

30 JAN FEB MAR ApR MAY I SEP I OCT I NCN IDEC FIGURE5. Five-year record of mean monthly salinities at the laboratory dock.

did not drop nearly as low as it did in those of the second 2 years (Fig. 5). Not only was the population greater in the first 2 years, but also there was a better spatfa]). However, if low summer salinity is disadvantageous, the drop in population should have been greater where the drop in salinity also was greater. Yet, at area B, which is closer to outside water, the salinity during the rainy months averaged 1.91%0 higher than at the land- ward area C. Furthermore, Cyclinella consistently attained a larger size at area C than at area B. Of individuals over 5 mm in length, only 0.5 per cent of all those taken at area B exceeded a length of 20 mm, while at area C, 11.0 per cent exceeded 20 mm. Cyclinella was also less abundant at C. Finally, there were no marked differences in temperature among the four years. 1970] Wright & Moore: Ecology of Cyclinella tenuis 801 Apparently Cyclinella tends to decrease its spawning at the time of year when the salinity is low, and has a poor spatfall and low density of popula- tion, in those years when the salinity is low. It has a lower populational density at the station where the salinity drops lower, but grows larger there. Its populational changes are greater at the station where the salinity changes are less. Perhaps the advantages of proximity to land partly outweighs the harmful effects of lowered salinity. We can only hope that further years of study may clear up the problem.

SUMARIO

CONTRIBUCI6N AL ESTUDIO DE LA ECOLOGIA DE Cyclinella tenuis (MOLLUSCA: BIVALVIA) C. tenuis constituye un componente de menor importancia de la infauna en dos zonas fangosas del sublitoral de Biscayne Bay, Florida. Parece reproducirse durante todo el ano, pero con tres period os mas importantes. Como resultado de ello, los estudios de la frecuencia de tamanos no han aportado datos seguros sobre el crecimiento. El exito del desove y la sub- siguiente densidad de poblacion han sido tentativamente asociados con las salinidades del verano, las cuales fueron excepcionalmente altas durante los primeros dos alios del estudio y bajas durante los dos siguientes. Las variaciones en la poblacion de Cyclinella parecen ser iguales a las de Dosinia elegans y Tellina martinicensis pero no muestran relacion alguna con las de T. alternata.

REFERENCES FRASER, T. H. 1967. Contribution to the biology of Tage/us divisus (Tellinacea: Pelecypoda) in Biscayne Bay, Florida. Bull. Mar. Sci., 17( 1): 111-132. JOHNSON, C. W. 1934. List of marine Mollusca of the Atlantic coast from Labrador to Texas. Proc. Boston Soc. nat. History., 40: 1-204. McNULTY, J. K., R. C. WORK, AND H. B. MOORE 1962a. Level sea bottom communities in Biscayne Bay and neighbouring areas. BulL Mar. Sci. Gulf & Carib., 12(2): 204-233. 1962b. Some relationships between the infauna of the level bottom and the sediment in south Florida. BulL Mar. Sci. Gulf & Carib., 12(3): 322-332. MOORE, H. B. AND N. LOPEZ 1966. The ecology and productivity of Moira atropos (Lamarck). Bull. Mar. Sci., 16(4): 648-667. 1969. The ecology of Chione cancel/ala. Bull. Mar. Sci., 19(1): 131-148. In press. A contribution to the ecology of Tellina alternata. Bull. Mar. Sci. PENZIAS, L. P. 1969. Tellina martinicensis (Mollusca: Bivalvia): Biology and productivity. Bull. Mar. Sci., 19(3): 568-579.