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<I>Dosinia Elegans</I>

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<I>Dosinia Elegans</I> A CONTRIBUTION TO THE ECOLOGY OF THE LAMELLIBRANCH DOSINIA ELEGANS1 HILARY B. MOORE AND NELIA N. L6PEZ University of Miami, Rosenstiel School of Marine and Atmospheric Sciences ABSTRACT Dosinia elegans is a venerid which is a constituent of sublittoral soft- bottom communities in Biscayne Bay, Florida. There are two settlements of young in the course of the year. A length of about ] 5 mm is reached in a year, and an animal with the maximum size of 25 to 30 mm is 2 to 3 years old. A mortality rate of 80 per cent per annum has been estimated. On the grounds studied, the biomass of Dosinia averaged 102 mg per square meter, and this produced the very high spawn output of J 60 mg. The ratio of annual productivity to standing crop was 2.8, which is about midway in the range of comparable local species. The growth rate was lower than that of most of these species. This study is based on materials collected in shallow sublittoral areas in Biscayne Bay from November 1963 to the present time. These areas and the methods employed have already been sufficiently described by Moore & Lopez (1966), and in reports of earlier studies of McNulty et al. (1962a, 1962b), on which we have drawn for some information. This study was initiated under grant number WP-00573 from the Public Health Service, later from the Federal Water Quality Administration, and continued under grant number WP-01433 from the latter agency. We are grateful to the agencies for their support and encouragement and also to many members of the Rosenstiel School who have shared in the work. Dosinia elegans Conrad is a member of the family Veneridae and the subfamily Dosiniinae. Of the three western Atlantic species, D. elegans occurs from Cape Hatteras, round Florida to Yucatan and Cuba (Clench, 1942); D. discus has a very similar distribution; D. concentrica occurs from Cuba and Panama south to Brazil. In the areas of Biscayne Bay from which this material was collected, D. etegans was present in only moderate numbers, and the analyses would have been more satisfactory had the numbers available been greater. McNulty et at. (1962a) recorded a density of 0.31 per square meter in the finest sedi- ments, and 0.12 in the next finest. The species was not sufficiently abundant to be placed in the first ten species in terms of biomass. 1Contribution No. 1281 from the University of Miami, Rosenstiel School of Marine and Atmospheric Sciences. 1970] Moore & Lopez: Ecology of Dosinia elegans 981 0%50 t I 30 E 20 E I •....J: l!)z W ...J 10 o FIGURE 1. Size frequencies of Dosinia elegans with all years and both stations combined. The growth of the autumn (A, A') and winter (W, W') broods is suggested. GROWTH In order to increase the numbers available for analysis, the data from sta- tions Band C have been combined, and the data from successive years have been grouped together by months (Fig. 1). The size frequencies are in agreement with the results of examinations of the gonads in indicating two spawning periods in the course of the year, one about January and the other about October. The individuals of the winter brood are about 15 mm long when they are a year old the following January, and about 25 mm long at 2 years old. Those of the October brood are somewhat larger at a year old- about 19 mm. The two broods together probably contribute to the very few individuals of over 25 mm, and it appears that few individuals live to be more than 2 years old. 982 Bulletin of Marine Science [20( 4) 80 w a... 60 a: w (!) c:t E I- 0> Z E 40 w u I a: r.: w ~ a... 20 50 0, o FIGURE 2. Seasonal changes in the tissue weight of a "standard" Dosinia elegans 15 mm long (solid lines) and periods in which ripe individuals were taken (dashed lines). GONADS Gonadal smears were examined in a proportion of the animals. The numbers are small, but by grouping these together (Fig. 2) we have ade- quate samples for four periods of the year. In April, and again in August, none were ripe. In October-November and again in January, about half were ripe. These cannot, of course, be taken as precise dates. The data from tissue-weight determinations supplement those from the smears. For each sample, individuals were measured, their tissues removed, dried, and weighed. Then, from the calculated regressions of dry tissue weight on the cube of the length of the shell, the weight corresponding to a length of 15 mm was estimated. This had been determined to be a length slightly greater than that at first sexual maturity. The tissue weights of a "standard" 15-mm- long animal rise to a peak at the end of October, drop rapidly, and then rise to a second peak in January. They are low through the summer. Thus size- frequency analyses, gonadal smears, and tissue weights all point to two spawning periods in the year. POPULATION DENSITY McNulty et ai. (1962a) recorded a population density of Dosinia of 0.31 per square meter in fine sediments such as that at stations Band C (i.e., particles with a median diameter of less than 0.2 mm), 0.12 per square meter at 0.2 to 0.4 mm, and none in coarser sediments. In the present study, the mean number per square meter was 1.73. For the entire data, the mean 1970J Moore & Lopez: Ecology of Dosinia elegans 983 length was 1.25 cm. However, if tissue weight is more or less linearly related to the cube of the length, we should consider the mean cubed length of the population. This was 4.27 cm:!, equivalent to a length of 1.62 cm. A combination of the various regressions of tissue weight on cubed length gives an annual mean weight of 59.2 mg for a 1.62-cm individual, and a biomass of 102.4 mg per square meter. The tissue weight dropped at one spawning from 92.3 to 42.7 mg, and at the other from 92.7 to 47.0. Thus, in the course of the year about 100 per cent of the maximum weight, or 160 per cent of the mean weight is released as spawn. The spawn output per square meter is therefore about 165 mg per year. DISCUSSION Very little has been published with regard to the biology of Dosinia. An- sell (1961, 1962) has given some account of its method of burrowing and special adaptations for this. It is a deeper burrowing form than the related Venus. The maximum size reached by the individuals of Dosinia elegans in this area is about half of that recorded by Clench (1942) for the whole range of the species. Data are not available on the nature of the habitat from which his largest specimens were taken, but it was probably from deeper water than our collections. The changes in abundance of Dosinia over a 4-year period have already been illustrated in a comparison of four species (Wright & Moore, 1970). The pattern for Dosinia (Fig. 3) was similar to that for Cyclinella tenuis and that for Tellina martinicensis, but different from that for T. alternata. On both areas Band C, the second two years showed a marked drop in population compared with the first two years. In the first two years there were good spatfalls, as indicated by the percentage of the population com- prising small individuals. In the first two summers, salinities remained high, instead of dropping during the typical rainy season. The size-frequency data allow a tentative estimate of a mortality rate of 80 per cent per year. If we start with a theoretical population of five hundred spat settled at each of the two settling periods, we can apply a mortality rate of 12.55 per month to these and, by following them through a number of years, reach a time when there are too few left to be significant. At the same time we can trace their size from the growth curves, and from this their tissue weight. Finally, we can calculate the standing crop of the resulting population for each month. Its mean value will be 84.85 gm. For each month, the product of the mortality rate, which we assume not to be size dependent, and the standing crop gives the somatic productivity, and the sum of the monthly values is the annual productivity, which in this case is ]27.79 gm. These values are based on a mean tissue-weight-length rela- tionship, and they do not include spawn output. Bulletin of Marine Science [20(4) 984 :J~=(Bl N .....E d Z :L~Y_(_C_) _ I , , , , o 200 I ~oo 600 800 1000 1200 14001 1600 1964 1965 1966 1967 FIGURE 3. Changes over a 4-year period in the population density of Dosinia elegans at stations Band C, in the percentage of individuals 10 mm long or less at the two stations combined, and in salinity. Cumulative time in days is indi- cated along the abscissa. Sexual maturity is reached at an age of about one year and a length of 15 mm. Individuals of this size or larger comprise about a third of the popula- tion numerically, but represent 71.6 per cent by weight. At both spawning periods the adults of Dosinia lose more than half their weight, or a total during the year of 113 per cent of the peak weight.
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