The ecology of two species of blood clams, Anadara granosa (L.) and Anadara antiquata (L.) in Central Java, Indonesia
Members of the family Arcidae (phylum Mollusca, Class Bivalvia) typically occur in habitats ranging from the intertidal zone on waveexposed sandy shores to the marginally subtidal areas of sheltered mudflats bordering mangroves. They also occur at much higher tidal levels within the mangrove system (e. g. tambaks) and even extend into deeper subtidal areas. This study confirms that Anadara granosa possesses remarkable behavioural and physiological adaptations to deal with short-term fluctuations in environmental conditions. Both A. granosa and A. antiquata are gonochoristic species. Planktotrophic post-fertilisation development is associated with the production of large numbers of small (45-65µm) eggs during their iteroparous life cycle. A few individuals were reported with both male and female gametes present within the same individual follicles; these appear to be sequential protaridric hermaphrodites with only a single sex change during their life history. However, A. antiquata grows to a larger size prior to the onset of reproduction. A. granosa exhibited major spawnings during July-September 1992 and June-August 1993 with other spawnings in February and March 1992 and 1993. Allometric growth in both A. granosa and A. antiquata seems to be associated with the development of a broad, light weight shell necessary for life in relatively soft sediments. Two distinct ecomorphs of A. granosa, one rounded and one elongated form, are described. Although the growth bands within the shell of A. antiquata are difficult to interpret, those in A. granosa are clearly defined and have been shown to have a tidal periodicity which reflects the predominantly diurnal component of the mixed semidiurnal tidal regime. These growth bands exhibit a semilunar pattern in which narrower growth increments are produced during or a few days after spring tides whilst wider increments are produced during neap tides. The width of the growth increments was also shown to vary seasonally with groups of narrow increments deposited during FebruaryMarch and July-September when the clams were spawning, whilst the widest increments were laid down during May-June. This pattern of narrow and wide increments was used to estimate the age of individual clams and the age composition of the population. Whereas the population of A. granosa at Wedung consited of younger clams (1-4 years old) the population at Tapak consisted mainly of older (6-12 years old) clams.