The occurrence of pinnotherid crab Arcotheres tivelae in the bivalve mollusc Amiantis umbonella was investigated for one year on the Bandar Abbas coast (Persian Gulf, Iran). Specimens of A. umbonella were collected monthly from two transects from April 2007 to March 2008 and were investigated for presence of the Arcotheres tivelae. Infestation frequency of A. tivelae was 9.18% in a sample of 893 clams. From a total of 89 specimens of crabs, only eight were male. They were observed in late February and early March, all of them but one in association with female crabs. There was no significant difference between the sexes of the infested clams that pea crabs choose as a host. The mean carapace width of the crab A. tivelae was 7.7 ± 1.7 mm and the mean length of the Amiantis umbonella was 39.84 ± 8.93 mm. Clams in the mid and low tidal zones were more infested. There was no significant relationship between clam length–crab width (R2 = 0.28). The mean fecundity of crabs was 2517 ± 864 eggs. Infection caused a reduction of flesh weight of clams. There was no significant correlation between the frequency of crabs' occurrence and the temporal variability of water temperature, salinity, dissolved oxygen and pH in different months. This study provides the first report of the distribution of pea crabs A. tivelae in A. umbonella, a new clam host record in Iran.

Amiantis umbonella is distributed throughout the coast of the northern Persian Gulf. This study of 893 clams provides data on reproduction, growth and production for the period April 2007 to March 2008 from two transects along the Bandar Abbas coast. Histological preparations showed six stages of gametogenic development: resting stage (Stage 0), early active (Stage I), late active (Stage II), ripe (Stage III), partially spawned (Stage IV) and spent (Stage V). The specimens were gonochoric and showed synchronized spawning. The reproductive cycle commenced in September and finished in March with one major spawning event in January which correlated with lower sea temperature. Von Bertalanffy growth parameters for the sample were asymptotic length (L∞) = 58–62 mm, growth constant (k) = 0.28–0.29 yr−1 and length zero (t0) = –0.48 –0.47. The mean annual clam abundance, mean biomass and production were 10 individuals. m−2, 5.7 g shell free dry weight (SFDW) m−2 and 0.495 g SFDW m−2 yr−1, respectively.