Long-term observations on population structure and body dimensions of two grapsid crabs inhabiting a boulder-shore

The study site is a stony shore consisting of pebbles (1-5cm), cobbles (5-10cm) and boulders (>10cm) situated on sandy-silt, and gravelly substrate on a rocky sea shore at the western slopes of Mt. Carmel, Haifa. The study spread over a period of 16 years between 1984-1999. During this study period crabs were sampled for 29 months. The purpose of this study was to find out whether changes in the population structure or in the dimensions of the two grapsids, take place during such long period. The two most abundant crabs were the grapsids: Pachygrapsus transversus with 366 specimens, P. marmoratus with 262 specimens. A minor drop in numbers of both genders was seen in P. transversus during the 1990s. In P. marmoratus there was a noticeable increase in numbers of both genders in 1999. Only long-term observations could reveal these changes. The largest number of crabs was captured during spring (P. transversus males n=83, and P. marmoratus females n=39 respectively). The lowest number was during autumn and winter. The sex ratio varied during the study period. In P. transversus is mostly male-biased ranging from a low of 44% to a high of 67% males in the population. The opposite was seen in P. marmoratus population which is generally female-biased ranging between 50-100% females in the population. The body dimensions of both grapsid species varied over the years.