Breeding in a rockpool: Larvae of the semiterrestrial crab Armases [ = Sesarma] miersii (Rathbun) (Decapoda: Grapsidae) develop in a highly variable environment

Abstract

The sesarmid crab Armases miersii (Rathbun) undergoes development in supratidal rockpools. We studied the ecological conditions in these pools, the breeding behavior of females, and the larval development and behavior on the Jamaican north coast in spring and summer 1992 to 1994. Individually marked females showed a high site fidelity, visited, molted and released their larvae repeatedly in the same pools. Larvae occurred in rockpools throughout the study periods. An abundant microfauna as well as detritus in the pools provided food resources for developing larvae. Experiments with a predatory corixid bug, which sometimes populates rockpools, indicate that larval survival in pools is significantly threatened by these bugs. Diurnal variations in pools were highest for temperature, ranging on average from 24.4 to 32.7 °C, and for oxygen saturation ranging from 20 to 155%. The mean pH values varied moderately between pH 8.3 to 8.9 and the salinities remained constant. Seasonal salinity variations were influenced by seawater spray, evaporation, precipitation and the distance of pools from the shore. Mean salinities and the pattern of their variation differed greatly between pools. In pools near the shore, mean salinities in 1992 ranged from 31.5 to 36.1 ppt and salinity varied between 0 to 120 ppt. In one pool distant from the shore mean salinity was 2.7 ppt and salinity varied between 0 to 8 ppt. Average salinities in the same pools were significantly higher in 1992 than in 1993, because of higher precipitation in 1993. Stratification in salinity and temperature occurred after heavy rains. Stratification in oxygen saturations occurred every night with on average 26% in the surface layer and 20% or less below the surface. Exposed to oxygen saturations below 30% for several hours, larvae did not survive in laboratory experiments. We observed that larvae in pools aggregated in the surface layer at night, whereas they were dispersed in the water column and feeding during the day. The mean duration of development to the first-crab stage in three rockpools was 11.5, 13 and 16.3 days at a mean temperature of 27.6 °C and shorter than the 17.5 days at 25 °C in the laboratory. However, the high salinity fluctuations in the pools, caused higher larvae mortality than the stable conditions in the laboratory. This study shows that the development of A. miersii larvae in Jamaican supratidal rockpools is subjected to highly variable and unpredictable abiotic conditions and that food availability is generally high.