The porcellanid crab, Neopetrolisthes spinatus Osawa & Fujita, 2001, is reported from Rutland Island, south Andaman Islands. The specimen represents the first record of the poorly known species from Indian waters and was collected from a large sea anemone Heteractis crispa (Hemprich & Ehrenberg in Ehrenberg, 1834).

The porcellanid crab Porcellana sayana is, for the first time, reported to have a symbiotic association with the crab Stratiolibinia bellicosa. This is also the first record on the association between the porcelain crab P. sayana and a brachyuran crab on the Brazilian coastline. In addition, this is the second record of association of P. sayana with a Majoidea crab.

Four porcellanid species, Petrolisthes coccineus (Owen, 1839), P. lamarckii (Leach, 1820), P. moluccensis (De Man, 1888) and P. tomentosus (Dana, 1852), are reported on the basis of specimens collected from an intertidal area of Agatti Island, Lakshadweep Archipelago in south-western India. All the four species are recorded from the archipelago for the first time, and P. moluccensis is new to the Indian fauna. Morphological diagnoses, notes on the habitats and distributions are provided for the four species.

The different stages of embryonic development of the porcellanid crab Pachycheles chubutensis are described, along with the chronology of each stage at 16±1°C. Five different developmental stages (I–V) were recognized including: (i) early cells and 100% of the vitellum; (ii) formation of the embryonic primordium at the animal pole of the egg; (iii) presence of dark pigmentation on the posterior part of the ocular globe; (iv) appearance of chromatophores on mouth parts and in the abdominal zone; and (v) eye pigmentation in circular–oval form, filling the entire surface. The embryonic development lasts from spawning to hatching approximately 21 days, with the second stage being the longest.

The social behaviour of symbiotic organisms is influenced by the density and distribution pattern of hosts. Herein we examined the host-use behaviour of the anemone-dwelling crab Allopetrolisthes spinifrons (Porcellanidae) in which adults usually live as solitary individuals on their hosts. Adults of this crab use two different sea anemone species, one intermediate-sized species that can be found at relatively high densities in the intertidal zone and another large one that occurs at significantly lower densities in the shallow subtidal zone along exposed rocky shores of the south-east Pacific. Mark–recapture experiments demonstrated that crabs in subtidal waters (low abundance of hosts) remained for long time periods on the same hosts while crabs in the intertidal environment (high abundance of hosts) frequently changed hosts. There were no differences in host fidelity between male and female crabs. In an immigration experiment in the intertidal zone, significantly more juveniles immigrated than had been present originally, indicating that host-use behaviour is age dependent. The sex ratio between resident and immigrated individuals did not change. In an additional experiment hosts were planted at two different densities (high and low) both in the shallow subtidal and the intertidal zone to test whether host fidelity of crabs depends on distance between hosts. At the subtidal site, about half the crabs remained on their sea anemones for 14 days while at the intertidal site most crabs disappeared within 1 day in both density treatments. At both sites crabs were seen changing hosts in high density treatments, but no such events could be witnessed at low densities of hosts. Although the results are not fully conclusive, they suggest that host movements are affected by host densities. This relationship may be mediated by host (anemone species, size, distance) and site-specific (predation pressure, exposure time) factors. In general, the present study indicates that host characteristics influence host fidelity of symbiotic organisms and thereby, their social behaviour.