Mottled shore crab

Pachygrapsus transversus is a small crab of the family Grapsidae that lives on intertidal rocky shores ^[1]. Its range covers the warm and warm-temperate zones of the eastern and western Atlantic and of the eastern Pacific ^[2], although the identity of Pacific specimens is under discussion ^[3]. In experiments done in the south-western Atlantic it behaves as an omnivore that prefers animal prey and whose presence changes how the rock is covered ^[1].

In this species clear sexual dimorphism is limited to the abdomen; in addition, as the crabs grow, the claws become larger in males and the carapace wider in females ^[4]. Males become pubescent, through a moult, when the carapace measures about 5.0 mm ^[4].

Its identity in the Pacific calls for caution. A phylogeographic analysis treats it as an Atlantic species, and Pachygrapsus socius was regarded as part of the eastern Pacific fauna ^[3]. At Easter Island, specimens that had been cited as Pachygrapsus transversus proved to be Planes laevimanus when the male gonopods were compared ^[3]. A phylogenetic study of the Grapsidae also showed that Pachygrapsus is not a natural group ^[3].

The larvae have been reared in the laboratory and pass through 7 zoeal stages, described in detail ^[5].

WoRMS records the species from the eastern and western Atlantic, the eastern Pacific, the Indo-West Pacific and the Mediterranean, with localities that include the Gulf of Mexico, the Lesser Antilles and the Canary Islands ^[6]. Other authors summarise its range as the warm and warm-temperate zones of the eastern and western Atlantic and of the eastern Pacific ^[2]. As explained above, a phylogeographic analysis regards it as Atlantic, so the identity of Pacific specimens is an open question ^[3].

In the Mediterranean it has been known from the east since the 1920s, and in the west there were only records from 1980 and 1994 with very few specimens, until it was found at Ustica and other localities of north-western Sicily, on vermetid reefs ^[2]. The authors could not tell whether that population arrived on its own or on ships, and because young crabs of about 5 mm and egg-bearing females were present they consider it stable ^[2].

It is a crab of the rocky intertidal: the field studies of its ecological role were done on subtropical rocky shores of the south-western Atlantic, in particular in the lower fringe of sheltered shores ^[1].

It is an omnivore. In laboratory experiments it preferred invertebrates to macroalgae and was opportunistic within each of those groups ^[1].

Males reach puberty by moulting at about 5.0 mm of carapace ^[4]. In a subtropical population egg-bearing females are present all year, but their relative abundance among mature females changes from under 10 % in winter to almost 80 % in summer, and photoperiod was the factor that best explained that pattern ^[7].

In the temperate zone of Brazil (Torres, Rio Grande do Sul) egg-bearing females were also found in every season, with a sharp drop in winter ^[8]. There fecundity was related to female size, and egg counts peaked in the spring and summer samples ^[8]. Egg volume grew during development and was higher in autumn than in the other seasons ^[8].

Larvae reared at 25 °C at salinities of 28, 32 and 35 per mil gave the best results at 35 per mil ^[5].

In field exclusion experiments, removing the crab changed the community of the lower fringe on sheltered shores: the dominant cover shifted from macroalgae to filter-feeding animals (ascidians, sponges and mussels) ^[1].

In contrast, limpets, predatory snails, large crabs and fish did not significantly change the community ^[1]. The authors conclude that the species plays an important role in the structure of the community on these shores and in controlling invasive species ^[1].