Nemertean worms of the genus Carcinonemertes are associated with various species of crabs. Although juvenile worms are found on males and immature females, the worms grow to maturity and reproduce only on female crabs brooding eggs. When the worms are abundant they can be important egg predators, consuming large numbers of host eggs. (Roe 1979 and references therein) Carcinonemertes epialti apparently has a wide range of hosts. Roe (1979) found it associated with both Hemigrapsus oregonensis and Pachygrapsus crassipes, but based on her study it appears that the life history of C. epialti is much more closely tied to the life history of H. oregonensis than to that of P. crassipes. Infection rates may be very high: in her study, Roe found worms on about half of P. crassipes individuals and about three quarters of H. oregonensis individuals examined (Roe 1979).
The Rhizocephala is an order of highly specialized parasitic barnacles whose hosts are mainly decapod crustacreans, such as crabs. Rhizocephalans have free-living nauplius and cypris larval stages in the life cycle, as commonly seen in cirripeds. However, the morphology of adult rhizocephalans, which lack segmentation and appendages typical of arthropods, has departed very far from that of most crustaceans. The adult consists of an external reproductive saclike body called the ‘externa’ and a ramifying (i.e., spreading with a branching pattern) nutrient-absorbing root-like body inside the host called the ‘interna’, which is connected by a narrow stalk to the externa. This morphological modification in rhizocephalans is extremely conspicuous. Among rhizocephalans, sacculinids are especially notable for severe biological effects on host crabs. Parasitization by sacculinids induces serious modifications in morphology, behavior, reproduction, and molt cycle of hosts. Although significant modifications in morphology and reproduction of a host are also induced by parasitization by parasitic bopyrid isopod crustaceans, the effects of sacculinids on the host are much more dramatic. Tsuchida et al. (2006) report on three different species of Sacculina parasitizing Pachygrapsus crassipes in Japan: S. confragosa, S. imberbis, and S. yatsui. In their study, the authors found that about a quarter of the 138 P. crassipes sampled were infected; examination of two other crab species (103 Hemigrapsus sanguineus and 113 Gaetice depressus) revealed no sacculinids. (Tsuchida et al. 2006)
Both field and laboratory observatins indicate a high level of intraspecific aggression in the Lined Shore Crab, with strong spacing tendencies in tide pool populations. However, when these crabs are out of the water, aggression is much reduced and individuals tend to aggregate (e.g., in rock crevices). (Bovbjerg 1960)
The Lined Shore Crab (Pachygrapsus crassipes) is common from southern Oregon (U.S.A.) to Baja California (Mexico), where it inhabits the upper portions of rocky shores. Its coloration is variable, ranging from greenish to blackish to reddish with transverse (often green or red) stripes running across the carapace. The large pincers are sometimes red or may have red or purple lines. The body width of a large specimen is about 3 to 5 cm. These crabs live under rocks and in crevices. They feed extensively on algae (e.g., green algae such as Ulva, see Sousa 1979), tearing them up with their pincers. Some animals are eaten as well, including limpets and small crabs. Typically, it feeds by bringing its left and right pincers alternately to its mouth. (Kozloff 1993; Sheldon 1999)
Pachygrapsus crassipes is widely distributed along rocky shores of the eastern Pacific Coast, from Charleston, Oregon, USA, to central Baja California, Mexico. It also inhabits the western Pacific Coasts of Japan and Korea, where it was first reported in 1890. Recent evidence suggests that P. crassipes is expanding its distribution northwards along the eastern Pacific Coast and has been found as far north as Bamfield, Vancouver Island, British Columbia, Canada, since 1997. (Cassone and Boulding 2006 and references therein)
The Lined Shore Crab (Pachygrapsus crassipes) is found along rocky shores, in bays, in mussel beds, in estuaries and tidal creeks, and on pilings. Along tidal creeks, this crab may burrow into the soft, sandy banks. It appears at ease both on land and in the water. (Hui 1992; Sheldon 1999)
Pachygrapsus crassipes is found in the high intertidal zone of both bays and exposed coastal habitats of the North Pacific (Cassone and Boulding 2006 and references therein).
Like many crustaceans, the life cycle of the Lined Shore Crab is complex and includes several phases. Larval development of this species is described in detail by Schlotterbeck (1976). Rice and Tsukimura (2007) provide an illustrated identification key to the zoae larvae of the brachyuran crabs found in the San Francisco Bay (U.S.A.) estuary, including recently inroduced species.
