Balanus amphitrite

Balanus eburneus

Chthamalus proteus

Gonodactylaceus falcatus

Ligia exotica

Pachygrapsus fakaravensis

Scylla serrata

Pachygrapsus fakaravensis (Rathbun, 1907)

Polynesian grapsid crab

Phylum Arthropoda
Subphylum Crustacea
Class Malacostraca
Order Decapoda
Infraorder Brachyura
Family Grapsidae

Description
Medium-sized crab, to about 5 cm carapce width. Carapace almost square (9/10 as long as wide), with its whole surface crossed by prominent, granulated lines, fringed by hairs lying flat on the carapace. Middle pair of suprafrontal lobes very prominent and elongate, separated from each other and from the lateral lobes by broad, deep, hairy furrows.
Rathbun (1907) notes that this species is near P. plicatus (a native Hawaiian species, pictured below), but is distinguished by its parallel sides, coarse striation, and great hairiness.
Habitat
Intertidal, on rocky shores.
Distribution
Hawaiian Islands
Only known from leeward harbors on Oahu
Present Distribution
French Polynesia, Japan, and Hawaii
Native Range
French Polynesia
Mechanism of Introduction
Unintentional, perhaps as larvae in ballast water.
Impact
Ecological impact unstudied, but probably competes with native intertidal grapsid crabs for food and shelter.
Ecology
Feeding
This crab is a scavenger, feeding on plant and animal debris in the intertidal zone.
Reproduction
With separate sexes. Fertilization is external, with planktonic larval stage.
Remarks
This rocky intertidal crab, previously known from French Polynesia (the type locality is Tuamotu) and Japan, was collected in 1996 from Barbers Point Harbor (Davie 1998). Peter Ng (pers. comm.) found it to be common at Kewalo, Oahu, in February 2000. Sakai (1976) notes the habitat as "coral reef, at high tidal mark", but in Hawaii, so far it seems to be rescricted to harbors and adjacent areas. We regard it as introduced by ballast water.
References
Davie, P. 1998. New records of crabs in Hawaii (Crustacea: Decapoda: Brachyura). Bishop Mus. Occ. Pap. 56: 63-64.
Sakai, T. 1976. Crabs of Japan and adjacent seas. Kodansha Ltd., Tokyo. 773 pp.
Rathbun, M.J. 1907. The Brachyura. Mem. Mus. Comp. Zool. 35(2): 21-74.