Western Rock Louse, Ligia occidentalis

Western Rock Louse, Ligia occidentalis. Collected off the coastal rocks of Loreto, Baja California Sur, July 2024. Size: 2.9 cm (1.1 inches) x 1.2 cm (0.5 inches).

Phylogeny: The Western Rock Louse, Ligia occidentalis, Dana, 1853, is a member of the Ligiidae Family of woodlice. The genus Ligia is one of two genera in this family, and there are fifty species in this genus. At least three of these species are found along the Pacific Coast of Mexico. They are also known as Rock Louse, Rock Isopod, Rock Slater and Western Sea Slater and in Mexico as cucaracha del mar.

Morphology:  The Western Rock Louse is oval in outline and somewhat flattened. It has two, long, forked, tail appendages (uropods), which are balanced by two long antennae in the front. The two eyes are separated by a space equal to the eye diameter. It is distinguished from the similar looking Pallas’s Rock Louse, Ligia pallasii, by its narrower shape and its much longer tail appendages. Western Rock Lice are pale gray during the day, becoming darker gray at night. They reach a maximum  of 3.2 cm (1.25 inches) in length.

Habitat and Distribution:  Western Rock Louse are found in rock crevasses, under stones, on pilings, and under algae piles. They generally try to avoid sunlight and are most active at night, in shaded areas, or on cloudy days. They are almost more of a terrestrial species than a marine species. They need to keep their gills moist, by dragging their belly through damp environments, but they will drown if submerged for a prolonged period. Consequently they spend most of their time in the high intertidal zone. They migrate with the tidal level. Western Rock Louse are found in the temperate and subtropical waters of the eastern Pacific Ocean. In Mexican waters, they are found along the Baja Peninsula, and south to Jalisco. They range throughout the Gulf of California.

Diet:  Western Rock Louse are scavengers, feeding on algae, carrion, and detritus.

Predators:  The Western Rock Louse is preyed upon by shore birds and crabs. During strong tidal events they can be swept into the sea and consumed by fish.

Reproduction:  Western Rock Lice are gonochoric (either male or female for life). Fertilization takes place internally, with the male’s sperm stored in the female’s oviduct until her eggs pass through the oviduct. The fertilized eggs and embryos are carried in the female’s marsupium (brood pouch). When the embryos develop to the post larval stage they are released to grow into adults. Mating usually occurs once a year, generally in the spring.

Ecosystem Interactions:  Information in this area is limited to the role the Western Rock Louse plays in the food web.

Human Interactions:  Western Rock Louse play no significant role in human activities. From a conservation perspective they have not been formally evaluated however they are common with a wide distribution and should be consider to be of Least Concern.

Synonym:  Lygia occidentalis.