Ligiidae Family of Woodlice

Western Rock Louse, Ligia occidentalis. A representative of the Ligiidae Family of Woodlice.

Phylogeny: Woodlice of the Ligiidae Family are in the phylum Arthropoda, meaning that they have jointed legs. They are in the subphylum Crustacea, the same as crabs and lobsters, and in the class Malacostra. Like terrestrial pill bugs and sow bugs, they are in the order Isopoda. The suborder Oniscidae is one of nine suborders in the order Isopoda. The Ligiidae Family is the only family under this suborder. The Ligiidae Family is comprised of two genera, three subgenera, and fifty-one species. The name Ligiidae comes from the name Ligeia who was a siren in Greek mythology. This refers to the rocky shoreline that both Ligeia and Ligiids inhabit. Species in the Ligiidae Family are also commonly called Rock Lice and Sea Slaters.

Morphology: Wood Lice are ovate in outline and flattened in profile. They have a rigid, segmented, exoskeleton, and seven pairs of limbs. The head (cephalothorax) is wider than it is long, and is widely rounded at the anterior margin. The head has long multiarticulate antennae and large compound eyes. The thorax (pereon) consists of seven segments  and the abdomen has five plus a shield shaped pleotelson. Forked uropods extend from the posterior margin and may be short or very long.  Some species in this family exhibit sexual dimorphism. Males are often larger and more narrow than females and they have longer, wider, antennae. Wood lice are largely semiterrestrial and are primarily air breathers. They have pseudo-lungs on the abdominal appendages (pleopods). These pleopodal lungs are modified gills that require moisture or high humidity to allow for adequate gas exchange. Some species can also breath through their skin. The gills do work under water if the louse inadvertently finds itself submerged. Ligiids are usually drably colored in shades of gray, brown, or olive. Woodlice are fairly small, with the largest species reaching a maximum of 3.85 cm (1.5 inches) in length.

Habitat and Distribution: Woodlice are semiterrestrial animals. They are generally found on coastal rocks, pilings, sea walls, mangroves, and coastal cliffs. They live in the high intertidal zone, and where the humidity is high enough, may be completely terrestrial. To maintain hydration they will visit rock pools and hide under rocks or vegetation during the day. They emerge at night to scavenge. Marine species of Woodlice are found worldwide along temperate to tropical seas. At least four species in the Ligiidae Family are found in Mexican waters.

Reproduction: Woodlice are gonochoric (male or female for life). Reproduction is sexual,  with internal fertilization. The male transfers sperm to the female’s oviduct using specialized pleopods. The female stores the sperm  until her eggs are ready to be fertilized. After fertilization, the female lays her eggs into a fluid-filled brood pouch (marsupium) on the underside of her body. Woodlice do not have a larval phase but are hatched as a juvenile version of the adult.

Ecosystem Roles: Woodlice may be scavengers, detritivores, or grazers that feed on algae or diatoms. They are eaten by crabs, coastal mammals, shore birds, and, if immersed, fish. They depend on staying hidden and being fast runners as their defense mechanisms.