Ligia oceanica, the sea slater, common sea slater, or sea roach,
Description
This species has a flat body that is shaped like an oval that is widest in the middle and twice as long as broad.
This isopod can range from 20 mm to 35 mm in length and from brown to slate grey or olive green in color.
The head is wider than long, with a width/length ratio of about 1.5 and a rounded anterior margin.
The head features a pair of large
with more than 40
Ommatidium.
This head also features a pair of long antennae that are two-thirds as long as the body.
The flagellum at the distal end of each of these antennae features 12 to 14 bead-like segments.
Behind the head are seven thoracic segments. The adult of this species features seven similar pairs of legs that are all used for walking.
Distribution
The native range of this species extends along the northeast Atlantic coast from southern
Norway to
Morocco and into the western Balic Sea as far as the island of
Bornholm in
Denmark and the city of
Rostock in
Germany.
This species has also been recorded on the
Faroe Islands and the southern coast of
Iceland.
This isopod has been introduced not only to the Atlantic coast of
North America,
where this species is found from
Cape Cod to
Maine,
but also to the
Azores and the
Canary Islands.
Humans have probably transported this species overseas in cargo or in the
ballast of ships. Although common in Europe, this species seems to be rare in North America.
Ecology and habitat
This species is found where the substrate of the littoral zone is rocky, such as on boulder beaches, at the base of rock cliffs, or on harbor walls or
Jetty.
This isopod is especially common in crevices and rock pools and under stones.
These isopods rely on the shelter of rocks or seawalls for temperature control and moisture, but they can tolerate temperatures near freezing in the winter.
This isopod breathes air and avoids immersion in water.
The vertical range of this species is limited to a narrow region of the shore from the splash zone to a few meters higher.
This species is furtive and nocturnal animal, hiding in crevices by day and coming out at night to feed.
Reproduction and development
These isopods breed during the spring and summer. Individuals live for 2.5 to 3 years and usually breed only once.
In
England, a female can produce 50 to 120 eggs, with larger females producing larger broods.
Although the embryo develops legs before its release from the brood pouch (
Peracarida) of the female, the first free-living juvenile (
Peracarida) stage lacks the last leg pair found in adults.
Phylogeny
Multiple studies based on molecular data suggest that species of
Ligia are more closely related to the marine isopods in the suborders Valvifera and Sphaeromatidea than to the terrestrial woodlice in the suborder Oniscidea.
This evidence places the genus
Ligia in a
clade with these marine isopods on a branch separate from other Oniscidea genera in a phylogenetic tree of isopods, indicating that the suborder Oniscidea as currently defined based on morphology is
Polyphyly rather than
Monophyly.
This evidence also suggests that
L. oceanica and the other species of
Ligia represent a transition from the marine to the terrestrial environment that is entirely separate from the invasion of the land by the ancestors of the other genera in Oniscidea.
Phylogenetic analysis based on molecular data also indicates that the species L. oceanica includes two distinct clades in Europe, a southern lineage and a northern lineage. The southern lineage is found from the penninsula of Brittany in France to Spain, whereas the northern lineage is found from Brittany and the British Isles to the Netherlands, Germany, and Norway.
Mitochondrial genome
The mitochondrial genome of
L. oceanica, which was sequenced in 2006, suggests a close relationship between this species and the suborder Valvifera.
This genome is a circular,
DNA, with a size of 15,289
base pairs.
Although gene order is not conserved among isopods,
L. oceanica shows a similarly derived gene order to the species
Idotea balthica,
a marine isopod in the suborder Valvifera,
compared to the arthropod ground pattern, but the positions of three tRNA genes differ in the two isopod species.
See also
-
List of woodlice of the British Isles
External links
