Common west-coast species of shore-dwelling isopods are found amongst seaweeds and sea grasses, under rocks, in wood, and in rock crevices.  The large Suborder Oniscidea, with over 4000 semiterrestrial and terrestrial species worldwide, is represented on the west-coast shores by species of semiterrestrial Ligia and Tylos, by the beach sand-inhabiting Alloniscus perconvexus, and by several species of terrestrial woodlice, which sometimes wander into the upper intertidal regions.  Species of parasitic bopyrids are found on the external surfaces of mud shrimps, and in the branchial chambers of ghost shrimps, hermit crabs, galatheids, and various shrimps and prawns, but little research on the effects of these on their hosts has been done on west-coast species.

The following accounts are presented alphabetically.

In the rocky coastal areas of British Columbia the sea slater Ligia pallasii occupies a zone from the supratidal splash zone to 2-3m higher.  It is mostly nocturnal, and emerges from daytime resting spots in late afternoon or dusk to feed on diatoms and algae in the upper-intertidal zone and supratidal splash zone.  Carefoot 1973 Mar Biol 18: 302.

NOTE  this term is used synonymously with “supralittoral” and “semiterrestrial” in the ODYSSEY

Several Ligia pallasii crawl on a vertical rock face
on the west coast of Vancouver Island, British Columbia.
The group comprises one large male and 3 others of
indeterminate sexes 0.3X

On open-coast areas at Port Renfrew, British Columbia juvenile Ligia pallasii newly emerged from the female's brood chamber (known as mancas: 2-3mm in length) seek out moist beds of green algae Enteromorpha spp. in which to shelter.  The risk is from drying and being eaten by larger conspecifics.  Only after growing to a size of about 5mm do the juveniles move into the seawall, rock-crevice habitat of the adults.  Carefoot 1979 Crustaceana 36: 209.

Newly emerged mancas of L. pallasii are 3-4mm in length. Two
are visible in the photo, one at 9 o'clock; the other, at 5 o'clock

Freshwater seepage on rocks in the high intertidal/supratidal
favours growth of green alga Enteromorpha sp. This, in turn,
is inhabited by juvenile L. pallasii. The vertical extent of
seaweed in the photograph is about 1.5m

A second Ligia species on the west coast is Ligia occidentalis. Previously known from Oregon south to southern California, it has recently been observed at Savary Island, British Columbia, between Vancouver Island and the mainland, and also in locations in southern Howe Sound, British Columbia.

NOTE latitude 49.93^o N

Ligia occidentalis on a granitic rock
in the intertidal zone at Savary
Island, British Columbia 1.3X

NOTE  cytochrome oxidase 1 and 16S ribosomal DNA

NOTE  the authors believe that the northern limit of distribution of L. occidentalis is central California, at about the level at which it overlaps with the more northerly species L. pallasii (the authors mis-spell the species name as "pallasi" throughout their text). In fact, L. occidentalis has been moving steadily northwards and is now (January, 2012) known to be at West Vancouver and Savary Island, British Columbia, the latter some 1400km north of San Francisco.  The most likely explanation for this movement is boat transport or rafting, the latter of which would certainly have been expected to occur after the formative events of the Gulf of Mexico, making the present results all the more remarkable

Genetic clades of Ligia occidentalis
and putative species distributions of
Ligia in southern California and Mexico

In a remarkable convergence of research effort, a second research team, from UC San Diego and Centro de Investgación en Alimentación y Desarrollo, Mexico, has similarly investigated the genetic makeup of populations of Ligia occidentalis in the same geographic area (southern California to Manzanillo, Mexico) using basically the same research techniques.  Neither report mentions the results of the other, but the two groups were definitely in communication with one another, so the omissions are puzzling. The congruent studies are a real bonus for isopodologists and other scientists for they provide independent replicates that otherwise probably would not have been done.  The scope of the study presented here is slightly smaller than the one outlined above in Research Study 4 (only a single gene analysed, 44 locations, fewer individuals), but the results are strikingly similar (see illustration).  The authors identify 15 clades or cryptic species (vs. 13 for the other Research Study), also partitioned into 4 main groups.  These authors also credit the high genetic differentiation of L. occidentalis in this geographical region to its limited dispersal capability and restricted ecological habitat-needs.  Markow & Pfeiler 2010 Molec Phylogenetics & Evol 56 (1): 468.

NOTE  sequencing analyses of mitochondrial cytochrome c oxidase subunit I

NOTE  see capital letters A-O in the illustration. The 15 clades separate into 4 major groups: A-C: outer Pacific coast of Baja & California, D-H: northern & central Gulf, I-N: central & southern Gulf, and O: Pacific coast of mainland Mexico

An unusual-seeming habitat for an isopod is boreholes in wood or algal substrata.  The commonest species of gribble on the west coast may be Limnoria lignorum, a wood borer, but at least 3 other species are present in California, including one, L. algarum, that bores into holdfasts of large kelps. Limnoria lignorum is a small species and its working of wood is easily recognised as shallow (to 1cm depth), closely packed interlacing burrows, each about 1mm in diameter. With increasing honey-combing the wood surfaces fragment, leading to successively deeper layers being attacked, and a gradual reduction in diameter of the wood.  Limnoria lignorum and perhaps other species of isopods may inhabit the same piece of wood as much larger shipworms Teredo spp. and Bankia setacea, but competition is minimised by the preference of Limnoria for surface layers and of the bivalves for deeper regions.  Nonetheless, the persistence of Limnoria in its slow but continuous attack on the common habitat, eventually exposes the shipworm burrows, which then slough off in waves or by battering of the wood on the shore.  The gribbles continue boring until the wood is entirely destroyed.  Miller 1926 Ecology 7: 247.

NOTE  in floating wood, shipworms Bankia setacea prefer upper surfaces, while Limnoria spp. prefer lower surfaces.  The isopods also tend to be more prevalent in the intertidal zone, while shipworms are found more subtidally. Walden et al. 1967 J Fish Res Bd Can 24: 261.

Gribble Limnoria sp. burrows interspersed by larger,
calcareous-lined Bankia setacea burrows 0.5X

A common boring/burrowing isopod genus on the west coast is Sphaeroma, generally found in marsh banks (peat, mud, clay), but also able to burrow into more solid substrata such as wood, styrofoam floats, and even friable rock.  Two of 3 species present are introductions from New Zealand/southern Australia and one of these, S. quoianum is now common in salt marshes such as in San Francisco Bay and San Diego Bay.  Researchers based primarily at the Scripps Institution of Oceanography, La Jolla, California provide comparative information on the 2 habitats in order to understand better the potential of the invader to erode California wetland resources.  The isopods are intertidal and form extensive anastomosing burrow systems beneath vegetated marsh ecosystems (see photograph on Right).  At densities in excess of 13,000 individuals ^. m^-2 in some San Diego Salicornia-vegetated habitats, Sphaeroma’s burrowing activity can undercut the marsh bank to levels exceeding 1m per year (see photograph on Left).  Talley et al. 2001 Mar Biol 138: 561. 

NOTE  the native species is S. walkeri; the 2 alien species, S. quoianum and Pseudosphaeroma campbellensis

NOTE  each burrow is 0.6 x 2cm in dimension

Wax cast of anastomosing burrow system of
Sphaeroma quoianum in a San Diego marsh

Marsh in San Francisco Bay showing erosive
effects of high densities of burrowing Sphaeroma

A later study shows that Sphaeroma quoianum is quite abundant in at least 16 estuarine embayment locations from northern Baja California to mid-Oregon (see map).  In some of these locations, populations exceed several thousands of individuals ^. m^-3, and their burrowing networks have seriously exacerbated shoreline erosion and caused damage to marine structures.  The species was initially introduced into San Francisco Bay via ship fouling/boring in the mid- to late-1800s.  The authors compare habitat characteristics in 3 locations, in Tasmania, Australia, and Coos Bay, Oregon in order to explain the species’ colonisation success in west-coast estuaries.  Davidson et al. 2008 Biol Invasions 10: 399; see also Davidson 2008 Crustaceana 81: 155 and Davidson et al. 2008 J Exp Mar Biol Ecol 354: 144, and Davidson & de Rivera 2010 Mar Ecol Progr Ser 419: 129.

NOTE  there is one native species on the west coast, S. walkeri, and 2 non-indigenous species, S. quoianum and Pseudosphaeroma campbellensis