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  Types & habitats

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.

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Research study 1

schematic showing latitudinal distributions of several species of isopod Idotea on the west coast of North America
At least 18 species of Idotea are found from San Diego to the Bering Sea.  The latitudinal distributions of 9 of these species are shown on the schematic.  Menzies 1950 Wasmann J Biol 8: 155.

NOTE  the bars correspond with the geographic names on the Left, not with the dotted latitudinal lines. In addition to these Idotea species, there are several related species in the genera Colidotea and Synidotea

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Research study 1

photograph of sea-slater isopods Ligia pallasii in their natural habitat at Port Renfrew, British Columbia
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

Research study 2

photograph of mancas being released from the brood chamber of a sea-slater isopod Ligia pallasiiOn 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 photograph of typical habitat of juvenile isopods Ligia pallasii at Port Renfrew, British Columbiaalgae 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

Research study 3

photograph of semiterrestrial isopod Ligia occidentalis in Savary Island, British ColumbiaA 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 49.93o N

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

Research study 4
  drawings showing geological formation of Baja CaliforniaA study by scientists at Texas A&M University, College Station provides new and quite startling insight into the phylogenetic relationships of Ligia occidentalis populations in the area of central California, Baja California, and mainland Mexico. The species inhabits the supralittoral zone of rocky shores in these regions.  The investigation reveals a remarkable level of allopatric diversity, especially in areas of the Gulf of California characterised by past geological change.  The investigation is “close focus”, indeed, involving collections from 122 sites between central California and central Mexico, with sequencing analyses of 2 mitochondrial genes.  The remarkable part of the study is the finding of 4 main clades subdivided into at least 13 smaller clades that group according to past geography.  Note in the main illustration the sharp phylogeographic breaks on either side of the Baja peninsula and the genetic similarities of isopods in the north-eastern part of the peninsula with those of the corresponding mainland Mexico, and those of the mid-easter peninsula with ones of the southernmost part of the mainland.  The phylogeographic breaks in the lineages accord with geological theories of the map showing distribution of genetic clades of the isopod Ligia occidentalisformation of the area in question (see drawings upper Left).   For example, the distribution of lineages along the Baja Peninsula is consistent with a past mid-Peninsula seaway, a geological event that while contentious, is supported by other phylogeographic evidence (e.g., distribution of certain lizards). 

The authors credit the species’ apparent limited potential for long-distance dispersal to its incubatory mode of development, lack of a pelagic distributional phase, and poor ability to walk on sand.  The astonishing conclusion put forward by the authors is that multiple species of Ligia may exist in the region (perhaps a dozen or more), a suggestion that is bound to catch the interest of other isopodologists and phylogeographers.  The work is wonderfully presented with eye-catching and informative cladograms and other illustrations, and leaves the reader ready for more.  Hurtado et al. 2010 PLoS ONE 5: 1.

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
in southern California & Mexico,
indicated by different colours

Research study 5

map showing genetic differentiation in isopods Ligia occidentalis in southern California/MexicoIn 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

Research study 6

map showing summer seawater temperatures around Point Conception, Californiamap showing distributions of different genetic clades from Oregon to Baja California of isopods Ligia occidentalisA third contribution to this wonderful series on phylogeography of Ligia occidentalis comes from researchers at the University of California, Davis in collaboration with the research group featured in Research Study 4 above.  The focus in this paper is on genetic differences in populations around the marine geographical boundary at Point Conception, California.  Although much has been written on the effect of this boundary on distributions of marine species with planktonic larvae, not so much is known about its effect on species, such as a semiterrestrial isopod with limited dispersal potential.  The authors collect specimens from 24 sites in Oregon and California, extract DNA from leg segments, assay sequences of the mitochondrial genes cytochrome c oxidase subunit I and 16S rDNA, and combine these new data with those published previously (Research Study 4 above).  Results show that all populations north of the Point belong to a single Clade A, while all mainland populations south of the Point belong to 2 other Clades B and C (see illustration on Right).  The geographical dividing line between the 2 most divergent clades (Central California A and Southern California B) occurs at Point Conception.  The authors conclude that the genetic separation owes primarily to strong differences in sea-surface temperatures on either side of the Point (see illustration above Left), but are unclear on how these differences would actually affect gene flow past the boundary.  Their suggestions lean to abiotic factors related to sea-surface temperatures such as air temperature, humidity, and coastal fogs, but admit that further study will be needed for clarification.  Studies on other taxa have shown a limited southward movement around the Point, but these have mostly involved species with planktonic larvae.  Based on their own results and those from an unpublished doctoral thesis showing reproductive incompatibility between Clades A and B, the authors reiterate an earlier suggestion that there may actually be several species of Ligia south of Point Conception, but further contend that only the genetic lineages in Clade A (those north of the Point) be considered L. occidentalis in the strictest sense.  The authors discuss possible geological events that may have caused or contributed to the genetic separation of the 2 main clades, but suggest that the greatly reduced genetic divergence among lineages north of the Point is more likely to reflect recent post-glacial range expansions, thought also to explain low genetic diversities in other species such as whelks with similarly limited dispersal potentials.  Certainly, L. occidentalis is rapidly expanding its northernmost boundary and is now abundant on islands at 50 N latitude, several hundred kilometers north of the most northerly site sampled in the present study Eberl et al. 2013 J Biogeogr 40 (12): 2361.

NOTE the occurrence of Clade A individuals in the Northern and Southern Channel Islands is thought by the authors likely to result from rafting

NOTE whether increasing access by L. occidentalis to more northerly habitats has been facilitated by favourable water currents (rafting), transport by birds or boats, or even by more energetic use of their own 14 legs is, of course, not known

  Research study 7

map showing distribution of phylogenetic clades of the isopod Ligia pallasii along the west coast of North AmericaFurther information on the phylogeography of Ligia pallasii comes from collections made at 21 sites from Aleutian Islands, Alaska to Monterey Bay, California, and analysed by a researcher at the University of California, Davis.  Results fit the same pattern as shown from earlier studies, that genetic diversity is lower at high-latitude sites and higher at low-latitude ones.  Of 86 haplotypes defined, 92% are unique to a given site, while the 2 most common ones are shared across many of the most northernmost sites.  Three reasonably distinct clades are revealed (Northern, Central, and Southern), separated by Cape Mendocino and Point Arena (see map).  The author notes that the area north of Cape Mendocino is characterised by long stretches of sandy beaches and a scarcity of protective rock crevices that may represent barriers to dispersal of L. pallasii.  The findings are consistent with current understanding of postglacial range expansion in coastal species.  Eberl 2013 J Crust Biol 33 (2): 253.

NOTE  mitochondrial DNA sequenced from portions of the cytochrome-c-oxidase subunit-1 gene

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Research study 1

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

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Research study 1

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 photograph of wax cast of isopod burrowsphotograph of isopod-damage to marsh in San Francisco Bayburrowing 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

Research study 2

drawing of sphaeromatid isopod Sphaeroma quoianumA 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 map showing native and introduced populations of isopods Sphaeroma quoianumexceed 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

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