Habitat & community ecology

The section on HABITAT & COMMUNITY ECOLOGY includes a "grab-bag" selection of topics such as gene flow, considered here, and INTRASPECIFIC COMPETITION, INTERSPECIFIC COMPETITION, COMMUNITY INTERACTIONS, and HERMIT-CRAB COMPETITION & SHELL SELECTION, considered in other sections.

As a start to the subject of habitats: if you wonder how well crabs would do in derelict fishing-net habitats in Puget Sound, Washington and the nearby Straits the answer is, moderately well. Although deadly for air-breathing mammals and birds while floating, such habitats can be ideal for many species of bottom-dwelling invertebrates, including many species of crustaceans, when lying derelict on the sea bottom, mainly owing to the large relative surface area they provide. Researchers from NOAA Fisheries along with specialists from northwestern Washington have collected and analysed 870 derelict gillnets from the Puget Sound, Washington area. Their report lists the presence and health of 76 species of invertebrates, about half of which are still alive at the time of recovery. Specifically, for 25 species of crabs, of over 10,000 collected from the nets about 35% are still alive. Good et al. 2010 Mar Poll Bull 60: 39. Photograph courtesy NOAA Southwest Fisheries.

photograph showing an abandoned crab pot filled with crabs and fishesNOTE the authors cite reports indicating that some 4000 fishing nets and perhaps as many as 20,000 crab pots lie derelict on the sea bottom of Puget Sound alone. The report is fascinating to read, as it indelibly underscores the many ways that humans have found to mistreat the world’s oceans

NOTE this, of course is misleading. Crabs live mostly on the sea bottom and would find a net habitat most suitable. Many individuals would live a natural life and, after death, their carapaces could remain intact for a long time, so statistics on "survival" as given above are going to be misleading


Derelict crab pots may provide handy surfaces for crinoids to cling to, but they
are conveyor belts of lethality that continue to "ghost-fish" for as along as their
cage structure remains intact. Fishes and other motile creatures are attracted to
the scent of death, enter, die themselves, attract others, and so on, and so on

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Gene Flow

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Research study 1

photograph of shore crab Pachygrapsus crassipes courtesy Jackie Soanes, Bodega Marine Laboratory, Californiamap showing collecting sites for shore crabs Pachygrapsus crassipes for genetic studiesThe lined shore crab Pachygrapsus crassipes is broadly distributed along the west coast1 from mid-Oregon to central Baja California and, as well, is found in Japan and Korea.  Invertebrate species with the capacity for high larval dispersal usually have low degrees of population differentiation.  Indeed, DNA-sequence analysis of the mitochondrial cytochrome c oxidase I gene2 (COI) in 296 individuals of P. crassipes from 6 populations along the west coast shows that genetic variation is, as expected, low.   A total of 154 haplotypes is identified, but there are close genetic similarities among haplotypes.  If 50 individuals from a population in Korea is included in the analysis, then genetic differentiation is highly significant, indicating that trans-Pacific gene flow is restricted (no haplotypes are shared between west-coast localities and Korea).  The authors estimate the time of divergence between the 2 trans-Pacific lineages to be 0.8-1.2my3 ago.  If the west-coast population is indigenous, how was the species introduced to Japan and Korea?  A time scale of a million years effectively rules out ship-board transport on hulls or in ballast water, but perhaps not rafting or, as suggested by the authors, progressive colonisation of island stepping-stones created during Pleistocene sea-level drops.  One notable and significant break in the geographical structure of P. crassipes occurs immediately north and south of Point Conception, California, likely due to oceanographic circulation patterns in this area that result in asymmetrical migration of haplotypes.  Cassone & Boulding 2006 Mar Biol 149: 213. Photo courtesy Jackie Sones, Bodega Marine Laboratory, California.

NOTE1   a fringe population in Barkley Sound, British Columbia became established during the 1997/98 El Niño but, after peaking in population numbers in 2000, has since disappeared

NOTE2   this gene is used in studies of population genetics of a wide variety of marine invertebrates

NOTE3   a time separation of millions of years is in stark contrast to anecdotal reports, which indicate that
P. crassipes
has only existed in Asia for about 100yr (since 1890).  The authors note that the sequence divergences observed in the present study do not appear to be high enough to justify separating the
east- and west-Pacific populations into separate species

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Research study 2

map showing colder, upwelling seawater along the west coastGenetic analysis by a researcher at the University of California, Berkeley of 8 populations of shore crabs Hemigrapsus oregonensis occurring from central California to northern Oregon reveals 2 main haplotypes, an older one of which (Haplotype B in map on Left)  extends throughout the entire range.  A recently derived northern haplotype A became widespread north of Cape Blanco only after the last glacial maximum about 100, 000-yr ago.  This group is presently restricted to the northern area, while southern haplotypes extend into the north without restriction.  The author discusses possible mechanisms that would explain this latitudinal break, including historical and contemporary coastal circulation patterns relating to larval dispersal.  The time of larval release in the study area (May-July) is characterised by the predominantly southward California Current, so a correspondingly southward gene flowmight be expected.  However, populations in southern and central California may release larvae as early as February, and some populations may produce a second brood.  Thus, some fraction of larvae produced could be exposed to markedly different water-circulation patterns map showing haplotype distributions of shore crabs Hemigrapsus oregonensisthan those released during the May spawning peak.  During summer there is strongupwelling with resultant offshore flows just south of Cape Blanco and other prominentories along the coast (see map on Right), and these are thought to present major obstacles to southern dispersal of larvae.  However, mixing has occurred in the past in ways not obstructed by the mechanisms maintaining the present barriers.  The study is the first exploration of genetic population structure in H. oregonensis, and the author provides much interesting speculation on its causes.  Petersen 2007 Mar Biol 150: 1289; see also Petersen 2006 Biol Invasions 8: 565 for a genetics-based distributional study of the crab Rhithropanopeus harrisi, introduced into California in the 1930s and morphologically similar to H. oregonensis.

NOTE  sequence variation at the mtDNA cytochrome-oxidase subunit 1 locus

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Research study 3

photograph of Dungeness crab Cancer magistermap showing collection site for study of genetic structure in Dungeness crabsAn investigation by researchers at the Pacific Biological Station, Nanaimo of population structure of Dungeness crabs Metacarcinus magister at 8 locations in British Columbia reveals general genetic homogeneity throughout the province save for a single population in Allison Sound (see map).  Crabs from Allison Sound have only about 65% of the alleles observed in the other 7 populations. The Sound is characterised by poor water exchange with outside waters, and so the results are consistent with a high level of retention of larval crabs within the Sound.  The researchers report a few other small differences among the remaining 7 populations but conclude that, overall, regional variation is absent in M. magister in British Columbia.  Beacham et al. 2008 J Shellf Res 27(4): 901.

NOTE  analysis of variation at 8 microsatellite loci

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Research study 4

An investigation by a consortium of EPA and university researchers on genetic structure in west-coast populations of green crabs1 Carcinus maenas finds high population connectivity in sites from British Columbia to California.  Results show low genetic2  diversity with expansion likely from a single introduction of a small number of founder individuals to San Francisco Bay from the east coast of North America.  From there, spread was mainly by current-transport of larvae.  The authors suggest that C. maenas will likely continue to expand northward into suitable habitats in northern British Columbia and southeastern Alaska3 . Tepolt et al. 2009 Diversity Distrib 15: 997.

NOTE1  in less than a decade after its introduction in 1989-90 the species spread over 1000km

map showing predicted range expansion of green crabs Carcinus maenas into the far reaches of the Aleutian Islands, AlaskaNOTE2  crabs are collected at 21 sites and analysed for  genetic diversity at 8 microsatellite loci

NOTE3  a later investigation by another group of researchers, using projections from “ecological-niche” models to predict future range expansion by C. maenas, concludes that Alaska and other high-latitude shorelines are currently vulnerable to colonisation (see map).  De Rivera et al. 2011 Diversity Distrib 17: 1198.


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Research study 5

map showing haplotype distributions for collections of sand crabs Emerita analoga from northern Washington to Baja California in the year 2000Sand crabs Emerita analoga have a relatively long planktonic larval stage (3-4mo duration), are widely distributed along the west coast, and would be predicted to have low population differentiation.  This is investigated by a research group primarily from campuses of the University of California, which finds that while there is no significant population subdivision among north-eastern Pacific populations over 2000km distance (Baja California Norte to northern Washington), the larger region (i.e., along the north-eastern and south-eastern Pacific coasts from Vancouver Island to Chile) hosts 2 distinct phylogroups.  The differentiation of these major clades occurred between 0.9-1.9mya.  Dawson et al. 2011 J Biogeogr 38: 1600.

NOTE  through sequencing of cytochrome c oxidase subunit I in 742 crabs collected at sites spanning 2000km of coastline from northern Washington to Baja California

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Research study 6

photograph of a shore crab Hemigrapsus oregonensisIt is not uncommon to find genetic patchiness1 in larvae or settled recruits, and between larvae and adults, of benthic invertebrate species with planktonic larvae. The cause of this will vary, but in species with large reproductive output it may owe to unusually successful larval survival/recruitment both spatially and temporally. Other causes could be variability in larval source and/or fitness of larvae. This is investigated by scientists in the Bodega Bay area of California for shore crabs Hemigrapsus oregonensis by genetic2 analysis of larvae and adults from various locations in the Bay and north into Oregon. They find marked genetic patchiness among larval samples and between larvae and adults. Although genetic differences in adults in the northern sites could explain the patchiness, the researchers point to generally low past gene flow from these sites. Also, as the analyses are done on first-stage zoea larvae, any larvae originating from distant sites would be too far advanced to have been included. Sweepstakes3 reproductive success is also discounted owing to relatively low fecundity in H. oregonensis, leaving natural selection as the fallback option. Cornwell et al. 2016 Mar Ecol Progr Ser 548: 139.

NOTE1  this is known as Chaotic Genetic Patchiness

NOTE2  allele frequencies at a single nucleotide polymorphism (SNP)

NOTE3 because recruitment to a population depends upon a series of successful matching of reproductive events (gamete maturation, fertilisation, larval development, and settlement) to the variables of oceanographic conditions that are conducive to survival of each stage, this is termed Sweepstakes Reproductive Success.In other words, when conditions are good, then survival of just a few individuals’ reproductive output can be very good and their genotypes will prevail in a given cohort. This chanciness, or sweepstakes, can lead to genetic patchiness over temporal and spatial scales

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