title for a learn-about section in A SNAIL'S ODYSSEY
  Habitats & ecology
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Nemertean parasites of crabs


Topics on habitats & ecology include nemertean parasites of crabs, considered here, while topics of INTERTIDAL/SEMITERRESTRIAL HABITS and PHYSIOLOGICAL ECOLOGY are dealt with in other sections.

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Research study 1

table showing host/parasite relationships in decapod crustaceans on the west coast of N.A.Several species of Carcinonemertes nemerteans commonly parasitise the eggs of crabs on the west coast. The accompanying table lists host-Carcinonemertes pairings that have been described for the west coast, although the list is by no means complete. Kuris 1978 Biol Bull 154: 121; Roe 1979 Biol Bull 156: 130; Wickham 1980 Biol Bull 159: 247; Wickham et al. 1984 Biol Bull 167: 331; Wickham 1986 Can J Fish Aquat Sci 43: 2295; Shields & Kuris 1988 Hydrobiologia 156: 31; Shields et al. 1990 Can J Fish Aquat Sci 47: 1275; Shields & Kuris 1990 Fish Bull 88: 279; Kuris et al. 1991 Can J Fish Aquat Sci 48: 559; Shields 1993 Hydrobiologia 266: 45; Torchin et al. 1996 J Parasitol 82: 449.

NOTE  the first confirmed description of C. errans as an egg-predator of Metacarcinus magister was apparently in 1979, although the author cited below notes that its predatory role was referred to in earlier publications.  Wickham 1979 J Fish Res Bd Can 36: 1319.

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Research study 2

graph showing feeding rate of nemertean parasites Carcinonemertes errans on eggs of Dungeness crabs Cancer magisterInfestation of commercially important Dungeness crabs Metacarcinus magister by Carcinonemertes errans in California may be in such numbers that the worms kill all the eggs in the female’s clutch.  In Bodega Bay, California, numbers of worms goes up with crab size, reaching almost 200 nemerteans on crabs of 6-7cm carapace width.  Collections of crabs in the San Francisco region disclose mean infestations of C. errans on ovigerous Dungeness crabs of 29,000 per individual. Each adult nemertean may consume up to 70 crab eggs, out of approximately 2 million carried by an average M. magister during its host’s breeding period.  Competition for food seems to exist within the egg mass, for consumption rates decline with increasing density of worms in the mass. Carcinonemertes errans and other parasitic nemertean species have been implicated in the demise of the California Dungeness photograph of gravid crab Cancer gracilis courtesy Dave Cowles, Walla Walla University, Washingtoncrab fisheries and may also be having deleterious effects on populations of Alaska king crabs Paralithodes camtschatica.  The author notes that this nemertean species is specific to its single host species M. magister.  Wickham 1979 Mar Biol 55: 45; Wickham 1980 Biol Bull 159: 247; Wickham 1986 Can J Fish Aquat Sci 43: 2295. Photograph courtesy Dave Cowles, Walla Walla University, Washington wallawalla.edu.

Gravid cancrid crab Cancer gracilis 0.6X

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Research study 3

The nemertean genus Carcinonemertes lives parasitically on several species of crabs including Hemigrapsus oregonensis, Pachygrapsus crassipes, Metacarcinus magister, and other species.  On non-breeding hosts the worms inhabit basal joints of the walking legs and chelipeds, live within the branchial chamber, and are often associated with the pericardial sacs posterior to the gills.  In gravid female crabs, about 90% of the worms frequent the undersurface of the abdominal flap where they feed on eggs.  Carcinonemertes epialti grow to maturity and reproduce only on female crabs brooding eggs.  In collections made at Elkhorn Slough, California throughout the year, about 70% of H. oregonensis carry C. epialti at a density of 15 worms per individual.  In comparison, 47% of P. crassipes are infested at a density of only 3 worms per individual.  Roe 1979 Biol Bull 156: 130.

NOTE  these spongy-textured organs are more prevalent in crabs showing terrestrialisation.  They can be pressed onto the substratum through a gap in the overhanging carapace of the branchial chamber, and appear to act in absorbing water directly from the substratum in order to keep the gills moist

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Research study 4

photograph showing nemerteans Carcinonemertes errans clustering around epimeral suture line of a crab that is preparing to moult, courtesy Wickham et al. 1984 Biol Bull 167: 331When the host crab moults, its load of nemertean parasites Carcinonemertes errans must transfer from the exuviae to the freshly moulted crab. In a study on Metacarcinus magister at the Bodega Marine Laboratory, California show that on average 88% of the resident worms migrate safely to the new exoskeleton.  Just prior to the moult the worms cluster around the epimeral suture on the posterio-lateral surface of the carapace (see photo).  It is at this suture that the carapace splits to allow the crab to divest itself of its exoskeleton.  Additonally, worms on the male must move to the female if they are to have access to eggs as food.  They do this during amplexus, and the transfer is almost wholesale.  For example, in 2 instances where such transfers were recorded, 90% of 5000 worms and 85% of 3200 worms moved from male to female M. magister during amplexus.  Prior to copulation the worms gather under the folded abdomen flap near the copulatory appendages, thus facilitating transfer to the female during mating. The cues that trigger this transfer are presently unknown.  Wickham 1980 Biol Bull 159: 247. Photograph courtesy Wickham et al. 1984 Biol Bull 167: 331.

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Research study 5

How are the egg-eating parasites able to subsist on male crabs or on non-gravid female crabs? A study at the Bodega Marine Laboratory, California shows that juvenile Carcinonemertes errans are capable of taking up several types of amino acids from dilute solutions.  The authors’ measurements show that concentrations of primary amines on Cancer-crab joints are greater than 200µM in the absence of juvenile nemerteans, but drop to 2-6µM in their presence, suggesting that the juveniles are using the amines as a nutrient and energy source.  Roe et al. 1981 Comp Biochem Physiol 69A: 423.

NOTE containing NH2+, as amino acids

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Research study 6

Adult nemerteans in the genus Carcinonemertes are parasitic on crabs and eat the eggs of gravid females.  The smallest juveniles of these worms, however, seem not to be able to feed.  Apparently, their probosces are too short to be be extended from the mouth.  The juveniles live for several months on their hosts, but are rarely seen in areas other than in water films on the arthrodial membranes, where they remain attached by mucus or scurry about until it is time to migrate to the abdomens of females, feed on eggs, and eventually reproduce.  The proximity of the juveniles to these potentially diffusible membranes gives a clue as to the source of their nutrition.  Scientists at Bodega Marine Laboratory, California show the concentration of primary amines in water films near the arthrodial membranes to be up to 1000-fold greater than in seawater.  Also, comparison of membrane films on crabs with and without juvenile nemerteans indicates lower concentrations of primary amines in the presence of the worms.  Removal of worms from selected membranes causes an immediate rise in concentration of amines in the associated water film, suggesting an active uptake by the nemerteans.  Next, by mounting small segments of membranes in miniature 2-celled chambers, the scientists are able to measure a considerable degree of amino-acid flux across them.  The high permeability of the membranes, especially to low molecular-mass amino acids, suggests to the authors that this photograph of soft suture on the joint of a walking leg of a Dungeness crab Cancer magisterleakage constitutes the primary nutritional source for the juvenile nemerteans.  Crowe et al. 1982 Am Zool 22: 671; Crowe et al. 1983 Bioscience 33: 192; see also Roe et al. 1981 Comp Biochem Physiol 69A: 423 and Wickham & Kuris 1985 Am Zool 25: 127; for review of nemertean parasites of crabs see Coe 1902 The Am Nat 36: 431.

NOTE the soft sutures that allow joints to articulate, as in the legs and eye sockets.  In Cancer they consist of numerous, loosely packed layers of cuticle, in total less than 0.1mm thick.  The layers are permeated with pores, presumably through which the amino acids diffuse


Arthrodial membrane on the walking leg of a Dungeness
crab Metacarcinus magister 2X. If the membrane is diffusible,
why don't we see hemolymph being forced through it by
the pressure of the circulatory system? Also, what is the
function of the diffusibility? In semi-terrestrial and terrestrial
crabs, it could serve to keep the membranes moist and
supple, but in a subtidal form like Cancer , this function
would be met by seawater bathing the membranes

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Research study 7

Do crabs have any defenses against Carcinonemertes parasites?  Apparently the worms have no protective toxins which suggests to some researchers that egg-predation is a low-risk venture.  Speculation on possible defenses by the host crabs includes protection conferred by the enveloping membranes on the eggs, and by possible aversive substances secreted by the female host or by the eggs themselves when they are attached to the host’s pleopods.  The possibility of such substances is suggested by the fact that eggs in vitro are eaten more readily than eggs in vivo.  The topic of defense would be a good subject for research.  For review of literature up to 1987 see Kuris & Wickham 1987 Bull Mar Sci 41: 151.

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Research study 8

photograph of a shore crab Hemigrapsus oregonensis bearing eggsPerhaps unsurprisingly, there can be considerable temporal variation in the intensity of infestation of Carcinonemertes on a specific crab host.  For example, in a “non-outbreak” period for the non-commercial shore crab Hemigrapsus oregonensis infested with C. epialti in Bodega Harbor, California, females suffer about 6% egg mortality.  In an “outbreak” period, however, as recorded during autumn 1982 in the same location, infestation may be such that over 80% of the female crabs experience 75-100% brood mortality.  During this time, worm density on the crabs may reach almost 300 per individual.  More worms occur on male crabs than females during the outbreak period, but maturation of the nemerteans occurs only on the ovigerous females.  The authors’ findings for the non-commercial grapsoid crab H. oregonensis suggest that fishing pressure, thought by some to be a factor in promoting epidemic outbreaks of Carcinonemertes spp. in certain heavily fished commercial crab species, may not necessarily be involved in such outbreaks.  Shields & Kuris 1988 Hydrobiologia 156: 31.

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Research study 9

photograph of green crab Carcinus maenas courtesy Fred Uglow, University of Hull, EnglandSurveys conducted in Bodega Harbor, California in 1995, 5 years after the green crab Carcinus maenas became established in California from Europe, indicate infestations of the parasitic nemertean Carcinonemertes epialti on this crab species comparable to those on one of its favoured indigenous host species Hemigrapsus oregonensis (79 vs. 98%, respectively).  In laboratory experiments, the authors confirm that the nemertean eats C. maenas eggs at similar rate as it does H. oregonensis eggs.  Torchin et al. 1996 J Parasitol 82: 449. Photograph courtesy Fred Uglow, University of Hull, England.

NOTE  Carcinus maenas arrived in San Francisco Bay,  California in 1989-90, and by 1996 had spread south to Monterey and north to Humboldt Bay.  In Europe the green crab is infested with its own nemertean egg predator, Carcinonemertes carcinophila, but there is no record up to 1996 that this nemertean was introduced along with its host

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