title for learn-about section of A SNAIL'S ODYSSEY
  Feeding & growth
  There are many bacterial and fungal diseases of mussels that are known to affect growth and survival.  These have been well studied in the context of mariculture and will not be considered in the ODYSSEY.  Parasites of mussels include intestinal copepods and mantle cavity-inhabiting crabs that must also deleteriously affect growth and survival, but no studies have yet been done to test this.
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Effect of parasites on growth & survival

  Studies on effect of parasites on growth & survival are presented in this section, while topics of FEEDING, and GROWTH are considered in other sections.
 
pair of Pinnixa sp. crabs in the mantle cavity of a mussel Mytilus californianus
A pair of pinnotherid crabs in the mantle cavity of a sea mussel Mytilus californianus, possibly a smaller male and larger female Fabia subquadrata. The crabs enter a mussel as larvae or juveniles 1X
  photograph of a Pinnixa-crab in the mantle cavity of a mussel Mytilus californianus
Female pea crab Fabia subquadrata tucked within the gills of a mussel Mytilus californianus. Pea crabs feed on material filtered from the water by their hosts 2.5X
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Research study 1
 

Sea mussels Mytilus californianus have all manner of parasitic epibionts inhabiting the external surfaces of their shells, such as seaweeds, barnacles, limpets, and so on. Most of these species also live on nearby bare rock surfaces, but the question arises as to the negative impact they may have on their hosts when they select mussel shells over rocks to settle on. Unfortunately, no-one seems to have paid much research attention to this, but such things as costs to move the mass of epibionts during normal feeding behaviour, interference of the epibionts with growth, robbery of food by them, predation on spawn (eggs) by ones that filter-feed, and fouling from fecal and urine release are a few things photograph of parasitic barnacles on shells of sea mussels Mytilus californianusthat come to mind, and there are sure to be others. One would guesss that such information would be useful to aquaculturists, and it may turn out that some studies of this type may already have been done as basic research. Lohse 1993 J Exp Mar Biol Ecol 173: 133.

NOTE the research paper cited here actually addresses the parasitic relationship from the standpoint of relative advantages to the epibionts (major ones being barnacles and limpets in the Santa Barbara, California area) in living on shells vs. rocks. It turns out that survival and recruitment of all 3 species are better on shell substratum than on rocks, but not growth or reproduction


Where have all the mussels gone? Three species of parasitic barnacles
crowd the shell surface of these sea mussels Mytillus californianus,
along with at least 8 predatory snails Nucella canaliculata and
a single goose barnacle Pollicipes polyymerus 1X

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Research study 2
 

photograph of copepod Mytilicola orientalis, a parasite of marine invertebrates including mussels Mytilus spp.The copepod species Mytilicola orientalis inhabits the intestinal regions of bivalves including oysters, clams, and mussels Mytilus spp.  A study on aspects of infestation in mussels Mytilus trossulus at the Bamfield Marine Sciences Centre, British Columbia shows that up to 80% of summer-collected mussels in sheltered habitats host the parasite.  No parasites are found in mussels from wave-exposed sites.  On average, only 2 copepods are present in each mussel, although large individuals living closest to the low tide mark may have 4 or more.  Why so few parasites per mussel and do their numbers reflect balanced sex ratios? The authors do not discuss this, but do remark on restricted distribution owing to limited transmission within sheltered, muddy estuaries.  Goater & Weber 1996 J Shellf Res 15: 681.

NOTE  both M. orientalis and M. ostrea may have been introduced to the west coast in shipments of oysters Crassostrea gigas from Japan

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