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Defenses of nudibranchs and their relatives include spicules, considered in this section, and
, and
NAVANAX: A SPECIAL CASE STUDY, considered in other sections. 

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Research study 1

Many species of dorid nudibranchs contain spicules made of calcium carbonate.  The role played by these spicules in defense, if any, is not clear.  Ideas considered in the literature include interference with feeding and/or defecation, abrasion of gut linings, and adding of non-nutritive components to a consumer’s mouthful of food.  Ash contents (including spicules) of 7 species of non-spiculated west-coast nudibranchs average about 3% of live body mass, while ash contents of 7 spiculated species average about 8%.  Given the many experiments on possible defensive role of sponge spicules in the literature, possibly similar studies could be done on nudibranch spicules. Penney 2002 Oecologia 132: 411.

NOTE with the exception of the dorid Triopha maculata these are all aeolid nudibranchs

NOTE these are all dorid nudibranchs. A more interesting and perhaps more valid comparison would be if the author had compared non-spiculated vs. spiculated DORIDS (if that were possible) instead of intermixing dorids and aeolids as is done here

Research study 2

photograph of dorid nudibranch Cadlina luteomarginata courtesy Jeff Goddard, Santa Barbarahistogram of % spicule content of different parts of the dorid nudibranch Cadlina luteomarginataAlternatively, but not exclusively, the spicules could serve a structural role.  Whether it be dorid nudibranchs, sea cucumbers, sponges, soft corals, or gorgonians, the presence of spicules confers stiffness, and the more there are, the stiffer is the tissue.  Depending upon their size, shape, and packing, spicules also confer varying degrees of toughness and strength. Recent studies at the Bamfield Marine Sciences Centre, British Columbia on the dorid nudibranch Cadlina luteomarginata reveal a complex system of spicules and connective tissue ramifying throughout the body.  An intricate system of musculature is associated with the system.  The overall construction is suggestive of a supporting role. The arrangement of spicules as short, incompressible elements in a connective-tissue sheath would also resist tearing while allowing deformation. Spicules are present in all parts of the body (23-36% dry mass1), but are in significantly higher concentration in the outer mantle tissue (45% dry mass). A supporting function for the spicules is also suggested by their arrangement in the gill pinnules and rhinophore lamellae where, respectively, the spicules2 are arranged in vertical rows or histogram showing proportion of different potential predators of the nudibranch Cadlina luteomarginata eating artifical pellets containing spicule/extract components of the nudibranchin dense parallel rows. 

Incorporation of spicules in an agar-based food at concentrations similar to those in Cadlina mantle tissue does not deter feeding by crabs3 Cancer productus and C. gracilis, and sea anemones Anthopleura elegantissima (see histograms lower Right). In comparison, secondary chemicals4 isolated from Cadlina and incorporated into “artificial” food in physiological concentrations significantly deters feeding by crabs, but not sea anemones (third bar from Left in each histogram).  Finally, combination of spicules and secondary chemicals does not increase the deterrence of the chemicals for crabs, and only weakly increases the deterrence for sea anemones. From these data the author does not exclude a defensive role for the spicules, but concludes that their primary function is likely in body support.  Penney 2006 Invert Biol 125: 222. Photo of Cadlina courtesy Jeff Goddard, Santa Barbara and seaslugforum.

NOTE1  the author explains that for technical reasons, these spicule masses may “slightly overestimate” the true values

NOTE2  while spicule concentrations in the mantle and foot tissues are isometric with body size (slope b=0.96-1.03), those in the gills (b=1.38) and rhinophores (b=0.80) are slightly, but significantly, allometric with increasing body size.  For the gills, this indicates a disproportionately greater investment of spicules into these organs as the nudibranch grows, and the reverse for the rhinophores (the author suggests that this negative allometry may have been a product of incorporating the rhinophore base in the calculations – spicules are few or absent in the base and would possiblly contribute disproportionately (and negatively) as the animal grows)

NOTE3  omission of fishes, e.g., tidepool sculpins from this array of potential predators is somewhat surprising, in view of the potential importance of fishes as predators of nudibranchs, and perhaps they should be included in any future study

NOTE4  body wall is extracted with a combination of methanol and carbon tetrachloride.  The food is prepared from a 1:1 mixture of squid flesh and water, combined with 2% sodium alginate as a binder (set with 0.25mol . l-1
).  The overall concentration of organic matter in the food is about 12% fresh mass, similar to that of dorid-nudibranch tissue.  Food is scored as eaten when 75% of the mass is consumed

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Research study 3

schematic outlining different spicule patterns in dorid nudibranchsLater studies by the same author show that spicule networks in dorid nudibranchs vary taxonomically.  Analyses of 16 species of dorids including 10 from the west coast of North America reveal 3 general patterns as shown in the accompanying diagrams: 1) a ramifying system of thick, spiculated tract, as in Cadlina luteomarginata, 2) a cobweb-like, unbraced framework of one or few spicules per side, as present in Diaulula sandiegensis, and 3) a lattice-like arrangement of distinct radial and circumferential tracts, as displayed in certain tropical Phyllidia species.  Additionally, there are spicule tufts in some species that form clusters just beneath the epidermis. The author does not comment on possible differences in defensive potentials of the different patterns, but does suggest that the different patterns may prove taxonomically useful.  Penney 2008 Acta Zoologica 89: 311. Photograph of Cadlina courtesy Dave Cowles & Kirk Onthank, Walla Walla University, Washington and photograph of Diaulula courtesy Linda Schroeder, Pacific Northwest Shell Club, Seattle, Washington PNWSC.

NOTE  some species spicules have no spicules

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