Defenses
 

Nudibranchs, and opisthobranchs in general, have a repertoire of defenses unmatched in any other invertebrate group.  Our knowledge of predators of nudibranchs & their relatives is scanty, even though there are numerous isolated examples of predation.

Defenses include camouflage (crypsis), considered in this section, and
FAST CRAWLING & SWIMMING,
MUCOUS COATINGS,
CERATAL AUTOTOMY,
NUTRITIONAL CONTENT
,
SPICULES,
NEMATOCYSTS
,
VACUOLATED SKIN WITH PROTECTIVE SPINDLES,

ACID SECRETIONS,
INK & OPALINE SECRETIONS
,
SECONDARY METABOLITES,
ALARM PHEROMONES
,
APOSEMATIC (WARNING) COLORATION & BATESIAN MIMICRY, and
NAVANAX: A SPECIAL CASE STUDY
, considered in other sections. 

NOTE  one known predator of opisthobranchs is the cephalaspidean Navanax inermis

NOTE  lit. “away signal” G.  Brightly demarcated colour patterns, as the yellow and black markings on wasps, warn potential visual predators, such as birds (and humans), to stay away.  In toxic marine fishes, on which most research on marine warning colorations has been done, such aposematic markings are often blue or yellow – colours that transmit well through seawater.  Each colour pattern that warns of toxicity has to be learned anew by each naïve predator (juvenile?), and each encounter can result in a loss of an individual to the predator – but with the potential for long-term benefit to the species

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  Camouflage (crypsis)
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Research study 0.0
 

An early report on the biology of the sacoglossan Olea hansineensis at Friday Harbor Laboratories, Washington notes a behaviour termed “death feigning” when disturbed.  This manifests as a complete cessation of movement for a few moments, then resumption of normal behaviour.  Whether it is a defensive strategy, or something else, is not known but, as it has been mentioned in reference to behaviour of other opisthobranchs, it might be deserving of further research.  Agersborg 1923 The Nautilus 36 (4): 133.

NOTE  the observation is included in this section because, if truly defensive,it could be considered a kind of “behavioral camouflage”

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Research study 0
 

photograph of sea hare Aplysia californica courtesy Kevin Lee, Fullerton, CaliforniaAn early publication on skin colour of sea hares Aplysia californica collected from around Palos Verdes, California notes variable brown, dark green, and grey coloration patterns in shallow-water specimens, and red coloration in deeper-water specimens.  From fecal-pellet analyses of these latter individuals, the author identifies the red alga Plocamium pacificum as the major dietary component.  In contrast, fecal pellets of the green- and grey-coloured individuals consist mainly of remnants of the red algae Ceramium eitonianum and Chondracanthus (Gigartina) canaliculata.  On the bases of this observation and experiments where different colour variants noted elsewhere could be changed to a common indistinguishable pattern by feeding them on a diet of parsley and celery leaves for 1-3mo, the author concludes that diet is the principal determinant of skin colour in AplysiaWinkler 1959 Pac Sci 13: 63. Photo courtesy Kevin Lee, Fullerton, California diverkevin.

NOTE  coloration change in European sea hares A. punctata was thought initially to be a camouflaging strategy for individuals that settled as larvae on deeper red-algal species, then migrated slowly through the shallower zones of brown and green algae, adopting the corresponding skin colours of these foods as they went.  Later investigations showed that such migrations do not occur, nor does several-months feeding on different types of algae markedly alter skin colour.  However, in view of these results on A. californica, perhaps the subject of colour in sea hares should be re-investigated

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Research study 1
 

photograph of opisthobranch Phyllaplysia taylori resting on a blade of seagrass Zostera marinaphotograph of nudibranch Corambe steinbergae camouflaged against its food prey bryozoan Membranipora sp.There are many examples of crypsis, that is, camouflage mimicry, in opisthobranchs. One of the best examples among west-coast nudibranchs is that of Corambe steinbergae.  In colour, body form, and transparency it almost perfectly mimics its bryozoan prey Membranipora spp.  Another example of crypsis involves Phyllaplysia taylori.  On its usual habitat of blades of seagrasses and eelgrasses, the narrow body shape of Phyllaplysia and white striping blend in convincingly. No work has been done on the functional significance of this apparently defensive mimicry. Photograph of Corambe courtesy Bill Rudman, Sea Slug Forum, Australia seaslugforum.

NOTE  this type of mimicry is also known as “concealing imitation”

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Research study 2
 

photograph of dorid nudibranch Rostanga pulchraphotograph of egg mass of dorid nudibranch Rostanga pulchraEgg ribbons of west-coast nudibranchs are frequently coloured.  For example, those of Doris montereyensis and Anisodoris nobilis are yellow; Rostanga pulchra, orange/red; Hopkinsia rosacea, rose; Diaulula sandiegensis and Triopha carpenteri, white; and Hermissenda crassicornis, white/pink.  Other than Rostanga, which often lays its eggs directly onto its prey sponges and which therefore may be camouflaging, the functions of other nudibranch egg-mass colours are not known. Costello 1938 J Morph 63: 319.

NOTE  derivation of food pigments and incorporation into egg masses is well known for anaspid sea hares Aplysia spp., but not so much for nudibranchs.  Egg masses in some aplysiids, most notably A. dactylomela in the Caribbean and Gulf of Mexico regions, and A. punctata in the north Atlantic/Mediterranean region take on subtle shades of colour from the algal pigments, and may function in camouflaging the eggs from predators

 
Research study 3
 

Rostanga pulchra mimics the colour of its prey sponge, such as Ophlitaspongia pennata, by taking up and sequestering carotenoid compounds in the same proportion as found in the sponge.  The carotenoid compounds are non-polar, that is, they are poorly soluble in water. For study they must be extracted with a solvent such as hexane.  The research question asked in an investigation at the Bamfield Marine Sciences Centre, British Columbia is whether the compounds that Rostanga sequesters are the same as those that attract them to their food.  The researchers use a Y-chamber to test the attractiveness of whole sponge O. pennata and 2 extracts1, the first extracted in methanol only, and the second in methanol followed by hexane.  As noted, the second extract will have the non-polar fraction of the sponge extracts, including carotenoids.  Tests2 of the non-polar extract in the Y-chamber result in 80% “correct” choices by Rostanga.   The authors are not certain if the attractant stimulus is a carotenoid compound, but consider it likely.  If so, it is the first report of a non-polar attractant3 for a carnivorous gastropod, and indicates extreme sensitivity to detect at a distance the non-soluble compound(s) it sequesters from its prey.  Ong et al. 2001 Veliger 44: 99.

NOTE1  the extracts are set in agar blocks to allow slow diffusion

NOTE2  a test individual is considered to have made a choice if it enters an arm of the Y-chamber within 30min.  Some individuals are multiply used in the experiments, so the results should be interpreted with this in mind

NOTE3  attractants used in studies to date are polar compounds with high solubility in seawater; hence, diffusing over a distance

 
Research study 3.1
 

photograph of sea hare Aplysia californica showing white skin-patchesWhite patches on the skin of the sea hare Aplysia californica and some other species (e.g., parvula, juliana, punctata, depilans), are so common that one hardly gives them a thought.  Fortunately, some Florida researchers have done some thinking, and have provided an excellent account of the makeup of the patches and at least one possible idea for their function.  The white patches appear early in juvenile development and consist of dense aggregations of extremely large, vase-shaped, vesiculated cells.  The vesicle within each cell is bound by layers of elastic collagen fibres and is filled with numerous spherules of an amorphous calcium-carbonate substance, possibly vaterite (see photographs).  As for function, the authors propose close-view photographs of the vesiculated cells and their contents from the white patches on the skin of sea hares Aplysia californicathat the vesicles are used for excretion of excess calcium.  Apparently, the tops of the vesicles eventually contact the outside of the skin and are broken off, at which time the calcium carbonate becomes hydrated, pressure builds, and the contents are forced out.  One immediately wonders, though, why such a complex system is needed to deal with calcium salts, which are not only physiologically innocuous, but which are soluble in seawater and would presumable diffuse as freely out of the hemolymph as they diffuse in.  Many species of Aplysia have similar-appearing spots, but many don’t, which adds to the puzzle.  The authors note that sea hares require calcium for production of egg capsules and briefly consider, but dismiss, the idea that the amorphous calcium-carbonate spherules could provide a steady source of readily metabolisable calcium for the millions of eggs required to be produced.  They do not consider, or at least do not comment on, the possibility that the patches are involved in pattern disruption or other kind of camouflaging.  Prince et al. 2006 J Moll Stud 72: 405.

NOTE  a less stable form of calcium carbonate and more soluble than calcite, the most common calcium compound in marine organisms (coral, mollusc shells).  The spherules resemble in appearance and chemistry those retained by intramoult terrestrial isopods as a means to conserve calcium salts for mineralisation of the new exoskeleton (see LEARNABOUT/ISOPOD/EVOLUTION TO LAND/MOULTING)

NOTE  the authors point out several features of these spherules and vesicles that differ significantly from those of other molluscs that do store and later recycle calcium for egg-capsule and shell formation

 
Research study 4
 

electron micrograph showing fine structure of skin of a sea hare Aplysia californicaSea hares Aplysia spp. feed on seaweeds including mostly reds, but also some green and brown species.  With a few exceptions, their colours are mostly bland and uniform, and a few authors have even termed them “camouflaging”, especially relevant to young stages.    Of the 2 west-coast species, A. californica eats mostly red seaweeds and is a red-purple colour with white patches1, while A. vaccaria eats a variety of algae including both greens and browns, and is dark brownish in colour, almost black.  A description of skin coloration in A. californica is provided by researchers at the University of Miami.  Colours owe mainly to 2 types2 of cells, undescribed prior to this time: a type of epidermal cell bearing many pigment-containing vacuoles and, to a lesser extent, a pigment cell located in the connective tissue underlying the epidermis.  Each type of cell contains numerous electron-dense spherical vacuoles (0.6mm diameter; see e-micrograph on Right).  The vacuoled are stuffed with particles of about 3nm diameter, similar in size to those in ink-release vesicles of the ink gland3Prince & Young 2010 Bull Mar Sci 86 (4): 803.

NOTE1  patches contain calcium crystals (see Research Study 5 below for details)

NOTE2   the authors describe 2 other cell types bearing “electron-dense material), rhoogcytes and packet cells, but their contribution to skin colour, if any, is uncertain

NOTE3  see LEARN ABOUT NUDIBRANCHS & RELATIVES/DEFENSE/INK & OPALINE SECRETIONS for details

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