Research Study 1

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.

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

Research Study 2

Fig. 1.  Is this a camouflaged Aplysia californica?

Courtesy Kevin Lee, Fullerton, California

An 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 Aplysia.  But is it camouflage?  If so, from what?

NOTE color 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 observations on A. californica, perhaps the subject of colour in sea hares should be re-investigated

Research Study 3

Courtesy Bill Rudman, Sea Slug Forum, Australia

Fig. 2.  This Phyllaplysia taylori actually seems to be crawling on degrading seaweed.  On its usual seagrass habitat, the mimicry is quite striking (to a human eye)

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 (Fig. 1).  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 appears to have been done on the functional significance of this apparently defensive mimicry.

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

Research Study 4

Fig 3.  Selection of eggs of the Atlantic sea hare Aplysia punctata showing dietarily derived colours: yellowish from green-algal diets and reddish from red-algal diets.  With practise the precise algal diets of each of the 6 animals producing the egg strings shown here can be determined (Carefoot, 1965. pers comm)

Fig. 1.  Rostanga pulchra

Fig. 2.  Egg mass of Rostanga pulchra

Egg ribbons of west-coast nudibranchs are frequently coloured.  For example, those of Doris montereyensis and Anisodoris nobilis are yellow; Rostanga pulchra (Figs. 1 - 2), reddish or 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. 

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 (Fig. 3) 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 5

Fig. 1.  Rostanga pulchra presumably feeding on its favoured food sponge Ophlitaspongia pennata

Later study at the Bamfield Marine Sciences Centre, British Columbia shows that Rostanga pulchra (Fig. 1) 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 the present study 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 two 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. 

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 used more than once in the experiments, leading to pseudoreplication and invalid statistics.  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, capable of diffusing over a distance

Research Study 6

Fig. 1.  Sea hare Aplysia californica with white patches

Fig. 2.  Details of white patches in skin of Aplysia californica

White patches on the skin of the sea hare Aplysia californica (Fig. 1) and some other sea-hare 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 (Fig. 2).  As for function, the authors propose that 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. Oddly, 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. 

NOTE this is a less stable form of calcium carbonate and is much more soluble than calcite (the most common calcium compound in marine organisms such as coral and 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 ISOPODS>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 7

Fig. 1.  Pigment-bearing skin cells in Aplysia californica

Sea hares Aplysia spp. feed on seaweeds including mostly reds, but also some green and brown species.  With a few exceptions, their skin colours are mostly bland and uniform, and a few authors have termed them “camouflaging”, especially relating to young stages.  Of the two 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 two 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 (Fig. 1). The vacuoles are stuffed with particles of about 3nm diameter, similar in size to those in ink-release vesicles of the ink gland³. 

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

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

NOTE³ see also NUDIBRANCHS & RELATIVES >PREDATORS & DEFENSES >INK & OPALINE SECRETIONS