The Sensory System

The majority of nudibranchs come in a dazzling array of colours and patterns but nudibranchs themselves cannot discern image or colours (Behrens et al., 2005). Rather, there is evidence that they have primitive eye tissue, usually a pigment spot imbedded anteriorly, which is sensitive to light. In terms of finding their way around, the nudibranchs are believed to employ a sense of touch through head/oral tentacles (Behrens et al., 2005). Aeolid nudibranchs, characterised by their dorsal cerata, extend and wave their head tentacles for guidance as they move  and it is thought that dorids do this with the cylindrical feeders on the sides of their mouth in a similar manner. However, rhinophores are also used extensively in reading the environment and locating food (Figure 1) .

While it is not known if nudibranchs can smell, the rhinophores can distinguish chemical molecules in the water and are connected to the brain via large nerves (Behrens et al., 2005). Two types of cell structures are present in these nerve endings. These include ciliated cells which sense vibrations and changes in water pressure functioning as mechanoreceptors and branched (dendritic) cells facilitate chemical reception. The ciliated cells also facilitate “hearing” as the vibrations and changes in water pressure that they detect are equivalent to sound waves in air. Nudibranchs also have octoconia or otoliths which are thought to function as hydrostatic and gravitational sensors. These small spherical calcareous bodies which are contained in an organ known as the statocyst, are the evolutionary precursor to human ear bones and don’t necessarily interpret sound but spatial orientation (Behrens et al., 2005). The dendritic cells and oral tentacles play an important role in sensing chemical compounds released by their prey.

Respiration

Dorid nudibranchs, bar the phyllidids, have an “anal gill” which is easily recognised by dorsal branchial plumes that encircle the posterior anus (Figure 2)(Brodie et al., 1997).  The gills, like many other marine organisms, extract oxygen from the seawater.

Figure 2: The gill plume of D.nigra. Original photo by Elsie-Mary Felix

Locomotion

D. nigra’s primary mode of locomotion is crawling using its foot located on the underside of its body  (Behrens et al., 2005). The foot is made up of an elongate inner strip of tissue (the sole) and a thicker outer band that runs all the way around the circumference of the animal and mostly keeps in contact with substrate (Figure 3). It is only when muscular contractions running from the head to the tail, that pull the slug forward, does the sole make contact with the substrate.  Through powerful contractions of the muscular outer foot the nudibranch is able to anchor itself to the substrate while the remainder of its body is stretched forward. The outer band can also be used to cling to irregular surfaces (Behrens et al., 2005). While muscular waves are almost exclusively used to crawl, the secretion of a slime trail and the beating of cilia can also be used to assist.

Figure 3: The underside of D.nigra which includes the foot and sole.

Feeding

Nudibranchs are carnivores with dorids having the highest number of species without hard mouth parts (Behrens et al., 2005). Most cryptobranchs feed on sponges and this is seen in species of Dendrodoris. D.nigra secretes digestive enzymes onto the sponge’s surface before sucking up the resulting “stew” (Young, 1966).

Reproduction

All adult nudibranchs are hermaphroditic with the reproductive organs of both sexes and genital opening located on their right hand side (Behrens et al., 2005, Valdés and Gosliner, 1999). Self-fertilization cannot be achieved so mating requires two nudibranchs align, head to tail, with their genital openings in close proximity.   As the genital papillae of the two nudibranchs touches, the penis of each individual enters the female duct of the other and fertilization occurs. Egg ribbons can then be laid by each individual. Once reaching sexual maturity, nudibranchs only have a short time to reproduce due to their relatively short lifespan.

Predation and Defences

Not much is known about predators of nudibranchs as it is difficult to distinguish their tissue in stomach contents due to their lack of shell and bones (Behrens et al., 2005). Those found to consume nudibranchs include pycnogonids, nemertean worms, crabs, starfish, some fishes and sea turtles. However there is uncertainty associated with these predators as it is not known whether the nudibranch was actively pursued as prey or whether it was scavenged after death.  In order to protect themselves, nudibranchs have evolved physical and chemical defences to compensate for the loss of their shell.  Most nudibranchs display their distastefulness through bright colours and patterns, known as “Aposematism”; however D.nigra lacks such a display.  Rather, D.nigra’s colouration may act as more of a camouflage to match the bottom environments. Nudibranchs that prey on sponges have also evolved the ability to molecularly modify deterrents they consume in sponge tissue and use it for their personal defences by storing it in their skin and egg masses (Behrens et al., 2005).