Feeding:
The four images above provide a cross-section of feeding methods. From left to right: Hypselodoris zephyra extruding its buccal mass placing its radula upon the sponge colony it is consuming. Melibe viridis casting its expanded oral hood over the substrate to trap small crustaceans. Nembrotha lineata has extended its oral tube down inside an ascidian to access and consume the internal organs. Bornella anguilla is using its jaws to crop off segments of hydroid to ingest.

Pooping:
The other end of the alimentary tract eliminates solid wastes from the feeding/digestive process. Here we have examples that also serve to illustrate the anal duality of the nudibranchs. Nearly all of the dorids possess an anal papilla located dorsally and posteriorly and surrounded by the gill branches, as shown here with Hexabranchus lacer at Left. The cladobranchs, on the other hand, have the anus located on the right side of the body wall, as shown here in the Right image with Tritoniopsis elegans. Both nudibranchs are caught in a scatological moment.

Swimming:
We all know that sea slugs crawl along on a slime trail but some are able to swim albeit inefficiently. Left: Sagaminopteron ornatum flaps its parapodia for emergency locomotion. Right: Aplysia reticulata is seen here lifting itself off the substrate by the same method. Other methods are lateral contractions by Bornella anguilla that see it swim with an eel-like motion while Hexabranchus (the Spanish Dancer nudibranchs) swims by dorso-ventral contractions.

Trailing:
It has been suggested that this behaviour may be associated with mating or finding a food source. Left: Trailing behaviour is shown here with two specimens of Hypselodoris tryoni. Right: Sometimes different, but closely related species, may be seen undertaking this behaviour such as this Hypselodoris pulchella tagging onto a Hypselodoris tryoni.

Sensing:
Left: Nembrotha kubaryana rears up in the water column the better to sense the chemical cues in the water detected by its rhinophores. Right: Kabeiro rubroreticulata reaching the end of a bare rock rears up in order to place its rhinophores higher in the water column hoping to detect in which direction its hydroid prey might be.

Mating:
Being simultaneous hermaphrodites sea slugs only have to find another of their species to mate rather than the opposite sex. This increases the probability of locating a mate and also doubles the egg production. Left: Here, two Hypselodoris jacksoni are coupled together via their swollen copulatory organs allowing the reciprocal simultaneous transfer of sperm. Right: A pair of Nembrotha chamberlaini exchanging sperm to produce the next generation.

Spawning:
Following simultaneous mating both parties will lay fertilised eggs. Left: A species of Hallaxa is commencing the laying of its spiral egg mass upon the substrate. The egg ribbon can be seen being extruded from the genital pore anteriorly on the left side of the body, beneath the mantle skirt. Right: A composite of a series of images showing Trapania sp. 03 laying and arranging ii’s spiral egg ribbon. The nudibranch travels in a tight anti-clockwise circle as it deposits.

Burrowing:
All of the headshield slugs are built for burrowing, both for feeding and defensive purposes. Left: There is group of aeolids that burrow for infaunal hunting – the Cerberilla. Here, Clockwise from upper left, Cerberilla ambonensis disappears into the sand before re-emerging while on the hunt for tube-dwelling anemones. Note the big oral tentacles but the small rhinophores. Right: The bubble-shell sea slug Hydatina physis creates a flurry of snow-like sand as it burrows beneath the surface to escape the attention of a diver’s light on a night dive.

Autotomizing:
Self-sacrificing of body portions as defensive behaviour is illustrated here with Tomoberthella martensi. Left: An entire animal – the fracture lines are visible. Right: A specimen that has cast off both of the lateral and the posterior segments of its mantle, exposing the gill. Many aeolids and species of Melibe are known to sacrificially autotomize their cerata.

Mantle Flapping:
Certain species of the Chromodorididae family engage in a behaviour of raising and lowering their mantle overhang. Left: Goniobranchus coi is caught in the act of raising the entire periphery of its mantle in the one motion an action it rhythmically repeats. Right: Goniobranchus geometricus usually raises and lowers only the anterior portion and only when on the move. The dark purple underside of the anterior portion of the mantle is revealed.

Startling:
A behaviour known as deimatic display is used by some sea slugs to startle potential predators. Left: Hexabranchus lacer suddenly unfurls its rolled mantle margin to reveal different colours and increase its size dramatically. Many aeolids such as this species of Unidentia, Right, will stiffen, erect and direct their cerata towards a threat.

Reaction:
Sea slugs react to many stimuli in the habitat. Here is an example of one exhibiting a dramatic response by reacting to a defensive worm bite. Left: A specimen of Hypselodoris jacksoni approaches its preferred sponge food with its buccal mass extended for feeding. Beneath the white buccal mass small polychaete worms residing within the sponge can be seen (arrowed). These will defend their territory and bite the nudibranch’s buccal mass as it presses against the sponge. Right: The nudibranch reacts by rearing back when bitten, taking a still attached worm with it (arrowed). This process occurred about six times before the nudibranch finally got the message and moved a little further along the sponge where it could feed in peace.