Adult A. villosus usually measure 8 to 12 mm in length however specimens have found to reach up to 15mm (Willan & Coleman, 1984). The juvenile specimen collected for analysis from Heron Island measured 8mm in length as seen in the below video.

Juvenile Aegires villous collected from Heron Island, Queensland Video: Kathleen Doody

 

A. villosus is a slender, high bodied nudibranch equipped with numerous elongated papillae covering the mantle and tail, as well as several smaller papillae which are concentrated around a snout-like mouth.

The mantle is thick and tough due to calcareous spicules ramifying throughout the body wall and papillae (Beesley et al. 1998; Cimino et al. 1999). The papillae are narrow at the base and terminate in an enlarged, rounded cap which is given a ‘furry’ appearance due to projecting spicules.

 

A ring of particularly long branching papillae surround, and protect the gills which are located postero-dorsally. A varying number (generally two or three) of partially transparent, bipinnate gills are arranged in a ring around the anus and cannot be retracted - a common characteristic of the superfamily Anadoridoidea (Phanerobranch nudibranchs) (Beesley et al. 1998). 

Two smooth rhinophores are located antero-dorsally on either side of the head and are protected by three smaller papillae which form a sheath into which the rhinophore can be retracted. The mantle skirt is reduced to a ridge of small papillae along the sides of the body and the anterior edge of the foot is blunt.

 

This species displays considerable colour variation between individuals however the general palette is a white body with yellow, pink and purple patches. In some individuals the yellow colouration can dominate and reach deeper golden-orange shades. Magenta spots or stripes are usually found on the dorsum and rhinophores are always either tipped with magenta or are uniformly magenta. The majority of papillae are also tipped with magenta however the smaller papillae lining the mantle skirt and the neck are white. 

Apart from individual variation in colour, this species also shows variation in morphology between life stages and location. The papillae of juveniles are proportionally shorter than that of adults and it has been noted that individuals found off the coast of South Australia tend to be more robust with shorter papillae than specimens found in tropical waters (Rudman, 2005).

 

A. villosus are benthic crawlers known to be associated with the cryptic community of coral reef habitats in temperate, tropical and sub tropical waters. They are most commonly found on the underside of dead coral boulders and amongst hydroid communities, usually at depths of 0-9m (Willan & Coleman, 1984). Specimens have however been photographed at up to 25m deep (Rudman, 2000a). 

 

The specimen collected from Heron Island was found on the underside of a coral slab taken from the reef crest. This slab also housed several species of Annelidia, Nematoda, Hydrozoa, Anthozoa, Cypraeidae, Crustacea, Ascidiacea, Rhodophyta and Polyplacophora.

 

A. villosus are predatory carnivores which are thought to feed exclusively on calcareous sponges (Bertsch, 1980 in Beesley et al. 1998). While it is not confirmed exactly what species of sponge A. villosus feeds on, specimens have been observed feeding on what is though to be the genus Leucetta (Yonow, 2016) and Clathrina (specifically Clathrina coriacea) (Cobb, 2016). Clathrina coriacea is an encrusting sponge which, similar to A. villosus, is found under coral boulders in the intertidal and under overhanging rocks or crevices in the subtidal (van Soest, 2016). Many species of Lecetta have an Indo-pacific distribution similar to A. villosus and are also found in crevices and under rocks in shallow marine habitats. Due to a high level of feeding specificity in opisthbranchs, specimens are usually found on or around their food source. Hence, the similarities in habitat and distribution between A. villosus and these sponges supports the hypothesis that they are a potential food source.

  

  

While the loss of a protective shell has led to spectacular radiation in nudibranchs, it also resulted in a soft and vulnerable body. As such, nudibranchs would have had to develop effective defensive strategies in parallel with the loss of a shell in order for this to be a viable evolutionary strategy. Historically however, defense in nudibranchs is not well understood due to a lack of precise knowledge about food sources and predators. In particular, the defensive strategies of A. villosus is yet to be investigated at all. 

Pedators that are known to prey on other species of nudibranch and which may therefore prey on A. villosus include turtles, other nudibranchs, crabs and birds (Rudman, 2000c).

One defensive strategy employed by some nudibranch species is crypsis, where the animal has evolved a body form similar in texture and colour to its food source. This makes them more difficult to spot and therefore less vulnerable to predation. The bright pink, yellow and magenta coloration of A. villosus however makes this species stand out quite significantly both from the coral rubble substrate as well as against the pale yellow colours of its food source. This therefore suggests that another method of defense is being employed and that their colouration may be more  aposematic than cryptic.

 

It is well documented that certain groups of Dorid nudibranchs, including the chromodorids and some Notodoris species (the second genera within Aegiridae along with Aegires) are able to store defensive chemicals obtained from their sponge prey in glands on their epithelium (Beesley et al., 1998). This ability has not been described in A. villosus however, as they too feed on sponges and as it is hypothesised that ancestral nudibranchs had some level of chemical defense and aposematic colouration (van den Burg, 2015), it is quite possible that this ability does exist.

Alternatively, the spicules found throughout the body of Aegiridae may present some defensive value by making this species less palatable. However spicules may be more structural than defensive, as their presence alone has often been found ineffective in deterring predators (Van Alstyne et al. 1992).

Another explanation for the warning colouration of A. villosus is that they are Batesian mimics and that they are copying the colouration of a defended species in order to gain the protective benefits of those colours. While this method of defense is observed in other nudibranchs, and to a certain extent A. villosus does share the colouration of a few other species, no obvious links have yet been made to a morphologically similar defended species. 

Nudibranchs are a semelaprous species which die after a single spawning event and therefore generally live for less than a year. It is not yet known how long the life span of A. villosus is, however lifespan, life history and developmental strategies of opisthobranchs is often linked to the stability of their primary food source. Species which feed on transient food sources such as hydroids tend to have shorter life cycles and faster embryonic development. In contrast, species such as A. villosus that feed on more stable sources including sponges, tend to have longer life cycles and slower development (Beesley et al., 1998). 

Repoduction in A. villosus is yet to be described however, like all opisthobranchs, this species is hermaphroditic. Self-fertilisation does not occur in opisthobranchs, instead an individual must find a mate with which to perform reciprocal copulation. 

Anatomical analyses of the A. villosus reproduction system by Fahey and Gosliner (2004) place the penis and the vagina close to each other, anteriorly on the right side of the body (see Anatomy and Physiology). This suggests that this species performs “head to tail” copulation, where two individuals line up their right sides and each inserts their penis into the partner’s vagina. No parental car has been observed in dorid nudibranchs and it is assumed but not confirmed that individuals mate with multiple partners throughout their lifetime (Beesley et al., 1998).

Fertilisation does not alway occur immediately following copulation, exogenous sperm can be stored and fertilisation delayed for up to several months until environmental conditions are optimal and/or the eggs become mature (Trueman & Clarke, 1985). Aegires are oviparous and lay flat ribbons of multiply encapsulated eggs, to which a thick layer of protective mucous is added (McKenna, 2016). In dorids, the ribbon is usually arranged in a spiral pattern and attached to the substrate along one edge (Beesley et al., 1998).

 

After approximately a week the eggs hatch into planktotrophic veliger larvae which disperse and feed within the water column until chemical detection of an appropriate food source induces settlement. Once settled, the planktotrophic larvae undergo metamorphosis, absorbing the soft body parts and casting off the shell to become a benthic predatory adult form (Bonar, 1978).

 

A. villosus is a slow moving benthic crawler which moves along the substrate using a combination of a wave like actions of the muscular foot along with cilliary creeping (Willan & Coleman, 1984).

Locomotion of a juvenile Aegires villous from Heron Island, Australia Video: Kathleen Doody

The eyes of opisthbranchs lie beneath the mantle and are thought to function only in detection of light (Beesley et al., 1998). This may aid in predator detection and maintenance of a circadian rhythms, however detection of nearby objects relies on the chemosensory and mechanosensory function of rhinophores and oral tentacles. Detecting distant objects relies solely on chemoreception and as such, nudibranch olfaction plays a vital role in many behaviours such as aggregation, feeding, mating, settlement and avoidance (Chase, 2002). 

 
 
 

A. villosus is acoelomate like all molluscs and instead has a blood-filled cavity called a haemocoel which provides gas, nutrient and waste transport as well as acts as a hydrostatic skeleton. In nudibranchs  there is only one haemocoelic space which is considered to be ‘open’ due to the lack of an endothelial lining. A two chambered heart (a single aorta and ventricle) pumps oxygenated haemolymph into these haemocoelic spaes where organs are bathed and gas and nutrient exchange occurs. Veins transport deoxygenated haemolymph to a nephridium before passing over the gills and back to the heart. Podocytes are found within the atrial wall and are the site of ultrafiltration (Ruppert et al. 2004).

Anadorid nudibranchs such as A. villosus are predatory carnivores which show specialisations in tooth and gut morphology for feeding on specific types of marine invertebrates. The digestive system of A. villosus is typical of that displayed in other Aegires species and follows the general anatomy shown below.

 

Inside the mouth is a muscular buccal bulb, which in A. villosus is oval shaped and contains a large radular sac. The oral tube is lined with ovoid oral glands which secrete digestive enzymes, while two salivary glands sit along the top edge of the buccal bulb to provide mucous in aid of digestion (Fahey & Gosliner, 2004; Behrens, 2005). An oesophagus leads to a stomach which is connected to a large posterior digestive gland which produces digestive enzymes, absorbs nutrients and excretes waste products (Rudman, 2000b). A. villosus has an unbranched or holohepatic digestive gland as opposed to a branched, cladohepatic gland (Beesley et al., 1998). A relatively short intestine leads off from the stomach and carries fecal pellets to the postero-dorsal anus.

 

The radula is the primary feeding structure in opisthobranchs and is made up of transverse rows of lateral teeth made of chiton attached to a radula ribbon.  Radula morphology is often used as a method of discerning phyletic relationships among opisthbranchs, as specialisation in a diverse range of food sources has lead to equally diverse and specialized morphologies (Gosliner, 1987). 

The radula of adult A. villosus is well developed and lacking a rachidian (midline) tooth. The tooth arrangement is described by the radula formula 16 x 17.0.17 which indicates that there are 16 rows of 34 teeth (17 on each side of the midline) with no midline/rachidian tooth present (represented by the ‘0’) (Fahey & Gosliner, 2004). Considerable variation in radula formula has been reported however some of this variation may have been due to differing ages of the specimens or past difficulties in counting the smallest teeth. All teeth are hooked with the innermost teeth and the second lateral tooth being smaller than the remaining teeth (Farran, 1905; Risbec, 1928; Edmunds, 1971; Fahey & Gosliner, 2004). The adult jaw of A. villosus consists of a thin chitinous lining of the buccal bulb and a single upper jaw plate (Beesley et al., 1998). 

       

The A. villosus reproductive system is traulic, meaning there are three ectodermal ducts connecting the ampulla to the exterior which work to prevent self-fertilisation (Fahey & Gosliner, 2004). These ducts, known as the vas deferens, the oviduct and the vaginal duct, respectively carry endogenous sperm (autosperm), eggs or zygotes and exogenous sperm (allosperm) (Trueman & Clarke, 1985). The ovoid ampulla stores autosperm and is connected to a long, thick prostate which adds a sticky secretion during copulation. After it narrows, a slight widening at the end of the prostate leads into the ejaculatory duct and the connecting penis is equipped with internal hooks at the distal end. This species has yet to be examined for hooks within the vagina, however the shape has been described and long and narrow. Proximally the vagina connects to the bursa copulatrix, which is large and round with a thin cell wall. This is joined by a long duct to the ovoid receptaculum seminis (distinguished by its thick muscular wall) and the female gland mass. The female gland mass in nudibranchs usually consists of three discrete regions, the albumen, membrane and mucous glands, however the presence of all three in A. villosus is yet to be determined (Trueman & Clarke, 1985; Fahey & Gosliner, 2004).

 

The central nervous system of A. villosus is typical of that in all Aegires species, and like all nudibranchs displays the most densely arranged nervous ganglia of all the opisthobranchs (Chase, 2002). The cerebral ganglia and pleural ganglia are fused to form a cerebro-pleural complex on top of which sits a pair of large, non-protruding eyes. The pedal ganglia are small and joined by the uncrossed visceral nerve loop, which is a synapomorphy of opisthobranchs (Chase, 2002; Fahey & Gosliner, 2004). Four nerves are connected to the cerebro-pleural complex while only three nerves are connected to the pedal ganglia. Lastly, a pair of buccal ganglia are located beneath the oesophagus (Fahey & Gosliner, 2004).

The body wall of Dorid nudibranchs is thick and embedded with mucous glands, while a collagenous matrix underlying the epithelium is supported by highly perfused calcareous spicules (Beesley et al. 1998). While the function of spicules in A. villosus has yet to be determined it is possible that they aid in defence by making the body wall less palatable to predators. However studies have found that the presence of spicules alone is often ineffective in deterring predators and as such, they have been proposed to be more structural than defensive (Penney, 2008). 

In A. villosus spicules are arranged in ovoidal patterns on the mantle (Cimino et al. 1999) but are also found in the rhinophores and the foot. Spicules can easily be seen on external inspection of A. vilosus, appearing as striations on the mantle, as well as projections at the top of papillae.