Hypselodoris maculosa like its companions from the order Nudibranchia are sea slugs. So they have a soft body but unlike other Opisthobranchs they do not possess a shell in their adult form. They are externally symmetrical but not internally. ‘Nudibranch’ refers to the unprotected and exposed gills. This gill structure is located on the dorsal part, on the posterior back of the mantle and forms a ‘circle’ around the anus. The thick mantle extends over a big foot, used for movement. H. Maculosa is part of the cryptobranch Doris and therefore can retract its gills into a pit or pocket barely visible on the dorsal part of the body (figure 2). H maculosa has an oval shape that can be elongated or retracted with a distinct head on the anterior end. They have an average length of 45mm. They also have two retractable perfoliate rhinophores (with set of flat plates on them) and oral tentacles on the head (figure 3). 

H. maculosa display wide range of colour and pattern variabilities depending on where they come from. The one found for this study comes from the Great Barrier Reef. The dorsal part (the upper mantle) is milky white in the middle surrounded by a reddish colours in waves-like pattern. There are few white lines and two dashed line on either side of the mantle going from the Head to the tail. Along the mantle there are lots of dots of different colours (pinkish-purplish) and smaller white ones. The gills, situated close to the posterior end and the rhinophores, situated on the head, are milky white with orange tips and one orange band. The head is pinkish-purple translucent in colour many white dots and few darker pinkish-purple dots. The tail (posterior end) is really similar to the head but less spots overall. 

Few studies has been done on H. maculosa regarding the life history of this species, but a lot is known regarding nudibranchs in general. Their life span is approximately one year which is relatively short so they quickly reach their maturity. They have both fully developed sex organs simultaneously (hermaphrodite). Despite the fact that they are hermaphrodite and have viable eggs and sperms, to transmit sperm from one nudibranch to the other, two mature individuals are required as they cannot self-fertilize. The genital apparatus are located on the right side of the body which means that to copulate, nudibranchs have to be next to each other with a head to tail position (figure 5). After that, genital papilla of each individuals get into contact and then each penis enter the female openings. Once sperms are released, both nudibranchs separate and once fertilized, they both can lay egg ribbons. Fertilization can occur a long time after copulation as nudibranchs have to be healthy as well as their surrounding environment. In order to successfully exchange sperms nudibranchs have developed mechanisms to remain stick to each other when copulation occurs. Chitinous spines are either on the penis or the vagina and function as hooks until the end of copulation. Egg ribbons vary in shape, size and colour depending on nudibranchs species. They are forced out through an oviduct on the same genital papilla where the penis and vagina are located (figure 6). The number of eggs vary between species and it also depends on the size of the nudibranchs, the larval development, the number of litters and the nudibranch nutritional conditions. Assuming H. maculosa is similar to its relative, Felimare californiensis (=Hypselodoris californiensis), the average egg size is about 100µm, the embryonic period is more than seven days and the mode of development is planktotrophic. It is still unclear if it is a type 1 (typical coiled shell) or type 2 (typical egg-shaped and inflated shell) planktotrophic larva (figure 7).

Fun Fact: courtship of some nudibranchs involves fight, bite, and nuzzle for over 30 minutes. 

The main way H. maculosa moves is via its flat and flexible foot, like the other members of Opisthobranchs. This foot is separated into two muscular parts: the first one is a thick outer band which is almost all the time in contact with the substrate. The second one is an elongated inner band more commonly called sole which touch the substrate only when the animal crawl. Crawling is defined by the waves of contractions of the foot muscle from the anterior to the posterior end which pull the H. maculosa forward. In addition, they use clear and sticky mucus, produced by specialized sole cells, to help crawling. They have thousands of cilia that helps the animal to slide over their slime trail. H. maculosa, like most sea slugs, possesses amazing strength and agility of movement. They use their outer band to grip firmly to substrate, via powerful contraction of the muscle, in order to let the rest of the body stretching out forward, to eventually find new substrate to move on. 

Fun fact: Nudibranchs sometimes crawl over the body of sedentary fishes, like scorpionfishes, releasing a slime over the head without disturbing the fishes. 

Another way of locomotion has been observed in many Opisthobranchs, called tailing. H. maculosa has not been observed but members of the genus Hypselodoris have (figure 8). Tailing or tailgating consists of one sea slug following the mucus trail of another one until reaching the posterior end of the first sea slug. Then, the second makes contact with the first sea slug and both keep moving attached. This way of movement first has been thought to be linked with mating, however tailing can occur in two different species which creates doubt in this hypothesis. 

Uncommon body movements have been observed in many sea slugs. One called mantle flapping (figure 9) which is the waving movement up and down of the mantle, has been observed in many Chromodorididae but not in the species H. maculosa. Rearing (figure 10) has been also a common body movement observed in many nudibranchs. It consists of elevation of the anterior part of the body to avoid irritating substrates or to better sense food or mates location. Once again, sea slugs from the family Chromodorididae but not the species H. maculosa have been observed.

Really few is known about H. maculosa feeding behaviour. Assuming that most of its fellows from the genus Hypselodoris have similar food sources, H. maculosa might have the same digestive system (a muscular oral tube with jaw rodlets and a pharynx with radula composed of innermost, lateral and outermost radular teeth). Most Hypselodoris feed on sponges but each species has specialised diets which is difficult to determine because proof of eating from Hypselodoris is difficult to obtain. It is believed that Nudibranchs inhabit their main food sources. H. maculosa has been seen many times around the sponge species Dysidea euryspongia and therefore, it is believed that this is its main food sources (figure 11). 

Nudibranchs have the disadvantage of being ‘naked’ compared to other gastropods which possess shells. Therefore, they have developed different methods and strategies to defend themselves. The first protective method applied by H. maculosa is the retractable gills and rhinophores into protective pockets (picture 2 and 3 from Physical Description section). They also display vivid colour throughout their body which is a warning given to any potential predator. In many nudibranchs, this vivid colour can basically be translated by having unpalatable skin. Nudibranchs also use chemical defence stored in their skin. However, this skin chemical defence is not innate in nudibranchs but it comes from their food sources, sponges. Sponges have deterrent chemicals that prevent predation from almost every fish species. Yet, nudibranchs are immune against those chemicals and they actually modify the chemicals structures, store the products in their skin and use it as deterrent chemicals. It is a form of stealing defensive strategies for its personal safety. 

Fun Fact: some nudibranchs of the Aeolidina suborder feed on Cnidarians and steal their venomous nematocyst to use it as a defence mechanism. 

Nudibranchs have five senses, similar to the human ones, the vision, the touch, the taste, the hearing and the smell. It is difficult to well describe and understand those senses but few is already known.

All nudibranchs have a pair of really basic eyes, less developed than the human ones. In most nudibranchs, the eyes are located near the rhinophores on the head tissue and is basically a pigment spot. The connection between the eyes and the brain is done by an unrefined optic nerve which works as a messenger. Unfortunately, those primitive eyes are unable to discern images and colours which is a shame knowing the incredible colours and patterns displayed by those species. However, it has been proved that nudibranchs eyes are responsive to difference in light intensity. Usually, and it is the case for H. maculosa, the eye pigments are really similar to the surrounding colours (or even the exact same colour) which makes the eyes difficult to locate. 

Most nudibranchs have two oral tentacles on either side of the mouth that are thought to give the ability of sensing the touch. H. maculosa has reducing feelers of the form of cylinders which probably help to guiding through the environment. They might aid locating prey and supporting feeding processes. The rest of the body is sensible to external stimuli which assumes that a nerve system is set up throughout the entire body (rhinophores and gills included). However, the most sensible part of the body remains the head including rhinophores and oral tentacles.

It is known that every species of nudibranch has a specific food sources. This suggests that they can taste what they eat. However, unlike more developed species, nudibranchs lack taste glands. They use their rhinophores and their head tentacles to taste chemical compounds coming from food sources which provoke a feeding reaction. They can also detect other chemicals such as waste water, reproduction chemicals or egg laying secretions.

Nudibranchs hear in a different way as we, humans, do. Underwater, vibration and pressure waves’ changes are equivalent to the hearing in air. Nudibranchs do that via their rhinophores and more precisely via the ciliated cells in it. Interestingly, they also have calcareous otoliths which are the forerunner of the human ear bones. However, they probably do not have the same function because nudibranchs might use those ‘ear bones’ as hydrostatic sensors, which is an organ helping in spatial orientation rather than sound detection.

Smell in nudibranchs is achieved by rhinophores. The two rhinophores on the head are able to distinguish chemical molecules in water, which is really similar to human’s nostrils that distinguish chemical molecules in the air. Therefore, detecting odours and analyses of chemicals in water are primary methods used by nudibranchs to locate food sources. Rhinophores are composed of two distinct cells: branched bells using in chemical reception (smell) and ciliated cells using as mechanoreceptors for vibrations and waves pressure (hearing). The connection between the brain and the rhinophores is done by a pair of significant nerves.

As mentioned before, H. maculosa has branchial plumes situated on the dorsal part which are better known as anus gills. Those gills encircled the anus located on the dorsal part close to the posterior end of the body. Those delicate organs can be retracted when threatened onto a protective pocket-like structure on the surface of the dorsal part. The branchlike plume structure is the most important respiratory organ. It has the function of exchanging gases between the water and the animal. This is how H. maculosa respires (pictures from Physical Description section). 

As hermaphrodite, nudibranchs possess both sexes, penis and vagina. This implicate the presence of both reproductive systems within the same individual which makes the overall system more complex. They have a temporal storage organ for sperm after copulation which is called the receptaculum seminis. The sperm keep travelling until reaching and fertilizing the eggs into the hermaphrodite duct and then the eggs are sent to the female gland mass which is similar to the ovary. The female gland mass is divided into three sections: the eggs acquire a nutritive layer from the albumen gland, then an outer capsule from the membrane gland and finally all the eggs aggregate to each other and form a ribbon from the mucous gland.