Jorunna parva

Kingdom: Animalia

Phylum: Mollusca

Class: Gastropoda

Order: Nudibranchia

Superfamily: Doridoidea

Family: Discodorididae

Genus: Jorunna

Species: Parva

Also called the "sea bunny," Jorunna parva was discovered by Japanese scientist Kikutaro Baba in 1934. It is a species of nudibranch, sometimes called sea slugs. Nudibranchs are marine opisthobranch (meaning the gills are behind and to the right of the heart) molluscs that have no shell as adults, though some varieties may have a coiled shell as a veliger which is shed at metamorphosis. Nudibranchs can be divided into two groups, dorids and aeolids, of which Jorunna Parva belongs to the former.

Habitat

Originally discovered off the coast of Japan, they have since been found in the Indian Ocean and near the Philippines, Tanzania, Papua New Guinea, Seychelles, and Réunion. There is some debate as to whether these finds, along with coloration variances, represent a species complex rather than one distributed species. They prefer shallow, warm water and are likely to be found in areas with coral and sponges, a primary food source. They may also consume cnidarians, such as anemones, but the sponges consumed by Jorunna parva provide it toxins it is able to retain and utilize for defensive purposes. Because of their diet, Jorunna parva are made less palatable, and even toxic, to predators. Some nudibranch toxins are used in cancer research, and other species even have the ability to capture and use cnidocytes from cnidarians!

Morphology

A. Inner lateral teeth. B. Lateral teeth. C. Outermost teeth. D. Caryophyllidia

As a mollusk, Jorunna parva's body consists of three main body sections: the visceral mass, foot, and head. The species grows to an adult size of less than an inch, approximately 2.5 cm. They are nearly bilaterally symmetrical externally, though both male and female sexual orifices open on the right side of the body. The eyes are small and simple, able to distinguish dark from light, and have five photoreceptors as well as a lens and cornea. A characteristic feature of dorid nudibranchs are the posterior feathery protrusions of the mantle. These are the external, "naked" gills from which nudibranchs take their name (nudus is Latin for nude, branchia is Greek for gills). The primary gills present in other mollusks have atrophied in favor of these secondary gills for respiration, though the process may also occur though the animal's skin.

Their coloration varies and they may be white, yellow, orange, green, or brown with black spots and "ears." The spots are elevated tubercles while the "ears" are actually sensory organs called rhinophores (meaning "nose bearer). They are retractable, able to be pulled into pockets and shielded from predators, and used to identify dissolved chemicals in the water for the finding food or mates. Jorunna parva's mantle has the illusion of fur due to a covering of cilliated fleshy papillae lined with pointed spicules, called caryophyllidia, which also function as sensory organs. To the above right is a photo of Jorunna parva's three types of teeth, inner lateral, lateral, and outer, as well as an image of the carophyllidia taken by a scanning electron microscope. All nudibranchs possess a pair of tentacles near the mouth used for sensing touch and taste, as well as smell.

Digestion

The labial cuticle is smooth and jawless, but the mouth still contains teeth upon the radula within a buccal bulb. To the right is an image of a nudibranch digestive system. There one can also see the salivary glands, which produce and release salivary fluids into the buccal bulb to facilitate digestion. From the bulb, food travels by way of the oesophagus to the stomach. Molluscs, such as Jorunna parva, also possess a digestive gland which releases digestive enzymes, absorbs nutrients, and excretes waste. This one gland does the work of three human organs, the stomach, intestine, and liver, for the mollusk, who also has an actual intestine for transporting fecal material to the anus. Jorunna parva, and other dorid nudibranchs, have the anal opening within the posterior gills.

Life and Reproduction

Jorunna parva lay eggs in a ribbon-like body of mucous typically attached to rocks or other benthic matter. The offspring hatch as floating veliger larvae which later settle to the bottom of the water column for metamorphosis. Opisthobranch mollusks experience detorsion, a life event in which the mantle cavity and its organs move from above the head to below and toward the posterior end of the animal. The lifespan of this species ranges from a few months to a year. They are simultaneous hermaphrodites with a high food intake rate, and this is thought to have a connection to the high energy cost of constantly producing both eggs and sperm. The reproductive system, shown below, features an ampulla, bursa copulatrix, seminal receptacle, deferent duct, female and prostate glands, penis and vagina, and an accessory gland with copulatory spine. The spine is used by nudibranchs to pierce a partner for anchoring during a mating, and in Jorunna parva, the spine is long in proportion to those of other species. When a mate is found a pair will copulate facing head to tail, right sides together, and can mutually exchange sperm via orifices on the right side of the body. Sperm created by an individual is stored within the ampulla. The sperm received from a partner may be stored until the individuals are ready to fertilize their held eggs within the seminal receptacle, though the bursa copulatrix may digest some. This selects for the strongest, most active sperm to be used in fertilization when appropriate.