Sea Bunny

Morphology

Jorunna parva, otherwise known as sea bunnies, are small organisms, being only a few centimeters long. Despite their furry appearance, they have no fur. Instead, they have have black and white spots, which are called papilla. Stiffened caryophyllida, or papilla with a sensory knob at the core, are also present. These caryophyllida is a key characteristic for species in the genus Jorunna (Heike, 2000). The function of caryophyllida is still unknown, but it is believed they play some sensory role. They have simple radial teeth and six bipinnate gills. The gills are on the animal's backside, looking like a tail or flower structure. They are commonly yellow or brown, with some occasionally being white (Camacho-García, 2008). The conspicuous coloring is due to their small body size and protection from predators. The bright color tells predators they are toxic. The coloring may be due to either environment or diet (Cheney 2014). Sea bunnies are also characterized by their shellless existence and smooth labial cuticle. They also have a visible foot with a line down the center.

Reproduction and Behavior

Sea bunnies are hermaphroditic with 13 haploid chromosomes. Sea bunnies fertilize gametes through cross-fertilization and copulation, meaning eggs are fertilized by another individual. Jorunna parva are also semelparous, meaning they reproduce once, then die (Heike, 2000). It is very important they reproduce, as their lifespan is only a few months to a year. They exchange sperm with each other, meaning both individuals involved will have offspring. This is done by using a long copulatory spine that exchanges sperm. During the mating process, they stab each other with the spine, ensuring sperm transfer and essentially holding the other in place during the transfer.

Toxicity

As stated before, Jorunna parva have a bright color, usually a yellow-brown. This color wards off predators by showing that the individual is toxic. Jorunna parva gets their toxicity from their diet. By eating other toxic animals, such as sponges, they retain the toxicity from their prey due to their specialized digestive system and immunity to the toxin. They can also steal cnidocytes from jellyfish as well, making them well-defended against predators.

The ability of these small creatures to obtain toxicity and stinging cells has been researched in an attempt to understand the direct method these animals are able to obtain stinging cells. These animals eat other animals with stinging cells. These cells are then transported through the gut to structures called cnidosacs. Here the cells are stored intact and fired upon touch (Proksch, 1994).

Studies done on many nudibranch species have found sponge metabolites in these animals. Metabolites from sponges travel from the digestive system to the skin glands, where the toxins can be used for defense. It has also been found that many species retain pigment from sponges; therefore, it is possible that Jorunna parva gets its yellow or brown color from the types of sponges it eats (Faulkner, 1983).

The toxic nature of sea bunnies is believed to be a cause and effect relationship. It is believed that the ability for Journna parva to sequester toxins and cnidocytes was an effect of the loss of its shell. The loss of its shell makes Jorunna parva in danger of predation. Due to this, Jorunna parva would have to have found another defense mechanism. This mechanism could have been the ability to retain toxins from sponges. This is still unknown though, and just a theory.

"Bunny Ears"

Sea bunnies are called bunnies due to the bunny ear-like extensions on their head. These structures are called rhinophores. Rhinophores in Jorunna parva are solid columns instead of the traditional rolled sheets. Originally, it was believed that these structures had a primarily olfactory, or smell, function. It is now known that these structures are, instead, general sensory organs that respond to a variety of stimuli. Rhinopores share their ability to sense with the rest of the body. The sensory input that can be detected ranges from smell and touch to chemical input (Arey, 1918). These rhinophores also help sense other sea bunnies for mating by sensing chemicals in the water, making it easier to find a mate in their short life cycle.

Habitat Threats

Jorunna parva are marine and free-living. Their diet consists of major animal groups (Heike 2000). They are found in the Indo-Pacific Ocean. They live on, or near, corals and coral reefs. Studies have been done to find out what is threatening the habitats of Jorunna parva. It has been found sedimentation in the Indo-Pacific Ocean near Indonesia is degrading corals. High sedimentation levels have also led to a lower number of mollusks living in these waters. Multiple factors including habitat destruction, sedimentation, and human activities have led many nudibranch species toward extinction. The stressful conditions created leads to a low number of species richness and evenness (Sabdono, 2021). Global climate change has also led to coral bleaching, further destroying habitats for nudibranch species, including Jorunna parva.