The Viral Nudibranch Jorunna parva, AKA the Sea Bunny

by Christopher Owen

A certain creature has recently gone viral. Many call it the sea bunny, but it is actually a tiny gastropod. Since it is not often that a gastropod of any size occupies a significant portion of the public consciousness, I've decided to take advantage of a felicitous situation join our little friend Jorunna parva's sudden wave of fame.

Image by crawl_ray on flickr

What are they?

You will have noticed by now that sea bunnies do not by any means belong in the order Lagomorpha, much less are they any sort of leporid. Rather, a sea bunny (more proprietously known as Jorunna parva) is actually a nudibranch (the word is a Latin/Greek mongrel meaning “nude branchia”, which is pretentious English for “naked gills”, referring to the fact that their gills are naked, but more about that in a bit). They are found throughout the coasts of the the Indian Ocean, as well as east to the central Pacific. Most individuals of J. parva are yellow with black spots, but the white ones are more famous.

Some would call nudibranchs slugs, while others prefer to refer to them as shell-less snails. Which one it really is depends on your personal views regarding the concepts of slug and snail, a timeless philosophical issue with powerfully convincing arguments on both sides. In the slug camp are those who believe that any gastropod without a shell belongs squarely in the set of slugs. However, a subtler distinction must be made when we examine the evolutionary history of these animals, for such an examination will reveal that the branching of those priapistic garden friends traditionally known as slugs occurred long after the split between the shelled snails and the nudibranchs. The issue is further complicated by the fact that larval nudibranchs retain the shells of their ancestors, while land slugs lose the shell well before they hatch. Does this mean that a nudibranch is a snail until its shell falls off, at which point it becomes a slug? Certainly further debate is necessary.

Image by crawl_ray on flickr

What are their parts?

Rather than providing an exhaustive list of structures found in J. parva, I will focus on their most bunny-like components.

The “fur” of the sea bunny is actually made of many hardened, skin-covered scaffolds called caryophyllidia. Many nudibranchs possess different versions of these structures. They are commonly assumed to play a sensory role, both because of their resemblance to known sensory organs and because of their close association with the nervous system.

The “ears”, called rhinophores, are actually more like noses. They contain a series of stacked membranes, called lamellae, which give them their feather-like appearance. The lamellae greatly increase the surface area of the rhinophores, allowing the nudibranch to detect very dilute chemicals in their watery surroundings.

Finally, the “tail” is actually J. parva's gills. If these animals were snails, the gills would be hidden inside the shell. It is the absence of this covering that gives nudibranchs their name.

Image by Christophe Cadet

What do they do?

Much like its mammalian namesake, J. parva spends most of its time eating and having sex. Fortunately, both of these behaviors happen to be pretty interesting among dorid nudibranchs, the group to which J. parva belongs. 

The dorid diet consists primarily of sea sponges. They incorporate several chemicals from the sponges into their own bodies, including various toxins and pigments, often without altering their chemical structures in any way. This makes dorids extremely toxic to almost all potential predators. Interestingly, the evolutionary loss of the shell in dorids and related groups tends to follow the ability to retain dietary toxins in the body. Even more interestingly, the sponges on which the animals feed have also lost much of their rigid skeletal structure in correlation with the ability to produce toxins.

Moving on to reproduction: J. parva are simultaneous hermaphrodites, containing both male and female reproductive systems (as opposed to serial hermaphrodites, who are able to change sexes, but belong to only one sex at any given time). The intromittent organ (equivalent to a penis) is a sharp spine located on the right side of the head. There is also a female port (equivalent to a vagina), which is located slightly left of the center line on the underside of the nudibranch. Copulation occurs between two animals, both of which fertilize each other. Sperm can be stored and used as needed. Unwanted sperm are digested. The animals lay ribbons of eggs in carefully crafted counterclockwise spirals. The larvae that hatch from the eggs are free-swimming, but will eventually settle to crawl around on their bellies, where they prefer to stay throughout their adult lives.

(Please feel free to leave corrections and questions in the comments.)

Sources:

Bouchet, P. & Rocroi, J. (2005). Classification and nomenclator of gastropod families. Malacologia, 47(1-2): 1-397. Retrieved from https://archive.org/stream/malacologia47122005inst#page/n3/mode/2up

Cardone, B. J. (2015). Sea slug courtship and reproduction. California Diving News. Retrieved from http://www.cadivingnews.com/marinelife/1204/Sea-Slug-Courtship-and-Reproduction--

DeYoung, A. (2007). Nudibranch Embryological Development. University of Oregon. Retrieved from https://scholarsbank.uoregon.edu/xmlui/bitstream/handle/1794/5332/embryology002.pdf?sequence=1

Faulkner, J. D. & Ghiselin, M. T. (1983). Chemical defense and evolutionary ecology of dorid nudibranchs and some other opisthobranch gastropods. Marine Ecology – Progress Series, 13: 295-301. Retrieved from http://www.int-res.com/articles/meps/13/m013p295.pdf

Kress, A. (1981). A scanning electron microscope study of notum structures in some dorid nudibranchs (Gastropoda: Opisthobranchia). Journal of the Marine Biological Association of the United Kingdom, 61(1): 177-191. (Abstract.) Retrieved from http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=4387472&fileId=S0025315400046002

Lee, J. J. (2015). Meet the adorable “sea bunny” taking over the internet. National Geographic. Retrieved from http://news.nationalgeographic.com/2015/07/150723-sea-slug-nudibranch-sea-bunny-ocean-animals-science/?sf11266065=1

Rudman, W.B. (1999). Rhinophore in nudibranchs. Sea Slug Forum. Retrieved from http://www.seaslugforum.net/find/rhinonud

Valdés, Á. & Gosliner, T. M. (2001). Systematics and phylogeny of the caryophyllidia-bearing dorids (Mollusca, Nudibranchia), with the description of a new genus and four new species from Indo-Pacific deep waters. Zoological Journal of the Linnean Society, 133: 103-198. Exerpt retrieved from http://rfbolland.com/okislugs/caryophy.html