Photo by Walter Hackerott

What is that yellow-spined thing? This is a species of sea slug, one of the strangest groups of organisms present on reefs worldwide. Sea slug is a pretty all encompassing term that includes groups such as Nudibranchs and Cephalaspidea. They are part of the “cryptic” fauna present on coral reefs. These interesting and strange creatures have been a popular study organism for a long time, and have recently become quite popular in the scientific press. In fact, they have been featured in three major articles within the last week. Perhaps this is why…

Many Nudibranchs feed on algae. They will ingest the algal cells and digest everything except for the chloroplasts, which they can store in a what is thought to be a digestive gland in their own body (these chloroplasts are then given the awesome name “kleptoplasts”). The kleptoplasts can then be used by the slug for energy, or so we thought. A new study in Nature this week reveals that some slugs who retain chloroplasts can live as long under starvation conditions in the dark as they can in the light. This seems to indicate that the organisms do not meet all of their metabolic demand by utilizing photosynthetic products from the kleptoplasts. The study reveals that we know even less about these cryptic organisms than previously thought.

What else can these strange little creatures do to get attention? How about this: “Sea slugs stab mates between the eyes to get them in the mood” was a headline on National Geographic earlier this week. The post has 12,000 likes on facebook and with a title like that, you can see why. Who could resist clicking on that? So… what behavior is it talking about? Sea slugs are hermaphroditic, that means that each individual has both male and female parts. For many slugs copulation is a bit of battle because each individual wants to place its own genetic material into another. One of the tactics that has evolved out of this struggle is the development of a needle like appendage that the slug aims at the head of potential mates. The appendage injects a chemical that somehow makes mating more successful, either by preventing the injected animal from copulating again, preventing competition from the sperm of other individuals. Now, as I mentioned, these organisms are hermaphroditic, so each individual would attempting to stab the other in the head, leading to a sort of awkward wrestling match. If you are convinced these things are strange and cool yet, just wait until you hear what else they can do…

Another species of sea slug was studied in Japan and researchers realized that the organisms often act as simultaneous sperm donors and receivers. That means that two hermaphroditic slugs impregnate each other at the same time. This is apparently a common practice among sea slugs. After this occurs, the slugs are unable to donate sperm for about 24 hours. Now a wait time between copulation events is not strange in the animal kingdom, but when the researchers investigated the mechanism behind this behavior they found something rather shocking. The sea slugs weren’t donating sperm because they lacked the anatomy to do so. It seems that they discard their used penis and grow a new one. Isn’t evolution funny? Why does this happen? Each penis has spines on it which are hypothesized to help remove rival sperm from a mate. It is possible that the appendages are not reusable and are therefore discarded after each use. Fun, right?