by B. N. Sullivan
Among the marine invertebrates that divers are likely to encounter in tropical and subtropical waters, some of the most colorful are the nudibranchs (pronounced NOOD-i-brank). These are marine gastropod snails without shells. These molluscs are also known as sea slugs -- a non-scientific term -- but divers and marine scientists alike often refer to nudibranchs by the short-form nickname "nudies."
Nudibranchs are wonderful subjects for underwater macro photography, because most of them are small and very colorful. We have encountered an assortment of nudies literally everywhere we ever have dived, and I have photographed many of those. We thought our readers would like to see and learn about some of the nudies we have come to know, so we decided to run a series on The Right Blue, featuring various 'exotic underwater nudies' (and you all can thank Jerry for coming up with the playful series title!).
To introduce the series, today's featured nudibranch is the Spanish Dancer (Hexabranchus sanguineus). This is the largest nudi we have ever encountered, and some say it is the largest nudibranch in the world, period, growing to a length of up to 40cm (about 15 in). The first three photos on this page are of the same individual Spanish Dancer, photographed during a night dive in the Red Sea. We estimate that it was roughly a foot long (30 cm). When these guys are flattened out on the reef, or crawling on the sand, they look a bit like red doormats!
We chose the Spanish Dancer to introduce our series about nudies because it is large (for a nudibranch) -- and since it is large, it's easier to see its most important external organs. The first photo on this page is a head-on shot of a Spanish Dancer. The two things sticking up are not horns, they are the critter's rhinophores.
Rhinophores are sensory organs that work much like our noses do. They sense 'smells' in the water. Nudies use their rhinophores to locate their food. (Spanish Dancers eat sponges, by the way.) They probably sniff out potential mates with their rhinophores, too.
You might notice that the critter does not seem to have any eyes. Nudies do have eyes of a sort, but no eyeballs. Instead they have little sensory specks embedded on their skin that sense light and dark, but that's all. So, you could conclude that their rhinophores actually are their most important sensory organs.
The name 'nudibranch' means "naked gills," and in the case of the Spanish Dancer, the gills are not only naked, they can't be retracted. Many species of nudies are able to retract their gills into a pouch on their back, but the Spanish Dancer's gills are always exposed. The second photo on this page is a macro image of the Spanish Dancer's six beautiful tree-like gills. The genus name of this creature, Hexabranchus, refers to the presence of six gills.
For comparison, have a look at the photo of two Risbecia pulchella nudibranchs that we posted recently. You can see their rhinophores and their gills quite clearly, but notice that the gills on this much smaller species are not nearly as elaborate as those on the Spanish Dancer.
You might be wondering how the Spanish Dancer got its common name. If you ever saw one of these creatures swimming underwater you would know the answer immediately.
While Spanish Dancers can crawl along surfaces just like any other snail, they have an alternative means of locomotion as well. They can launch themselves into the water column, and 'swim' for short distances by flexing their bodies rhythmically to achieve an undulating motion. The white ruffled margin of their mantle usually is tucked in when they are crawling or at rest, but when they swim, it is exposed. The visual effect is reminiscent of the swirl of a female flamenco dancer's skirt, thus the name Spanish Dancer.
The third photo on this page shows a Spanish Dancer nudibranch as it swam, with its lovely mantle ruffle fluttering this way and that. I have to say, though, that still photos are insufficient to illustrate this behavior. I don't shoot video underwater, but I did manage to find a brief Spanish Dancer video on YouTube, which I will post tomorrow.
Nudibranchs reproduce sexually. They line up side by side to mate, and then lay eggs in a ribbon-like mass. Some nudibranch species lay eggs as a flat ribbon. Others, the Spanish Dancer included, attach the edge their egg mass ribbons to solid surfaces in a spiral pattern. The Spanish Dancer's egg mass is easy to recognize. It is distinctive in that the ribbons are comparatively wide, and once set down in a spiral, they look like a reddish ruffled rosette.
The first photo below shows a Spanish Dancer's egg mass rosette. The second photo below is a macro image that shows the texture.