Underwater Photo of the Week

Hi Gang!

This week’s POTW is an image I captured during the last dive of a recent trip to the Midriff Islands in Mexico’s Sea of Cortez aboard a very nice, relatively new liveaboard named Rocio Del Mar. Earlier in the day I had been swimming with a pair of small whale sharks in fairly green water in Bahia de Los Angeles, but one of those images will have to wait until later. Because I have enjoyed the great luxury of having seen whale sharks on a number of previous occasions, the “find of the day” was this bright yellow sidegill slug known to scientists as Tylodina fungina. While this slug occurs from California to Ecuador’s Galapagos Islands, the sighting was a first for me. The species is sometimes referred to as a Chinaman’s hat due to the resemblance of this gastropod mollusk’s shell to the style of hat often worn by Chinese field laborers.

Sidegill slugs are further classified as being types of opisthobranchs known as Notasipideans, a group of animals that are sometimes called false limpets. Limpets have a smaller whitish foot and their shell is usually clamped down tight against the sea floor. Some people mistakenly think this sidegill slug is a type of nudibranch, but the presence of an external shell should prevent a misidentification of that nature.

The Chinaman’s hat feeds exclusively on the yellow sulphur sponge Aplysina fistularis, and when it is feeding the animal is often “clamped down” on the sponge in a way that makes it very difficult to impossible to use a low shooting angle. But the specimen I encountered was not feeding, and was instead having a leisurely paced crawl along the edge of a rocky ledge. That fortuitous position gave me the opportunity to shoot at a “head on” angle and create this week’s POTW.

I used a Nikon 60 mm lens behind a flat port in a Subal housing with two DS-160 strobes to capture the shot. The slug was less than three inches long, but because of the “head on” angle and the close shooting distance I was fearful that a lack of depth-of-field would be a distraction. To prevent this problem I set my strobes onto the full power setting so I could close down my aperture as much as possible in order to maximize my depth-of-field. I knew the bright yellow slug was likely to be highly reflective and I was able to close my aperture down to f/38, a setting that gave me a good depth-of-field.

Hope you enjoy this week’s POTW!

See you next week,

Marty