Photo of the Week: March Week 2009 4

Hi gang! Last week I shared a photograph of the face of a southern right whale. This week I thought I would submit a subject from the other end of the size spectrum so I selected a shot of a pair of mating nudibranchs. Sometimes referred to as sea slugs, nudibranchs are shell-less mollusks. The word nudibranch is derived from the Latin word for nude (snails without shells and, therefore, nude) and the Greek word for gills as the gills of these gastropods are often a prominent feature on the outside of their body.  

Nudibranchs have been described as both the most bizarre looking and the most beautiful of all marine animals. There are four suborders of nudibranchs. They are Aeolidacea, Doridacea, Dendronotacea and Arminacea. The general body shape of the members of each suborder is significantly different than the bodies of the species described in the other suborders. The food preferences of each suborder also tend to differ. The nudibranchs that are the most commonly encountered by divers are described in the suborders Aeolidacea (the eolids) and Doridacea (the dorids). 

The nudibranchs pictures here were photographed off the island of Mabul (near Sipadan) in Malaysia. At least one, and perhaps several species of nudibranchs are commonly referred to as royal hypselodoris. I am confident that the animal's genus is Hypselodoris while the exact species is questionable. 

The bodies of dorid nudibranchs are generally oval and flattened. Two sets of projecting structures extend upward from their backs. These structures toward the front of the body are called rhinophores while those near the back are the gills, or cerata. Rhinophores are chemosensory antennae that allow dorids to smell one another and to smell their prey. The gills are delicate, flower-like looking appendages that absorb oxygen from the water. While some dorids are able to retract their gills into a protective pocket, others leave the gills extended at all times. It is their exposed gills that gave rise to the name nudibranch when these animals were first studied many years ago. 

As adults, nudibranchs are hermaphrodites, meaning that individual animals simultaneously possess functioning male and female reproductive organs. This means that when two animals get together it is truly a case of boy meets girl meets boy meets girl with each animal supplying sperm that fertilizes the eggs of the other. In nudibranchs, copulation lasts anywhere from a few minutes to several hours. As many as several thousand eggs are laid in spiral shaped masses of mucus in a single event. The mucus causes the eggs to attach to each other en masse, and "glues" them to the substrate. 

From a photographic perspective, I tried to underexpose the scene by almost one f/stop so that I did not grossly over expose the white border. I raised the exposure of the body and gills in Photoshop. This allowed me to keep the vivid colors of body while keeping some detail in the border area. I also made an effort to compose the shot in a way that allowed me to see exactly where the bodies of these two animals were joined as that is the key element that allows you to know that they were actually mating and not just in close proximity.