Every good Dive Master/photographer has a problem. They must look after their clients while still taking pictures. They have no time to vary the depth of focus, no time to change camera settings; their dive clients must always come first. Colin Ogden of Amoray Diving at Sodwana Bay solved the problem by shooting nudibranchs, and his study of these amazing creatures has become a passion. I spent a spellbinding evening with him learning about them.

Nudibranchs come in brilliant colours, with fabulous markings, and they can vary in size from 1mm to 600mm. They are primarily carnivores and will eat hydroids and other species of nudibranchs but most of them feed on sponges; all of which are animals. A few will eat algae or seaweed. They feed where food is available so they frequent areas where coral growth is limited; even the cold Northern European waters harbour these colourful sea slugs and nudibranch hunting has become a global passion.

Nudibranch means exposed gills. Hexabranchus, for example, has six sets of exposed gills. All but two nudibranchs are called by their Latin names. The scientific community frowns on them being given common names so the study of these creatures is not for the faint-hearted.

Most of them breath using the exposed bunches of gills on their backs, and their sensual organs are contained in a pair of rhinophores, on the front of their heads. These look like horns. All are hermaphrodite, with both male and female sex organs, which are always on the right hand side, below the neck. These can be expelled to accommodate a large meal, and then re-ingested. All of them are poisonous to fish, and a single nudibranch can kill an entire aquarium full of fish if it becomes stressed and lets off a toxin. This occasionally benefits the palatable flatworm. He sometimes mimics the shape and rhinophores of the poisonous nudibranch, escaping predators and protecting himself.

There is even a species that farms algae in its gills.  It is long and thin and has many sets of gills or cerata which he uses to collect algae. You will sometimes find it spread out along the top of the reef on a sunny day, cerata exposed to catch the sun so the algae will grow faster.

The spectacular Spanish Dancer (Hexabranchus sanguineus) is one of the very few nudibranchs that goes by its common name. They grow up to 600mm long, so are clearly identifiable. “They are nocturnal animals in our waters, so we rarely spot them in Sodwana Bay. However, we found two on the point of mating on Seven Mile in broad daylight. They lined up neck to neck, and then they exchanged sperm, fertilising each other. After five or six days each lays a long egg ribbon in a continuous circle, and you can see these on the reef looking like soft rosettes.”

They are normally pink, reddish or orange. These ribbons are preyed upon by other nudibranchs of the Favourinus family and the survivors hatch into tiny nudibranchs that look nothing like their imposing parents.

Other species lay egg ribbons that hatch into tiny veligers, which are a larval stage, and they can float around in the ocean, following currents for months on end. When they sense that there is a food source, they will descend onto the reef and metamorphose into small nudibranchs. The study of these creatures is still in its infancy, although some were described as early as the 1700s, but they are fascinating creatures, and there are still many unnamed and quite rare species in our waters. Once you know what you are looking for, you can find them almost everywhere.

Words: Jill Holloway

Pictures and technical data: Colin Ogden, Amoray Diving

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