Some of the most beautiful and diverse animals inhabiting the Pacific Northwest are the Opisthobranchs, or more commonly known as nudibranchs. There are approximately 200 species of these sea slugs along our coastline, with around 6000 existing worldwide.
The attraction for underwater photographers and divers alike stems from the nudibranchs' spectacular array of colours and markings. This colouration, combined with the extroverted behavior of certain species, can make for a wonderful underwater encounter.
Nudibranchs feed on a variety of sponges, bryozoans, hydroids, colonial ascidians, and in certain cases, barnacles, and the animals' colouration is often affected by the food it consumes.
Nudibranchs have a pair of stalked shaped sensory organs called rhinophores on their head. They have retractable plume shaped gills for breathing that may resemble fingers, paddles, or clubs. Additional appendages assist in breathing and can provide camouflage or hide sharp rods for protection.
Dorids
Dorids are the largest group of nudibranchs inhabiting the Pacific Northwest. This species is typically identified by their flattened shape, and most have retractable gills. They are commonly found consuming a variety of sponges.
Dendronotids are some of the larger nudibranchs inhabiting the coast. This species have numerous branching gill tufts that run along their backs. Their rhinophores are covered by a cup like sheath, and typically have branches at the end of them. The individual species within this group are very unique to each other. The Giant Dendronotid is an amazing swimmer, and if disturbed, will dance around in the water column. Hooded nudibranchs, resembling an underwater Venus flytrap, will wave back and forth in the water collecting food but will also frequently float up and dance through the water.
This group of nudibranchs feed on a wide range of other plants and animals. Giant Dendronotids enjoy tube dwelling anemones, Orange Peels feast on hydroids and soft coral, and Tritonias cut down orange and white sea pens.
Arminids are the smallest group of nudibranchs found in the Pacific Northwest. What differentiates these species from others is the placement of their gills along their bodies. Some have the gills at the back of their body as can be found on the Striped Nudibranch, or under their mantle in the case of the Alabaster Nudibranch.
Aeolids are typically longish in body shape and have a pair of cephala tentacles which are distinct from rhinophores, but what stands out for this species are the rows of gills on their back. These animals prey mainly on hydroids and anemones, storing their prey’s stinging cells in their gill tips and using them for defense.
Images: Japanese Aeolid, Three-lined Aeolid, Long-mouthed Aeolid
Cephalaspidea - Shield Head Slugs
This group of opisthobranchs is very unique to all other nudibranchs. What separates these nudibranchs from the others is their shield. Several members of this family have a heavy external spiral shaped shell that they can withdraw into, while others have a thinner and softer shell that is covered by their mantle. One particular group appears to have no shell at all, however the shell is actually hidden internally on these creatures.
The majority of this family are burrowers, feeding on worms, bivalves and other nudibranchs. Most can be found just beneath the surface in the mud and sand, which is why they lack the brilliant colouration of other nudibranchs. The Aglaja is one of the species that possess internal shields and are voracious carnivores that feed on relatively fast moving food. These predators have a highly developed sensory system and detect can the slime trails left behind by its prey.
