Nudibranch Anatomy

Every diver loves nudibranchs. But do you know your nudibranch?

Kingdom: Animal, Phylum: Mollusca, Class: Gastropoda. Subclass: Heterobranchia,
Clade: Nudipleura.

Nudibranchs meaning “naked gills” are invertebrates. These mostly shell-less mollusks appear in dozens of different shapes, sizes and colours. Up to this day there have been at least 2.000 species of nudibranchs (short: nudis) discovered.

Nudis are amongst macro photographers’ most favourite subjects because of their extraordinary appearance. Generally, nudis can be divided into two classes: AEOLID & DORID NUDIBRANCH

AEOLID NUDIBRANCHIA

Aeolid nudibranchs are characterised by having long, narrow bodies with numerous finger-like projections, called cerata.  They have a pair of oral tentacles, parapodial tentacles, and rhinophores on their head (McDonald 1999).

A) Oral Tentacles are sensory feelers used to help the slug feel its way over the terrain

B) Rhinophores are chemical sensors on the head that are used to detect chemicals in the water (Ellis 2001)

C,D) Cerata are thin, finger-like projections of the digestive gland of the slug. Some nudibranchs have photosynthetic zooxanthellae from the coral in its cerata (Tackett and Tackett 2003).  The Cerata are also used in defense.  The tips of some cerata can contain poisonous substances (D) (Picton and Morrow 1994).  Cerata can also be cast off when the nudibranch is alarmed. 

E) Acid Glands produce defensive acid secretions against predators. 


DORID NUDIBRANCHIA

A) The Branchial Plume allows Dorid Nudis to filter oxygen from the water and breathe

B) Rhinophores are chemical sensors on the head that are used to detect chemicals in the water (Ellis 2001)

C) Oral Tentacles are sensory feelers used to help the slug feel its way over the terrain

D) Nudibranchs are hermaphroditic, meaning they are both male and female at the same time, but they cannot fertilize themselves. For reproduction, nudibranchs stick together in a dance-like act. Eggs are being deposited in each individual. Some nudibranchs have their anus on the same area as where the reproductive opening is located, others have their anus located in between the branchial plume.

E) Acid Glands produce defensive acid secretions against predators.