Nudibranch Fact Sheet
SUB CLASS: Opisthobranchia (Sea Slugs)
Classification, Description & Behaviour
The word nudibranch is often misused by scientists and lay persons alike. Taxonomically, nudibranchs are one of five orders beneath the subclass Opisthobranchia (sea slugs). Therefore, when referring to nudibranch type animals, it is correct to call them sea slugs or Opisthobranchs rather than nudibranch (s) unless referring specifically to the Nudibranch Order.
I recommend you open the sea slug classification sheet (.pdf file) and study the self explanatory flow chart which will explain the taxonomic positioning of sea slugs within the classification system.
I have attempted to simplify the classification of Opisthobranchs, however, it still remains confusing, especially to those not well versed in biological nonclamature.
Sea slugs belong to the Phylum Mollusca which is derived from Latin meaning “mollus”, for soft bodies, a characteristic common to all members of this group. The next link in the taxonomic nonclamature is Class and sea slugs fall within the Class Gastropoda.
Gastropoda, “gastro” means stomach and “poda” means foot. Broadly translated this name denotes animals that have a well-developed crawling foot, which contains the stomach.
Gastropoda is further subdivided into two subclasses: Subclass Prosobranchia which are the spiral-shelled animals such as cowries, snails, conches, etc, and Subclass Opisthobranchia (Sea Slugs) which is further divided into five orders and includes the headshield slugs, seahares, sapsucking slugs, sidegill slugs and nudibranchs.
The Nudibranch Group
Nudibranchs comprise approximately 2,000 species. The Order Nudibranchia (nudibranchs) is subdivided further into four sub orders: Aeolodina, Arminina, Dendronotina and Doridina. These sub orders are then further divided before eventually reaching species level.
The Aeolidoidea and Doridoidea are the two largest suborders of nudibranchs.
Aeolidoidea (Aeolids) have long, narrow bodies, lack gills, and have a number of projections on the dorsum called cerata. These sea slugs are generally smaller in size.
Arminoidea (Arminina) is made up of a motley of species with few consistent characteristics other than, a lack of rhinophoral sheaths and, often, oral tentacles. These broad characteristics of the suborders are helpful in determining which suborder an individual belongs to so that it may be identified further.
Dendronotoidea (Dendronotina) characteristics include rhinophoral sheaths and a mid-lateral anus, but resemble aeolids.
Doridoidea (Dorids) characteristics include a broad, flat foot, a thick, fleshy mantle, and a circle of gills on the posterior end of the dorsal surface. The gill surrounds the anus. These nudibranchs are also generally the larger species.
Most nudibranchs, with the exception of those belonging to the Suborder Arminoidea have rhinopores.
Aeolids make up the second largest suborder of nudibranchs. Members of this group look similar to terrestrial centipedes; their bodies are tapered and two rather large tentacles and rhinopores are evident. Clusters of respiratory organs called cerata extend the length of their back. Some species of aeolids store the nematocysts (stinging cells) captured from cnidarians on which they prey. The nematocysts are used for defence.
Several variants belong to this group that do not taxonomically fit into other groups. Many species exhibit striated lines along their backs and have a hidden gill between the foot and the mantle. Several species have numerous gill cerata covering their back and are often confused with Aeolid Nudibranchs.
Species belonging to this suborder are typically long and tapered with gills forming a number of identical appendages. Gill appendage shape is varied and can be cusped, cigars, spindles or branched tufts. Rhinopores are usually cup-shaped or fluted.
Suborder Doridina (Dorids)
The suborder Doridina contains more species then the other three suborders of nudibranch combined. Typically they are large in size and with striking flamboyant colours.
Dorids can be unofficially subdivided into three distinctly different sub categories: Cryptobranch Dorids, Porostome Dorids and Phanerobranch Dorids.
1. Cryptobranch Dorids Main characteristics of this group are perfoliate rhinophores, a distinct lateral foot and flowery anal gill (respiratory gill feathers located on the upper rear of the animal). Cryptobranch Dorids have the ability to retract their gills into their body when harassed or in danger.
2. Porostome Dorids Porostomes are similar to Cryptobranch Dorids, however, their heads merge more uniformly with their bodies (porostome means pore-mouth). Feathery gills are common on nearly all dorids in this category. A distinctive characteristic of this category can be knobbly tubercles and/or ridges which can aid in identification to family and species level.
3. Phanerobranch Dorids Phanerobranch means evident gill and nudibranchs belonging to this category are long, slender species with very distinct heads. Phanerobranchs exhibit a wide variety of colour patterns and body ornamentation and can have several differing styles of rhinopores. Of importance is that they do not have the ability to retract their gills as other nudibranchs do. Usually the gills are protected by fleshy protrusions or appendages.
Sea slugs move by muscular action or by ciliary action which causes the slug to move quite slowly. In a slug that moves via muscular action, contraction and expansion of the muscles move the slug forward. In ciliary motion, cilia in the foot beat against the substrate causing the slug to move forward.
A common observation amongst some species is what is termed rearing. Rearing is when the animals adapt a pose not dissimilar to that of a curled snake ready to strike. Various hypothesises have been suggested for rearing, however, based on the sessile nature of Sea Slug prey it would appear that the posture maybe used to assist in olfactory capture of chemical signals within the water table. The chemical signatures of prey animals maybe easier to capture when water above the substratum is analysed.
World Range & Habitat
Sea Slugs are found in all oceans in both tropical and temperate environments. Habitats are extremely varied, however, usually correspond with local food sources. Certain species are known to only inhabit particular depth ranges.
Feeding Behaviour (Ecology)
All nudibranchs are carnivorous feeding on sessile or sedentary organisms. Each suborder of nudibranch has a major food source. For dorids, most food comes from preying on bryozoans and sponges (Dysidea and Aplysilla) while aeolids and dendronotaceans prey upon cnidarian species.
Just as the suborder Arminacea is diverse, so is the food they consume, with some species specialising in Sea Slug eggs. Sea slugs have been observed eating the eggs of a fellow sea slug before the slug has completed laying. It is important to remember that the substrate an animal is found on may not necessarily be its food source.
All sea slugs are hermaphroditic, but rarely will they fertilize themselves. Normally nudibranchs will copulate and each individual will lay an egg mass, a nidosome.
Egg masses of a number of sea slugs are quite distinctive and can be found on substrates where sea slugs feed or are often found. When the eggs hatch, a veliger larva (larva with a protective shell and a ciliated flap-shaped foot used for swimming and feeding) is usually released. The veliger has a shell, but once the larvae has settled out of the plankton, the shell is released and they enter a juvenile stage.
Confusion and Identification
Sea slugs are notoriously difficult to identify in the field to species level. Like many animals, nudibranchs through the process of convergent evolution, can differ markedly in colour, texture, pattern, size and habitat. This causes confusion as to which species they belong.
Some species may be quite different in appearance with differing colours and knobbly protrusions but be the same species. Colouration cannot be used as a definite method for identification; many aeolid nudibranch colours are derived from the colour of the algae that are present within the food supply they consume – as such, you may find three identical species with differing colours.
Overall size also cannot be the main determinate for identification, as the animal maybe a juvenile or an adult, however, body shape or plan can be used as part of the identification process.
Characteristics to aid in identification are:
Rhinopore style (rolled, smooth, verrucose, annuate, sail-like, branched, striated bulbous, pulpit, corambid, tritonid, perfoliate & rostanga).
Location of feathery gill structure and whether it is retractable.
Presence of protective assemblages to protect the delicate gill structures.
Defined head structure.
Presence of ridges, bumps and/or knobs on the back of the animal.
Body shape (long, short, tapered, dumpy, flat, high or low).
Presence of respiratory gill structures running the length of the body.
Behrens D. W., 2005. Nudibranch Behaviour. New World Publications Nudibranch Photographic Gallery [Top of page]