Guide to Opisthobranchs

The opisthobranchs (Opisthobranchia) comprises more than 3000 species, almost exclusively marine, a variety of shapes, which have reduced the shell to varying degrees (until its demise in adults) and the mantle cavity and gill, have coated its shell with tissue in a greater or lesser degree, in most cases have developed parapodia (lateral extensions of the foot with diverse functionality) and cerata (body excrescences with defensive functionality, etc.). changes that eventually give the animal an external bilateral symmetry that does not correspond to the internal organization of the body.

Felimida purpurea @ Cala Montgó, L'Escala, Spain 5-03-2016 by Miquel Pontes

The digestive system of the opisthobranchs consists of a buccal vestibule, which sometimes has a lip armor, a pharynx and radula and sometimes chitinous jaws, then there is an esophagus (sometimes with masticatory adaptations as a gizzard), then a stomach where it meets the digestive gland, then the intestine and ending at the anus, located on the right side of the body, or on the back of the animal.

The heart is composed of only one ventricle and atrium, and is located in front of the gill. Often bulky blood or lymph glands appear near the cerebral ganglion. The ctenidium (typical gill molluscs) is well developed only in the groups that is well developed the mantle cavity (cefalaspideans, anaspideans and notaspideans). Groups that do not have a mantle cavity have secondary gills as the dorsal papillae in dendronotaceans and sacoglossans or cerata in aeolidaceans.

The kidney evacuates waste products through the ureter or directly to the outside through an excretory pore.

The nervous system consists of a esophageal collar that combines pedal and cerebral ganglion, the buccal ganglion and the visceral handle (where pleural, parietal, intestinal and abdominal ganglions are found). The visceral handle is only fully developed in cefalaspideans, while in the other groups this handle is short and ganglions are fused to a greater or lesser extent. Nudibranchs have the highest concentration of ganglions.

Sensory elements can vary greatly, from free nerve endings to complex organs like the Hancock’s organ (two groups of chemoreceptors sensors located in the sides of the mouth of cefalaspideans). The rest of the nudibranchs have concentrated the chemoreceptors on the rhinophores, since those groups having mantle cavity presented osphradium (another quemoreceptor organ) . The eyes may be located on the surface, or at various levels of depth into the tissues, but they are always near the cerebral ganglion. Statocysts (which give them sense of balance) are always near pedal ganglions.

Opisthobranchs are generally hermaphrodites. The reproductive system varies greatly among groups, but basically consists of a hermaphrodite gonad (usually bulky and attached to the digestive gland), gonoducts and glands. The deepest part of gonoduct is called hermaphrodite conduit, allowing the passage of both sperm and eggs themselves. Then it is separated in the oviduct and the sperm duct. Throughout the oviduct nidamental glands are found (they cover fertilized eggs with nutritive material necessary for survival), the copulatory bursa (which receives the sperm of another individual during copulation) and seminal receptacle (which keeps the sperm until the egg maturation). Along the sperm duct the prostate is found (needed for spermatophores formation and the transfer of sperm during the intercourse) and the penis.

After fertilization, the eggs are encompassed in a gelatinous substance, usually transparent, and go abroad where they are fixed to the substrate. The colors and shapes of the proposals vary among species, can be coiled, scalloped, kidney-shaped, in a disorderly way, attached to the substrate by a stalk, etc.. The number of eggs also varies greatly from one species to another, and can range from tens to millions. The location of the specific species is usually placed (determined sponges, algae or cnidarians).

Except in some cefalaspideans, sacoglossans and nudibranchs, in which the development is direct, the other opisthobranchs have indirect development, with the intermediate formation of larval type called “veliger“. In these cases there is always the metamorphosis, which involves the loss of the operculum and, often, the disappearance of the larval shell. This disappearance has forced shell to refine the opisthobranchs’ defense strategy, with mechanical, chemical (acids and terpenoidal secondary metabolites extracted from their food ) and behavior.

The opisthobranchs are generally benthic animals (tied to the seabed), but some are able to swim if they are threatened, as Aplysia and Tritonia. Other species are pelagic such as tecosomates and gymnosomates.

Most anaspideans and sacoglossans are herbivores and tecosomates are filter feeders, but the rest of the opisthobranch are carnivores, although there are curious cases (such as Tylodina perversa), where studies have shown that an apparently spongivore animal actually has a clear preference for sponges filled with blue-green algae.

Main opisthobranch groups:

Acochlidiacea

Acochlidiimorpha

Aplysia punctata

Aplysiida

Acteon tornatilis by Manuel Ballesteros

Acteonimorpha

Melanochlamys miqueli by Enric Madrenas

Cephalaspidea

Felimare tricolor by Enric Madrenas

Nudibranchia

Berthella aurantiaca

Pleurobranchida

Limacina helicina by Alexander Semenov

Pteropoda

Ringicula doliaris CC-BY Kano et al (2016)

Ringiculimorpha

Runcina coronata

Runcinida

Elysia timida

Sacoglossa

Umbraculum umbraculum

Umbraculida

Classification of the opisthobranchs