The pleurobranchids have a fairly homogeneous body organization, highlighted by the presence of a voluminous bipectinate gill to the right of the body, between the foot and the dorsal mantle. In fact the name of the group is related to this organ’s position (Pleurobranchid = gill aside).

Berthella aurantiaca by Enric Madrenas

Animals are medium to large size. The head has a pair of rolled rhinophores and a flattened frontal veil that usually differentiates short oral tentacles. The dorsal mantle is well developed and can be smooth or covered with tubercules, and completely covers the body. In the genus Pleurobranchus and Berthella the shell is internal as it is covered by the mantle, while the Pleurobranchaea lack any shell. In some species there are microscopic spicules in the mantle. The buccal bulb has a lip with masticatory independent elements and the radula always exist but has no rachideal or medium tooth . The mantle cavity is a kind of narrow groove between the mantle and the foot on the right side of the body and holds the gill. The foot is very large, allowing swimming in some species, such as Pleurobranchus membranaceus, and in the back it has an apparent metapodial mucus-secreting gland.

Some species of pleurobranchids are big sized (about 20 cm for the largest specimens of Pleurobranchus testudinarius and Bathyberthella antarctica) and use to be very visible on their habitats. They are usually very sedentary and many, as the species of the genus Berthella, live under the stones on the seabed, even at very shallow depths. Some species of the Pleurobranchidae family may swim with undulating movements of the edge of the mantle and the foot. They are hermaphrodites, have internal fertilization, and copula is reciprocal. Shortly after mating, the animals lay a straight or spiral coiled ribbon containing from one to 30 egg capsules, deppending on the species. Veliger larvae that hatch from the eggs are planktonic and planktotrophic (feed on plankton). They are carnivorous and can feed on sponges, colonial or solitary tunicates and other invertebrates. Pleurobranchaea meckeli could prey on smaller specimens of the same species, in laboratory conditions. Many species are able to release acid substances from the mantle glands when disturbed by a potential predator or under experimental conditions.

Most species live in temperate and tropical waters of all oceans. Known species (4) of the genus Bathyberthella live in the deep and cold waters of the Southern Hemisphere and Antarctica oceans. Several species have been cited in European waters, most of them are also found in Iberian waters .

Formerly known as Pleurobranchomorpha, this group belonged, until recently, to the traditional order of Notaspideans within the Opisthobranchia, which is no longer accepted. This is because, as it has been recently demostrated, comparative morphological analysis and molecular studies, the two included families (Pleurobranchidae and Umbraculidae) represent two different lineages that may not share common ancestors. Phylogenetic analysis conducted in recent years tend to place the Pleurobranchida as the sister group of the Nudibranchia , forming with them a group with the category of Superorder called Nudipleura .

The taxonomy of this group according to Bouchet et al. (2017) and published at WoRMS is as follows:

  • Order Pleurobranchida
    • Superfamily Pleurobranchoidea  Gray, 1827
      • Family Pleurobranchaeidae Pilsbry, 1896
        • Genus Euselenops Pilsbry, 1896
        • Genus Pleurobranchaea Leue, 1813
        • Genus Pleurobranchella Thiele, 1925
      • Family Pleurobranchidae Gray, 1827
        • Genus Bathyberthella Willan, 1883
        • Genus Berthella Blainville, 1824
        • Genus Berthellina Gardner, 1936
        • Genus Boreoberthella Martynov & Schrödl, 2009
        • Genus Pleurehdera Marcus & Marcus, 1970
        • Genus Pleurobranchus Cuvier, 1804
        • Genus Tomthompsonia Wägele & Hain, 1991
      • Family Quijotidae Ortea, Moro & Bacallado, 2016
        • Genus Quijote Ortea, Moro & Bacallado, 2016

There are about 71 known and accepted species. The genus Euselenops, Pleurobranchella, Boreoberthella, Pleurehdera and Tomthompsonia are monospecific while each of the genus Berthella and Pleurobranchus have about 20 known species.

The genus Pleurobranchaea, Berthella, Berthellina and Pleurobranchus are the only known in european waters.

Pleurobranchida species cited in the Mediterranean or around the Iberian Peninsula:

Berthella aurantiaca

Berthella aurantiaca

Berthella africana @ Taliarte, Gran Canaria 25-07-2014 by Jose Juan Calderín Peñate

Berthella africana

Berthella ocellata by Enric Madrenas

Berthella ocellata

Berthella perforata @ Malta by Constantino Mifsud

Berthella perforata

Berthella plumula (Portugal) by Joao Pedro Silva

Berthella plumula

Berthella stellata

Berthella stellata

Berthella sideralis by Klas Malmberg (Sweden)

Berthella sideralis

Berthellina citrina 15mm @ Mistra Bay, Malta 1m depth 19-09-1992 by Carmel Sammut

Berthellina citrina

Berthellina edwardsii by Enric Madrenas

Berthellina edwardsii

Pleurobranchaea meckeli by Enric Madrenas

Pleurobranchaea meckeli

Pleurobranchus forskalii ( by Jim Anderson

Pleurobranchus forskalii

Pleurobranchus membranaceus 65mm 14-12-2014 @ Ardcastle, Loch Fyne, Scotland 12m by Jim Anderson

Pleurobranchus membranaceus

Pleurobranchus testudinarius by Enric Madrenas

Pleurobranchus testudinarius

Pleurobranchus reticulatus by Peter Wirtz

Pleurobranchus reticulatus

Quijote cervantesi @ Taliarte, Gran Canaria, 7-08-2014 by Pablo Samper Méndez

Quijote cervantesi