Opisthobranchs are a group of marine molluscs that, although no longer considered a ‘formal’ group in modern scientific classification, are still well known for their diversity and striking shapes. Their name literally means ‘gills towards the back’, because most have their gills located behind the heart. It is estimated that there are between 3,000 and 6,000 described species, and new ones are discovered each year in different seas around the world.
Opisthobranchs belong to the class Gastropoda, within the subclass Heterobranchia. They are grouped into several main orders:
- Anaspidea: known as ‘sea hares’, such as the family Aplysiidae, large in size and with parapodia (extensions of the mantle) that help them swim.
- Nudibranchia: the famous ‘nudibranchs’ or sea slugs, lacking shells and displaying bright colors; families such as Chromodorididae or Flabellinidae are very popular in underwater photography.
- Sacoglossa: slugs specialized in feeding on algae, capable of retaining chloroplasts to perform photosynthesis (a phenomenon called ‘kleptoplasty’).
- Cephalaspidea: known as ‘bubble snails’ or ‘shield slugs’, some with small internal or external shells.
- Thecosomata and Gymnosomata: ‘sea butterflies’ and ‘sea angels’, pelagic gastropods that swim by flapping lateral extensions of the body.
- Notaspidea: side-gilled slugs, with the mantle partially covering the body and reduced internal shells.
In terms of morphology, most opisthobranchs have reduced or completely lost their shell, leaving a soft body. They display highly varied forms, with sensory tentacles (rhinophores) to detect smells and, in some cases, cerata or dorsal extensions used for breathing or defense. Their bright colors can serve as camouflage or as a warning of toxicity.
They are distributed across all oceans, from intertidal zones to significant depths, but are more abundant in warm waters rich in algae or sponges, which form part of their diet.
Their feeding habits are highly varied: some are herbivores (especially sacoglossans), others are predators of sponges, anemones, soft corals, or even other opisthobranchs. Many have developed the ability to incorporate chemical defenses from their prey, such as toxins or stinging cells, and use them against their own predators.
Regarding reproduction, all are simultaneous hermaphrodites, meaning that each individual has both male and female organs and can both fertilize and be fertilized. They usually mate in pairs, exchanging sperm, and lay gelatinous masses of eggs that release planktonic larvae called ‘veligers’, before transforming into adults.
