Opisthobranchia
Opisthobranchs, formerly classified as a subclass, are now recognized as a diverse group of specialized complex gastropods. The subclass Opisthobranchia, which previously unified these organisms, is no longer considered a monophyletic grouping. Instead, a taxon called Euopisthobranchia has been established to include a revised collection of opisthobranchs, which is considered monophyletic. However, Euopisthobranchia does not include certain "traditional" opisthobranchs like the Sacoglossa and the Acochlidiacea. The species that were once assigned to Opisthobranchia, along with those in the Pulmonata, are now classified under the subclass Heterobranchia.
Opisthobranchia previously encompassed several orders including Cephalaspidea, sacoglossans, anaspidean sea hares, pelagic sea angels, sea butterflies, and many families of the Nudibranchia. The term "opisthobranch" refers to the location of the gills, which are positioned behind and to the right of the heart. In contrast, "prosobranch" indicates gills in front of the heart. Opisthobranchs are characterized by two pairs of tentacles and a single gill behind the heart. The group's lack of a heavily mineralized shell has resulted in a limited fossil record. However, molecular clock studies suggest that Opisthobranchia emerged as early as the Carboniferous period.
Opisthobranchs exhibit evolutionary trends such as the loss of a shell, the elaboration of the head, foot, or mantle, and the development of chemical defenses. The reduction or loss of the shell has occurred independently on multiple occasions, showcasing parallel evolution. Opisthobranchs have also undergone detorsion, a reversal of the torsion found in their ancestors. This detorsion separates the visceral ganglia and is described as euthyneurous, contrasting with the more common streptoneurous condition in gastropods.
Opisthobranchs do not have a clear distinction between the head and mantle. The tentacles, located near the mouth, aid in orientation, while the rhinophores serve as olfactory organs, often possessing intricate forms. The sole, located in the middle part of the foot, is used for locomotion, and the sides of the foot have evolved into parapodia, wing-like structures seen in several suborders. These parapodia enable certain opisthobranchs, such as those in the Thecosomata and Gymnosomata, to swim. The eyes of opisthobranchs are simple pit-cup eyes capable of detecting light and shadows but not forming coherent images.
Opisthobranchs are mainly soft-bodied marine creatures with reduced or absent shells and no operculum. They employ various methods for protection, exhibiting a diverse array of highly specialized defensive mechanisms. Due to their exceptional camouflage and aggressive toxicity, opisthobranchs face fewer predators. However, some opisthobranchs use warning coloration as a defense. Predators that do consume opisthobranchs include other opisthobranchs and toxin-resistant organisms like sea spiders. Opisthobranchs produce irritants such as strong acids or acquire toxins from their food. Aeolidioidea, for example, pilfer the stinging cells from cnidarian prey and utilize them for their own defense.
Opisthobranchs can be herbivores, detritivores, or carnivores. The carnivorous opisthobranchs, being slow-moving, tend to hunt sedentary prey such as bryozoans, Cnidaria, or sponges. They absorb the toxins from sponges for their own defense. Some opisthobranchs maintain the zooxanthellae of their coral prey and utilize their metabolic products. Certain herbivorous slugs in the Sacoglossa suborder also acquire chloroplasts from the algae they consume.
Chemical cues play a significant role in the life cycle of opisthobranchs, as with most organisms. Planktonic larvae float until they detect a pheromone indicating a suitable settling site. In some cases, metamorphosis is delayed until favorable chemicals, such as prey pheromones, are detected. Mating opisthobranchs release chemicals to attract conspecifics.
Opisthobranchs are hermaphrodites and employ complex reproductive strategies, often involving reciprocal sperm transfer and storage until eggs are ready for fertilization. Eggs are typically laid in ribbons of varying structure, which are often unique to each species and serve as a distinguishing characteristic.
