Collecting opisthobranchs is a difficult task that requires much skill and training. The study of this group is often more difficult and complicated than the study of shelled mollusks. For one, many important taxonomic characters such as coloration are external and must be studied while they are still alive. The preservation process leaches the animal of its brilliant colors, and its body becomes an amorphous mass.

Furthermore, they must be relaxed before preservation; otherwise, their bodies will contract violently making specimens unusable for later anatomical studies. After fixing specimens in Bouins or formalin solution, they must be transferred to alcohol, which is the most adequate preservation medium (for morphological studies) because it prevents the dissolution of the calcareous shell (for those that have one) and spicules of taxonomic interest.

Opisthobranchs can be found in all marine ecosystem types and at any depth. However, they are most biologically diverse from intertidal rocky zones to 30 meters deep. For species that prefer soft bottoms or deep waters, dredging is the most appropriate collecting technique. Once collected, specimens are transferred to the laboratory.

In the case of cryptically colored opisthobranchs that live in marine algae, gorgonian corals, or hydroids, indirect collecting methods normally offer better results. These methods consist of collecting substrate, animals, or plants where opisthobranchs live, and transferring it to the laboratory, submerging it in water for a variable period of time. Substrate decomposition and organism respiration will quickly decrease the dissolved oxygen in the water. Little by little the opisthobranchs will leave the protection of the substrate and look for areas with greater oxygen near the surface of the container, at which point it is very easy to collect them.

Indirect methods are highly destructive and must only be used by trained personnel when absolutely necessary, and when there is evidence of opisthobranchs (for example, the presence of egg masses or animals). Many species of opisthobranch are not in danger of extinction; nevertheless, collection must solely be done for scientific studies. When rocks are moved in search of opisthobranchs, these must be placed in the same position in which they were found, eliminating the possibility of leaving other marine organisms exposed to predators or to unfavorable conditions.

Previous knowledge of the places being sampled and the type of bottom substrate greatly increases the success of collecting opisthobranchs.