The larvae of the two hookworm species can also be distinguished microscopically, although this would not be done routinely, but usually for research purposes. Adult worms are rarely seen (except via endoscopy, surgery or autopsy), but if found, would allow definitive identification of the species. Classification can be performed based on the length of the buccal cavity, the space between the oral opening and the esophagus: hookworm rhabditoform larvae have long buccal cavities whereas Strongyloides rhabditoform larvae have short buccal cavities. Recent research has focused on the development of DNA-based tools for diagnosis of infection, specific identification of hookworm, and analysis of genetic variability within hookworm populations.
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These fish live in shoals or schools. Otocinclus are able to breathe air. Prior to surfacing, they will release air through their gills and mouth. Upon returning to the bottom, the fish do not need to pump their buccal cavities, indicating they are absorbing oxygen from the swallowed air.
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The small pectoral fins move constantly to control pitch, while the clavus is employed as a rudder. Juvenile sharptail molas are known to feed on benthic annelids and sponges. Adults likely feed on medusae, siphonophores, ctenophores, and salps, as well as some fishes, crustaceans, and molluscs. Sharptail molas sometimes have remoras attached to the surface of their bodies or inside their buccal cavities.
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Little is known of the feeding habits of this species. Although some members of the Brisingida have been photographed with their arms raised, apparently filter feeding, those in the genus Freyella have only been observed lying flat on the sediment and are therefore probably not suspension feeders; some have been found to have the exoskeletons of small crustaceans within their buccal cavities.
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Stilbonematines can be up to 10 mm long, with a club-like head. The worms are completely covered in a coat of ectosymbiotic sulfur-oxidizing bacteria except for the anterior region. The presence of the bacteria, which often contain intracellular inclusions of elemental sulfur, gives the worms a bright white appearance under incident light. They have small mouths and buccal cavities, and short pharynges.
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Other species devour smaller organisms, which they may catch with their tube feet. They may also feed on dead fish and other animal matter. Sand dollars may perform suspension feeding and feed on phytoplankton, detritus, algal pieces and the bacterial layer surrounding grains of sand. Many sea cucumbers are mobile deposit or suspension feeders, using their buccal podia to actively capture food and then stuffing the particles individually into their buccal cavities.
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Orthacanthus and Triodus have a predator-prey relationship where specifically there is predation of Orthacanthus on Triodus. Cranial remains of specimens of both Orthacanthus and Triodus were from the Upper Carboniferous in Puertollano basin, Spain, and give evidence of this predator-prey relationship. Numerous and well preserved cephalic elements of Triodus were associated with the cranial remains of Orthacanthus, and is explained by the inclusion of occipital spines of Triodus in the buccal cavity of Orthacanthus. Additional evidence is the co-occurrence of one Orthacanthus spine with many Triodus spines, which likely penetrated the soft tissue and cartilage of the mouth of Orthacanthus, similarly to modern sharks that feed on stingrays where the spines of stingrays have been found within and around the buccal cavities of Carcharhinus, Galeocerdo, Negaprion and Sphyrna.
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