Mucus function and crossflow filtration in a fish with gill rakers removed versus intact

Filtration mechanisms are known for only two species of suspension-feeding tilapia, each of which relies on a different method of particle retention. We used high-speed video endoscopy to assess whether a third species of tilapia, Oreochromis aureus, with gill rakers intact as well as surgically removed, uses mucus in the oropharyngeal cavity for hydrosol filtration or uses crossflow filtration to retain particles during suspension feeding. Although a large amount of mucus was visible during feeding with rakers intact, particles were rarely retained in the mucus. The hypothesis that the presence of mucus results in particle entrapment by hydrosol filtration is rejected for O. aureus. Rather than functioning as a sticky filter, mucus is proposed to function in this species to regulate the loss of water between the rakers and between the anterior branchial arches, increasing crossflow speed and thereby increasing the inertial lift force that transports particles radially away from the arches. Gill raker removal resulted in an almost complete lack of observable mucus in the oropharyngeal cavity, probably due to the removal of mucus-secreting cells attached to the gill rakers. However, endoscopic videotapes showed that crossflow filtration continued to operate in the absence of gill rakers and mucus, indicating that the surfaces of the branchial arches play an important role in crossflow filtration.