Functional Morphology of the Branchial Basket Structures Associated with Feeding in the Atlantic Menhaden, Brevoortia tyrannus (Pisces: Clupeidae)

Abstract

Structures of the branchial basket associated with filter-feeding in Atlantic menhaden, Brevoortia tyrannus were studied with light and scanning electron microscopy. Surface ultrastructure, epithelial organization and gross morphology were analyzed with respect to food-particle capture, transport of food from point of capture to point of ingestion and potential gustatory reception. Predictions of the fluid dynamics involved in particle capture and handling were based on histological characterization of the mucous glycoproteins produced in the epithelium of the feeding structures. Branchiospinules, the sites of small-particle capture, lack mucous cells, suggesting that food is captured primarily by mechanical sieving. Particles probably move down the length of the raker blade by two methods: (1) one side of the raker has mucous cells and particles apparently are held in dynamic balance as they are transported the length of the blade, either over, or complexed with, a mucous layer; (2) the other side of the raker lacks mucous cells and particles apparently move down the blade in a vortex formed in a recessed groove. The arch sections where most filtration takes place have a cutaneous fold over the mesial raker elements. The epithelium on the lateral surface of the cutaneous fold is rich in mucous cells. The positioning of this surface relative to the base of the filtering raker suggests that particles coming off the raker blades are held in a dynamic balance and are transported back into the buccal cavity over, or complexed with, a mucous layer. Particles apparently cascade to the more interior arches via the epi-ceratobranchial joint. The mucus associated with the higher energy filtering environments was more viscous. Taste buds were found in two locations in the buccal cavity: those on the glossohyal probably are mechanoreceptors, while those on the crest of the fifth branchial arch probably are chemoreceptors.