Feeding behavior, epidermal structure and mucus cytochemistry of the scleractinian Mycetophyllia reesi, a coral without tentacles

Abstract

The scleractinian Mycetophyllia reesi lacks even the vestiges of tentacles, but quickly captures particulate food by mucus entanglement. Mesenterial filaments emerge through the oral opening, collect the mucus-embedded particulates, and withdraw to the gastrovascular system within 15 min. Mucocytes dominate the outer epidermis with about 3000 cells/mm 2 and are capable of apocrine discharge en masse. Mucocytes are spumous, typically with web-like inclusions, which for the most part lack electron opacity with ordinary staining, and are only weakly PAS positive. In contrast, the mucus reacts strongly to diamine and other reagents that suggest an appreciable acidic mucopolysaccharide component. The strongest staining reaction occurs in the presence of high iron diamine, suggesting with other tests that the mucus contains significant quantities of sulfated polysaccharides. Cells with cilia anchored by spiriliform microvilli flank the mucocytes and possess small, spumous inclusions that contain acidic, sulfated, and neutral polysaccharides that do not appear to discharge during feeding. These support cells are closely intertwined with narrow, sinuous, secretory cells containing an electron-opaque cytoplasm of unknown composition that is discharged along with mucus during feeding. The outer epidermis also contains scattered cnidae, rather than the clusters or batteries typical of tentacles. The overwhelming abundance of mucocytes is consistent with their importance in feeding. Likewise, the small number of epidermal cnidae suggest they play a minor role in acquiring food. An inner epidermal layer associated with the mesoglea contains epitheliomuscular cells, nerve cells and pigment cells. The two epidermal layers form an essentially pseudostratified, architecturally simple epithelium.