Gastrodermal structure and feeding responses in the scleractinian Mycetophyllia reesi, a coral with novel digestive filaments.

Mycetophyllia reesi Wells is a colonial scleractinian coral whose outer surface consists of a series of oral-pharyngeal openings that lack tentacles. The polyps also lack a column and cannot protrude from the colonial surface. Correspondingly, there is no central digestive cavity. Instead, the pharynx is directly connected to a series of radially arranged mesenterial ducts lying parallel to the skeleton. The ducts, composed primarily of ciliated cells with small mucus inclusions and large, compartmentalized mucocytes, house filaments that protrude through the oral apertures during feeding. The filaments may or may not be directly connected with or originate from the mesenterial ducts and are histologically distinct from them. They are therefore referred to as digestive, rather than mesenterial filaments. In contrast with other scleractinians, the digestive filaments are thin, unequally bilobed stalks with spatulate ends. The cnidoglandular (CG) lobe, the larger of the two, exhibits a distinct cellular zonation. Large mastigophore cnidae and elongated zymogen-like cells are clustered at its distal end. Neither of these cells appear to respond to particulate food material, suggesting that they may be employed in alternative modes of nutrition and/or competition. Behind the distal region, the CG lobe exhibits typical zymogen, mucus, and collar cells as well as numerous atrichous nematocysts. The atrichs and zymogen cells discharge during particulate feeding. Tracts of collar cells with particularly well-defined cilia, elongated rootlets, and mucus inclusions are found at the outer edge of the CG lobe. These cells disgorge their contents during feeding and appear to function in food transport. The smaller lobe of the filament is a muscular sheet containing well-defined fields of circular and longitudinal myofibrils along with associated neurons. Collar cells with lysosome-like inclusions and large, compartmentalized mucocytes are also characteristic of this region. There are no zooxanthellae in the filaments, but these endosymbionts are present as a thin layer in the oral-most portion of the gastrodermis. The cellular zonation and multi-functionality of these digestive filaments suggest another example of a cnidarian structure at the organ level of complexity.