Fecal Pellet Disposal in Marine Bryozoans

Bryozoans rid themselves of fecal pellets using a more complex and varied set of behaviors than might be expected from their body plan and feeding behavior. In bryozoans from the northern Adriatic Sea and in Bogue Sound, North Carolina, expelled pellets traveled along one of four pathways to leave the zooid. In two of these pathways, fecal pellets were drawn into the lophophore in the direction opposite to the feeding current. Each pellet entered the lophophore through a temporary gap created between the two tentacles closest to the anus. Once in the lophophore, the pellet was propelled through the lophophore beyond the tentacle tips or it was drawn back through the entry gap and carried away in the outflow of filtered water. In other zooids, pellets did not enter the lophophore but were carried away, along the surface of the colony, in the flow of filtered water or were moved along the exterior of the extended lophophore to escape beyond the tentacle tips. A large majority of the Adriatic zooids and nearly half of the Carolina zooids expelled pellets that traveled into the lophophore. In many species studied, zooids rid themselves of pellets along at least two different pathways. Apparently, zooids respond to local flow environment and direct fecal pellets in appropriate directions to flush them away from the colony surface. Additional key words: ciliary currents, Ectoprocta, waste removal Bryozoans are colonial animals that feed by extracting food particles from currents of water drawn through the funnel-shaped, tentacle-bearing lophophores of the autozooids, hereafter referred to as zooids (Ryland 1970; Winston 1977, 1978, 1979; McKinney 1990). Food particles are directed to each zooid's mouth, located within the base of the lophophore, and are selectively ingested (Winston 1977, 1978, 1979). Beating of lateral cilia on the tentacles produces the feeding currents (Atkins 1932; Strathmann 1973). The filtered water flows between the tentacles (Fig. Id) and continues to the colony surface and away from the zooid until it reaches an excurrent site (Banta et al. 1974; Winston 1979; Lidgard 1981; McKinney 1989, 1990). Excurrent sites may be edges of colonies, fenestrae, or chimneys. Fenestrae are open spaces in erect laminate colonies. Chimneys are regularly spaced areas on the colony surface where shape and bending of the lophophores allows water to escape from the colony (Winston 1978, 1979). Zooids' guts are U-shaped, ending in an anus located outside and just beyond the base of the lophophore and within the tentacle sheath (Hyman 1959; Ryland 1970). The position of the anus outside the a E-mail: mckinnymj @appstate.edu lophophore would seem to provide an effective and expedient means of allowing a zooid to expel waste and have it swept away in the outflow of filtered water. Thus, the ectoproct body plan gives the impression of a better means of avoiding the mixing of incoming food with outgoing waste than that of the entoprocts (kamptozoans), in which the anus opens within the lophophore. Findings reported here, however, do not fit this simple view. Aspects of bryozoan waste removal recorded by earlier authors include the means by which pellets move through the rectum (Winston 1977), physical characteristics of and rate of pellet production (Best & Thorpe 1987), composition of fecal pellets (Gordon et al. 1987), and position of the lophophore during defecation (Silen 1944; Winston 1977; Best & Thorpe 1987; Boardman et al. 1992). Behaviors associated with waste removal are complex and varied. The study reported here was undertaken to describe surprising and previously unreported paths taken by fecal pellets after expulsion from the anus.