The Ctenophore Mnemiopsis leidyi Has a Flow-Through System for Digestion with Three Consecutive Phases of Extracellular Digestion

Abstract

The ctenophore (comb jelly) Mnemiopsis leidyi is a periodically abundant and voracious predator in U.S. coastal waters.Mnemiopsis leidyi is especially competitive at high prey concentrations because of its very efficient extracellular digestion. W e investigated the functional basis for these outstanding digestion capabilities. Extracellular digestion takes place in the pharynx and consists of three distinct and consecutive phases. The three phases take place in different regions of the pharynx so that various prey items can be treated simultaneously in each phase. The first phase is acidic, while the second and the third are alkaline. Extracellular digestion is completed by ciliary currents that mechanically disrupt the predigested food. Bulky indigestible food fragments are expelled through the mouth. Except for a small area, the paths for ingestion and egestion are separate. Hence, both ingestion and egestion can occur simultaneously. The flattened and elongated shape of the pharynx provides the morphological basis for this flow-through system with various regions for different digestive treatments of the food. This system is highly elaborated compared with those of other lower invertebrates and allows for an efficient, fast, and simultaneous digestion of many prey items, which accounts for the outstanding feeding capabilities of M. leidyi.

Developmental expression of homeobox genes in the ctenophore Mnemiopsis leidyi

“…Although there are no obvious morphological differences along the pharynx early in development, there is a great deal of differential gene expression that may signify a greater complexity at the molecular level. This is also supported by a study of the mechanisms of extracellular digestion in adult M. leidyi (Bumann and Puls 1997). They have shown that the flattened and elongated shape of the pharynx allows for a flow-through system where different regions correspond to different digestive processes.…”

Defecation by the ctenophore Mnemiopsis leidyi occurs with an ultradian rhythm through a single transient anal pore

“…Evolution of the gut is a prime example. Contrary to a recent version of the historical record (Presnell et al., ), the prevailing consensus in both the scientific literature and textbooks is that the ctenophore gut is not a blind sac with a single opening (bottle‐gut), but a through‐gut with mouth and separate anus (Barnes, ; Brusca & Brusca, ; Bumann & Puls, ; Dunn, Leys, & Haddock, ; Franc, ; Hernandez‐Nicaise, ; Hyman, ; Main, ; Mayer, ; Pearse, Pearse, Buchsbaum, & Buchsbaum, ; Sullivan, ; Tamm, , ). This has been well known since the classic studies by Agassiz () and Chun (), and was one of the reasons why Hyman () set up Ctenophora as a separate phylum instead of remaining a subgroup of Coelenterata (now Cnidaria).…”

“…Following ingestion of prey by various means in different orders (Colin, Costello, Hansson, Titelman, & Dabiri, ; Haddock, ; Harbison & Madin, ; Harbison et al., ; Moss, ; Swanberg, ; Tamm, , ; Tamm & Moss, ; Tamm & Tamm, , ; Waggett & Costello, ), consecutive phases of extracellular digestion take place in an oral–aboral sequence along the stomodaeum (Bumann & Puls, ). Bulky indigestible food fragments (exoskeletons, bones, etc.)…”

Messages from the Inside. The Dynamic Environment that Favors Intestinal Homeostasis

“…An important step in the evolution of the digestive system is represented by the digestive sac. Cnidarians are multicellular organisms that represent a crucial step for the evolution of the gastrointestinal tract as they evolved a single opening followed by a cavity that served as a digestive space where extracellular digestion produces products ultimately distributed to the entire body (3). Hydra in particular are elegant multicellular organisms that use their tentacles to introduce food in their gastrovascular cavity (4).…”