Anaerobic zoospore-producing fungi that inhabit the gastrointestinal tract of herbivores, especially ruminants, have recently been discovered. These fungi have been isolated from the rumen, hind gut, and the feces of ruminants. Thirteen species, belonging to five genera, of these fungi have been assigned to the class Chytridiomycetes. These species are classified according to the number of flagella on the zoospores and the types of thalli that develop from the zoospores. Their life cycle consists of a zoospore that encysts and develops into a vegetative thallus with zoosporangia, which at times become resting sporangia. These fungi produce a wide range of active hydrolytic enzymes, notably cellulases and xylanases, that provide them with the potential to degrade the major structural polysaccharides in plant cell walls. Their cellulases are among the most active reported to date and solubilize both amorphous and highly ordered cellulose. Their esterases are active against both feruloyl and p-coumaroyl arabinoxylans, which provides an advantage in degrading poorly biodegradable cell walls. They degrade lignin-containing cell walls, but do not metabolize the lignin moiety. Rhizoids of vegetative thalli penetrate cell walls, and they are better able than bacteria or protozoa to attack recalcitrant tissues and weaken the textural strength of plant material.

Fresh gut contents of the wood-eating loricariid Panaque and a generalized loricariid Liposarcus sp. had enzymatic activity directed against both cellulose and hemicellulose. Aerobic cultures made from the guts of Panaque exhibited growth on a minimal salts medium containing only crystalline cellulose as a carbon source as well as on a variety of other substrates containing carbon polymers found in wood. Anaerobic cultures made from Panaque guts only grew with glucose as a carbon source. Cultures of whole gut contents grown on a yeast extract basal salts medium had significant cellulolytic activity. However, no culture of individual microbes had significant cellulolytic activity, suggesting that any cellulose breakdown which occurs in loricariid guts is by a consortium of micro-organisms. A variety of aerobes, microaerophiles and facultative anaerobes were found in the guts of Panaque; several of these bacteria appear to be new species. 1999 The Fisheries Society of the British Isles

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The development from zoospore to a mature thallus in Neocallimastix sp. isolated from a Georgia cow was studied at the light microscope level. The zoospore had 9–14 posteriorly directed flagella, and its shape varied from amoeboid in agar to ovoid in broth. Encysted zoospores developed endogenously into extramatrical ovoid or spherical incipient zoosporangia with extensively branched intramatrical rhizoids that often had constrictions. Sessile mature zoosporangia varied in shape, and zoospores were fully formed within zoosporangia before release through an apical pore. In agar, zoospores encysted close to the parent zoosporangium and developed endogenously into second generation zoosporangia or exogenously into elongate thalli. At maturity, an elongate thallus was made up of a sporangium, a sporangial stalk, a cyst, and branched rhizoids. Elongate thalli were sometimes formed in broth. Melanized resting sporangia were formed on branched thalli in old (> 36 h) cultures. Two isolates of Neocallimastix frontalis from a cow and sheep and Neocallimastix patriciarum were grown under the same conditions as our isolate, and the morphology of zoospore, zoosporangium, and melanized sporangium of the four isolates were compared. In broth, the isolates developed in the same manner and formed elongate thalli and melanized sporangia as described for our isolate. There is insufficient justification, based on morphology alone, for separating the four isolates. The importance of basic light microscopy is discussed. Key words: Neocallimastix, development, morphology.