A remarkable pyloric caecum in the evermannellid genus Coccorella with notes on gut structure and function in alepisauroid fishes (Pisces, Myctophiformes)

Gross morphology and development of the gut in the circumtropical genus Coccorella Roule (Alepisauroidei, Evermannellidae; two species C. atrata (Alcock, 1893) and C. atlantica (Parr, 1928)) is described and the gut structure of this genus is compared with that of other alepisauroid and myctophoid fishes. Alepisauroids characteristically have an enormous sac-shaped stomach and a simple, straight intestine. It appears that this gut morphology is a specialization for the ingestion of very large food particles and is an adaptation to the food-limited midwater environment.
Coccorella is remarkable in having a medial pyloric (intestinal) appendage that extends forward from the ventroanterior margin of the intestine, passes between the cleithra, beneath the heart, dorsal to the isthmian musculature, and ends blindly beneath the first basibranchial. This caecum is surrounded by the darkly pigmented peritoneal lining of the coelomic cavity and is easily visible in the floor of the orobranchial cavity. The caecum can be seen in the smallest available larva (6.3 mm SL) but does not develop its complete cephalic extension until the larvae are 20 to 25 mm SL.
Portions of the alimentary tract in Coccorella were examined with light and scanning electron microscopy. No substantial histological differences between caecum and intestine have been found. It is hypothesized that the caecum in Coccorella has a function similar to that proposed for pyloric caeca in other fishes, viz. to increase the resportive and storage capacity of the intestine.
Although pyloric caeca vary enormously in size, number and morphology among teleosts, Coccorella is the only genus known with any appendage of the alimentary tract extending into the head. General hypotheses on the adaptive significance of pyloric caeca are reviewed but none satisfactorily explain the appearance of this unique neomorphous feature in Coccorella.