A Comparison of Olfactory Organ Development and Feeding Behavior in Larval Fathead Minnows, Pimephales promelas Rafinesque
The early recruitment and use of the chemoreceptive senses, olfaction and gustation, are important for early behavior development in fathead minnows and the ability of larvae to survive after hatching. Chemoreception is used in the location of food, avoidance of predators, and for intraspecific communication. Furthermore, some studies have shown the olfactory system to be adversely impacted by environmental contaminants and low pH. Since the fathead minnow is a commercially raised baitfish in the aquaculture industry, and serves as a standard test organism for bioassays and reaction studies of aquatic contamination by the United States Environmental Protection Agency, an understanding of the early development of chemoreception in this species is crucial to an appreciation of its reactions to environmental insults. The development of the olfactory organ in conjunction with feeding behavior was investigated in larval fathead minnows during the first fifteen days posthatching. Larvae were presented with dead food organisms in the dark in order to isolate the use of chemoreception in feeding. Immediately following the feeding trials, the test organisms were measured, inspected for the presence of food in the gut, and separated into feeding and nonfeeding groups. These fish were then fixed and examined histologically using light microscopy (LM) or scanning electron microscopy (SEM) to determine the state of development of the olfactory organ. Ingestion of dead food organisms in the dark was first observed on Day 4 posthatching (PH) and feeding activity increased until Day 8 when 100% successful feeding in the dark occurred. After that, all test fish fed successfully in all trials with the exception of Day 11 when two fish failed to feed. In comparing size to feeding success, the average size of fish first feeding using chemoreception alone was approximately 6 mm. Regardless of age, size, and/or feeding status, LM and SEM observations revealed all specimens to have a complete olfactory system consisting of a ciliated olfactory epithelium and neuronal connections to the brain. The overall development exhibited by larval fathead minnows is not unusual for precocial species that must rapidly develop the senses to begin feeding shortly after hatching. Observations made by several authors were similar to those made in the current study, these are: the olfactory organ is present as a recognizable epithelium at hatching, the cells of which become more organized over time; a visible nerve connection between the olfactory epithelium and the brain exists at hatching; and ciliated cells are present in the epithelium at hatching, and those cells increase in number with age.