Free—living fathead minnows rapidly learn to recognize pike as predators

Individuals from a natural population of approximately 20 000 fathead minnows from a pike–free pond did not respond with appropriate anti–predator behaviour upon encountering pike odour in laboratory tests. However, 14 days after 10 pike were stocked into the pond, minnows had acquired recognition of pike odour. Laboratory studies have indicated several possible mechanisms for acquiring predator recognition in fathead minnows. This study indicates that these, or similar processes, can produce major changes in predator recognition in the wild.     

Epidermal club cells and the innate immune system of minnows

Thousands of fish species belonging to the Superorder Ostariophysi possess specialized club cells in their epidermis. Damage to the cells, as would occur during a predator attack, releases chemical substances that evoke antipredator responses in nearby shoalmates. These chemical substances have often been referred to as alarm substances and the cells that release them as alarm cells. Understanding the evolution of the cells in an alarm context has been difficult. The fish needs to be captured prior to the chemicals being released, hence the benefit to the receiver is unclear. Recent studies have suggested that the club cells are part of the immune system and are maintained by natural selection owing to the benefits that they confer against pathogens, parasites, and general injury to the epidermis. In the present study, we gave fathead minnows intraperitoneal injections of cortisol, a known immunosuppressant, or injections of a control substance (corn oil). We found that fish exposed to cortisol had suppressed immune systems (as measured by a respiratory burst assay) and that they also reduced their investment in club cells. This is the best evidence to date indicating that the club cells of Ostariophysan fishes are part of the innate immune system and that the alarm function of the cells evolved secondarily. © 2009 The Linnean Society of London, Biological Journal of the Linnean Society, 2009, 98 , 891–897.     

Generalization of learned predator recognition: an experimental test and framework for future studies

While some prey species possess an innate recognition of their predators, others require learning to recognize their predators. The specific characteristics of the predators that prey learn and whether prey can generalize this learning to similar predatory threats have been virtually ignored. Here, we investigated whether fathead minnows that learned to chemically recognize a specific predator species as a threat has the ability to generalize their recognition to closely related predators. We found that minnows trained to recognize the odour of a lake trout as a threat (the reference predator) generalized their responses to brook trout (same genus as lake trout) and rainbow trout (same family), but did not generalize to a distantly related predatory pike or non-predatory suckers. We also found that the intensity of antipredator responses to the other species was correlated with the phylogenetic distance to the reference predator; minnows responded with a higher intensity response to brook trout than rainbow trout. This is the first study showing that prey have the ability to exhibit generalization of predator odour recognition. We discuss these results and provide a theoretical framework for future studies of generalization of predator recognition.     

Effects of handling on endocrinology and reproductive performance of the fathead minnow

Anaesthesia with MS-222 followed by intra-peritoneal (ip) injection (with a 10% ethanol in corn oil carrier) of fathead minnow either as one or three (weekly) treatments did not affect survival, behaviour or secondary sexual characteristics of the fish. Fecundity of the fish, as indicated by fertility and hatching success, was also unaffected. Gonadal condition (relative gonad mass, histopathology) was not altered in either sex. Male and female plasma sex steroids (β;-oestradiol, testosterone, 11-ketotestosterone) and male vitellogenin concentrations were not significantly affected by the treatments. Females subjected to either ip treatment regime had significantly higher plasma vitellogenin concentrations than control females. However, based on previous data, this difference did not appear to be treatment-related. Overall, exposure of fathead minnows to chemicals via the ip route should not confound the interpretation of toxicity tests with potential endocrine disrupting chemicals.     

The effects of ultraviolet radiation on a freshwater prey fish: physiological stress response,club cell investment,and alarm cue production

Recent anthropogenic activities have caused deleterious effects to the stratospheric ozone layer, resulting in a global increase in the level of ultraviolet radiation (UVR). Understanding the way that organisms respond to such stressors is key to predicting the effects of anthropogenic activities on aquatic ecosystems and the species that inhabit them. The epidermal layer of the skin of fishes is not keratinized and acts as the primary interface between the fish and its environment. The skin of many species of fishes contains large epidermal club cells (ECCs) that are known to release chemicals (alarm cues) serving to warn other fishes of danger. However, the alarm role of the cells is likely secondary to their role in the immune system. Recent research suggests that ECCs in the epidermis may play a role in protecting the fish from damage caused by UVR. In the present study, we examined the effects of in vivo exposure to UVR on fathead minnows (Pimephales promelas), specifically investigating ECC investment, physiological stress responses, and alarm cue production. We found that fish exposed to UVR showed an increase in cortisol levels and a substantive decrease in ECC investment compared to non‐exposed controls. Unexpectedly, our subsequent analysis of the behavioural response of fish to alarm cues revealed no difference in the potency of the cues prepared from the skin of UV‐exposed or non‐exposed minnows. Our results indicate that, although nonlethal, UVR exposure may lead to secondary mortality by altering the fish immune system, although this same exposure may have little influence on chemically‐mediated predator–prey interactions. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 105 , 832–841.     

Influence of dissolved organic matter on copper binding, and calcium on cadmium binding, by gills of rainbow trout

Complexation of Cu by 5 mg Cl⁻¹ dissolved organic matter (DOM) from a marsh kept Cu from binding to gills of small rainbow trout Oncorhynchus mykiss in 9-day exposures to 0.5 μM Cu in soft water. The protective effect of DOM occurs because the formation of Cu-DOM complexes reduces the amount of free Cu in the water, so the disruptive effects of Cu on ionoregulation, such as inhibited Na uptake, cannot develop. The Cu-DOM complexes themselves do not bind to the gills. Calcium (1100 μm) reduced the accumulation of Cd by trout gills in short, 2-h exposures through competition for gill binding sites but not over longer, 7-day exposures to 0–14 μM Cd. However, the protective effect of Ca against Cd toxicity persisted throughout the longer experiment, likely due to the decrease in the electrochemical gradient for diffusive loss of Ca from the fish to the water. Rainbow trout and fathead minnows Pimephales promelas accumulated Cu and Cd on their gills in a similar manner; thus, binding constants for metal-gill interactions determined for one species of fish can be generalized to other fish species. When literature binding constants determined for fathead minnows were applied to our studies with rainbow trout, computer modelling of Cu-gill and Cu-DOM interactions simulated our results well. In contrast Cd-gill and Ca-gill modelling predicted the initial competitive effect of Ca against Cd accumulation by trout gills, but did not predict the longer-term accumulation of Cd by trout gills.     

Behavior of fish predators and their prey: habitat choice between open water and dense vegetation

Synopsis Behavior of largemouth bass, Micropterus salmoides, and northern pike, Esox lucius, foraging on fathead minnows, Pimephales promelas, or bluegills, Lepomis macrochirus, was quantified in pools with 50% cover (half the pool had artificial stems at a density of 1000 stems m⁻²). Both predators spent most of their time in the vegetation. Largemouth bass searched for bluegills and ambushed minnows, whereas the relatively immobile northern pike ambushed all prey. Minnows were closer to predators and were captured more frequently than bluegills. Even when minnows dispersed, they moved continually and eventually wandered within striking distance of a predator. Bluegills dispersed in the cover with predators. Bass captured the few bluegills that strayed into the open and pike captured those that approached too closely in the cover. The ability of predators to capture prey while residing in habitats containing patches of dense cover may explain their residence in areas often considered to be poor ones for foraging. The unit is sponsored jointly by the United States Fish and Wildlife Service, Ohio Department of NaturalResources, The Ohio State University, and the Wildlife Management Institute