Abstract
Epidermal club cells (ECCs), along with mucus cells, are present in the skin of many fishes, particularly in the well-studied Ostariophysan family Cyprinidae. Most ECC-associated literature has focused on the potential role of ECCs as a component of chemical alarm cues released passively when a predator damages the skin of its prey, alerting nearby prey to the presence of an active predator. Because this warning system is maintained by receiver-side selection (senders are eaten), there is want of a mechanism to confer fitness benefits to the individual that invests in ECCs to explain their evolutionary origin and maintenance in this speciose group of fishes. In an attempt to understand the fitness benefits that accrue from investment in ECCs, we reviewed the phylogenetic distribution of ECCs and their histochemical properties. ECCs are found in various forms in all teleost superorders and in the chondrostei inferring either early or multiple independent origins over evolutionary time. We noted that ECCs respond to several environmental stressors/immunomodulators including parasites and pathogens, are suppressed by immunomodulators such as testosterone and cortisol, and their density covaries with food ration, demonstrating a dynamic metabolic cost to maintaining these cells. ECC density varies widely among and within fish populations, suggesting that ECCs may be a convenient tool with which to assay ecoimmunological tradeoffs between immune stress and foraging activity, reproductive state, and predator–prey interactions. Here, we review the case for ECC immune function, immune functions in fishes generally, and encourage future work describing the precise role of ECCs in the immune system and life history evolution in fishes.
Highlights
Epidermal club cells (ECCs) have been extensively studied in the context predator–prey ecology, because they are the presumed source of chemical alarm cues released during predator attacks [1,2]
Pfeiffer published a review [4] that included much of his own research, showing that alarm reactions were widespread among fish species in the superorder Ostariophysi, and absent in the non-Ostariophysans tested
ECCs were unique to the Ostariophysi and concluded that these club cells were a strong candidate for the source of the alarm cue
Summary
Epidermal club cells (ECCs) have been extensively studied in the context predator–. prey ecology, because they are the presumed source of chemical alarm cues released during predator attacks [1,2]. ECCs were unique to the Ostariophysi and concluded that these club cells were a strong candidate for the source of the alarm cue He labeled the cells “alarm substance cells”, arguing that ECCs, being on the surface of the body, thin walled, and having no duct with which to release their contents to the external environment, ECCs would be among the first cells ruptured in an attack by a predator and release of their contents would thereby indicate the presence of an actively foraging predator. Smith [5] hypothesized that senders may benefit from their own injury-released compounds if alarm cues attracted additional predators which in turn increased the prey item’s survival probability [7,8] In these specific cases, ECCs may be considered exaptations [9]. We discuss the basic biology and distribution of ECCs, the existing evidence for immunological function of club cells, and propose that ECCs provide a convenient tool for ecoimmunological studies to investigate interactions among immune function and trade-offs with other ecological functions such as predator avoidance and reproduction
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