The effects of pathogens, parasites, and familiarity on alarm cell investment in fathead minnows, pimephales promelas

Abstract

Fishes in the Superorder Ostariophysi have specialized epidermal club cells that contain an alarm substance. Damage to these cells causes the release of the alarm substance which can serve as a useful indicator of predation risk for nearby conspecifics. The majority of research involving alarm substances has investigated the roles that cues play in anti-predator contexts including learned predator avoidance. In this study I tested the effects of non-predatory stressors including pathogens, pathogen conditioned water, social dynamics, and skin-burrowing parasites on epidermal variables including alarm cell investment. In experiment 1, fathead minnows, Pimephales promelas, were exposed to different levels of the pathogenic water-mold, Saprolgenia ferax to determine the effect it would have on the epidermis. Minnows exposed to Saprolgenia had significantly more alarm cells than those exposed to the control solutions. The treatments had no effect on body condition, alarm cell size, mucous cell density, mucous cell size, or epidermal thickness. In experiment 2, social dynamics were manipulated by pairing minnows with either a familiar partner or an unfamiliar individual and exposing them to Saprolgenia ferax. The treatments had no effect on body condition, alarm cell density, alarm cell size, mucous cell density, mucous cell size, or epidermal thickness. In experiment 3, minnows were exposed to either Saprolgenia or Saprolgenia conditioned water to determine whether the physical presence of zoospores was required to induce a change in epidermal properties. The treatments had no effect on body condition, alarm cell density, alarm cell size, mucous cell density, mucous cell size, or epidermal thickness; suggesting that water conditioned by Saprolgenia may be sufficient to induce a change in alarm cell investment. In experiment 4, minnows were exposed to three different infection rates of skin burrowing parasites (trematode cercariae) to determine its effect on the epidermis. Minnows exposed to high levels of tramatode cercariae had significantly more alarm cells than those exposed to either low levels or those in the control treatment. The treatments had no effect on body condition, alarm cell size, mucous cell density, mucous cell size, or epidermal thickness. In experiment 5 and 6, cultures of Saprolegnia ferax were exposed to skin extracts from various fishes. S. ferax cultures exposed to fathead minnow skin extracts, which contain alarm cells showed the least amount of growth, while cultures exposed to swordtail skin extracts, which lack alarm cells, showed an intermediate amount of growth. Cultures exposed to fathead minnow skin extracts showed the least amount of growth compared to cultures exposed to either the synthetic alarm cue or the control. The results of these experiments suggest that disease does appear to influence alarm cell investment and there appears to be some property found in fathead minnow skin that inhibits S. ferax growth.