Abstract
In the late 1930s, Karl von Frisch reported that semiochemicals released upon injury, act as alarm substances (Schreckstoff) in fish. In Ostariophysi species, club cells in the epidermis are believed to contain cues related to alarm substance; however, the function of club cells, primarily as reservoirs of alarm substance has been debated. Here, I describe an alarm response in the Japanese rice fish Oryzias latipes (medaka), a member of the order Beloniformes. The response to alarm substance (Schreckreaction) in medaka is characterized by bouts of immobility and an increase in cortisol levels within minutes of exposure to conspecific skin extract. Histological analysis, however, suggests that club cells are either rare or absent in the medaka epidermis. In addition to describing an uncharacterized behavior in a vertebrate popular for genetic and developmental studies, these results support the hypothesis that the primary function of epidermal club cells may be unrelated to a role as alarm substance cells. The existence of similar behavioral responses in two evolutionarily distant but well established laboratory models, the zebrafish and the medaka, offers the possibility of comparative analyses of neural circuits encoding innate fear.
Highlights
In the late 1930s, Karl von Frisch reported that semiochemicals released upon injury, act as alarm substances (Schreckstoff) in fish
In a pilot experiment using 2 adults each as experimental and control subjects, I found that after exposure to Consepecific Skin Extract (CSE), at least two of the three parameters previously described as characteristics of alarm behavior of fish reared in laboratories[32,37], were evident in experimental subjects (Supplementary Figure S1)
The same procedure failed to reveal morphologically matching cell type in any of the medaka examined (Fig. 3b,d, Supplementary Figure 2). Other secretory cells such as the goblet mucous cell could be detected in both zebrafish and medaka, albeit there were differences in numbers and size. In this set of experiments, I show that like many Ostariophysi, medaka display an alarm response to chemical cues released upon injury to conspecifics
Summary
In the late 1930s, Karl von Frisch reported that semiochemicals released upon injury, act as alarm substances (Schreckstoff) in fish. A student of von Frisch, suggested that a type of large secretory club cell (Kolbenzellen) in the teleost skin that do not open on the epidermal surface, are associated with alarm substance and termed them Schreckstoffzellen (or fright substance cells; reviewed[22]). He arrived at this conclusion having collated observations of fright response, or the lack of it, in species of fish from over 70 families. The presence of club cells was considered morphologically unique at that time and became one criteria for merging Ostariophysi and Gonorynchiformes into a single superorder[23]
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