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Counter-illumination behavior in the midshipman fish, Porichthys notatus.

Robert Harper

Keywords: Behavior, Bioluminescence, Porichthys, midshipman

Porichthys notatus occurs off the western coast of North America at depths ranging from intertidal to 400m.  Its several hundred ventrally directed photophores have long been suggested to function in counter-illumination, matching the intensity of the downwelling light to mask the silhouette of the fish as viewed from below, but this has never been demonstrated.  Porichthys bury in the sediment during the day and rise into the water column at night to feed, thus rendering counter-illumination an appropriate behavior.

Using juvenile, laboratory grown fish, we find a direct relationship between downwelling irradiance and the luminescent responses of Porichthys.  Insufficient light is produced to replace that blocked by the fish under lighting conditions typical of the natural environment.  Consequently, the effectiveness of counter-illumination probably relies on the disruptive luminous pattern of the photophores.   In further support of the hypothesis that the photophores of Porichthys function in counter-illumination, we find that the angular distribution of the light produced by Porichthys matches that typical of downwelling light.  Unlike in other counter-illumination systems, Porichthys does not obligatorily counter-illuminate when exposed to downwelling light. In addition to appropriate light, application of a mechanical stimulus is required to induce counter-illumination. That the fish does not luminesce at all times in appropriate light is perhaps an adaptation to conserve luciferin, which they must obtain in the diet.

Porichthys from the geographically isolated Puget Sound population, which lacks a Vargula-type luciferin source and is non-luminous, show counter-illumination behavior when the luminescent system is activated, either by cannibalism of luminescent Porichthys or force-feeding dried Vargula tsujii.  Similarly, juvenile Porichthys from the Southern California population whose luciferin reserves have been allowed to deplete naturally in the laboratory also regain counter-illumination behavior when fed luciferin.

Submitted: 24 Oct 97

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Marine Science Institute | phone: (805) 893-3639
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