BEHAVIORAL RESPONSES OF BALANUS IMPROVISUS NAUPLII TO LIGHT INTENSITY AND SPECTRUM

¹ Environmental Protection Agency, Environmental Research Laboratory, South Ferry Road, Narragansett, Rhode Island 02882

A video-computer behavioral analysis system—the "Bugwatcher"—was found to be capable of rapidly and accurately analyzing the phototactic movements of stage II, III, and VI nauplii of the barnacle Balanus improvisus. Under the test conditions these larvae only display a positive phototactic response; a negative response was not observed. The response spectrum of dark-adapted stage II shows a plateau of strongest positive phototaxis between 480 and 520 nm with about 60% of test larvae swimming ± 15° toward the light stimulus. In contrast, stage VI nauplii are generally less phototactic. The response spectrum changes to have a depression at 520 nm and enhanced responsiveness to longer wavelengths of 540 to 580 nm.

Responsiveness to different light intensities changed upon light adaptation. Upon stimulation with 480 nm lights, dark-adapted stage II nauplii show a significant positive phototaxis at 6.2 x 10^-7 W/m². Peak response occurs between 10^-4 and 10^-2 W/m^-2. A signficant increase in mean linear velocity (MLV) accompanies the maximal phototactic response. In contrast, upon light adaptation, stage II nauplii show a significant positive phototaxis only to intensities above 10^-4 W/m², 480 nm. Peak response occurs between 10^-3 and 10^-1 W/m². A significant increase in MLV occurs from about 10^-3 to at least 27 W/m². Light-adapted stage III nauplii show intensity sensitivity similar to stage II nauplii; however, no increase in MLV occurs during light stimulation. Stage VI nauplii show a reduced percent phototactic response at all intensities as compared to stage II-III. Light- or dark-adaptation does not result in clean shifts in intensity sensitivity as evident in stage II responses. Differences in stage VI naupliar light responses may be related to impending metamorphisis to cyprid. A study of cyprid light response is needed to clarify this.

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