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The numbers of various plankton animals, drawn from an experimental vessel by a siphon tube, were much less than the numbers which would have been caught had the animals been distributed uniformly and shown no tendency to escape from the siphon intake.Inequalities in distribution were generally insufficient to account for the differences observed, which must generally be attributed to the escaping capacities of the animals. They escaped most readily in still water and general illumination. Catches were increased in darkness and, still more so by putting a small light in the siphon intake.Shaking the experimental vessel resulted in increased catches, in various conditions of light and darkness, the only exception being Tisbe furcata. When illuminated (though not in darkness) this harpacticoid responded to turbulence by seeking the walls and bottom of the vessel and thus escaped the intake even more readily in those conditions.Of four copepods and two types of larvae studied, Temora longicornis was caught least readily, provided the water was still. The others, in apparent order of escaping capacity, were zoea larvae of Portunus, Tisbe furcata, metatrochophore larvae of Poly dor a, Centropages hamatus and Acartia clausi. In certain conditions this order was disturbed, for instance through the effect of turbulence on Tisbe and through the zoea larvae being caught in particularly large numbers, when the intake tube was illuminated.Ability to escape in total darkness suggests sensitivity to small gradients of currents or pressure. Observations and previous work on copepods suggest that the antennules may be involved in this.

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Using a single-beam, wavelength-scanning, dichroic microspectrophotometer, we measured absolute absorbance, bleaching difference absorbance, and linear dichroism spectra from isolated retinal receptors of the black sea bass, Centropristis striata. We determined, among other properties, the wavelength of peak α-band absorbance (λmax) of the pigment of the receptor cells. Out of well over 100 recordings, we found only 3 spectral types of visual pigment. The shortest-wavelength-absorbing type (λmax = 463 +/- 2 nm) was present only in single cones. Both members of the double cones contained the longest-wavelength-absorbing pigment of the three, with λmax = 527 +/- 5 nm. Rods were found to bear a typical rhodopsin, with λmax = 498 +/- 2 nm. Thus, the retina of this predatory demersal fish appears to use a set of three closely spaced visual pigments, with λmax clustering about 500 +/- 30 nm. This remarkable feature is discussed in relation to photic conditions in the habitat.

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