ffn-^'dii \Yhile many opisthobranchs have special feeding preferences, the sensory mechanisms involved in their food choices are mostly unstudied (Kohn, 1961 ;Paine, 1963). Aplysia Juliana, a common sea-hare of Hawaiian waters, is almost monophagous, feeding only on Ulva lactuca, if available, but taking Ulva fasciata. if given only this. On the latter, however, the animals do not grow normally. These sea-hares may nibble on other algae, but never eat enough to grow or survive for long. They live well in marine aquaria when fed Ulva lactuca, and make good subjects for research on sensory physiology. The animals we tested were maintained in the laboratory in ten-gallon aquaria or one-gallon jars furnished with sub-sand filters. Sea water was obtained locally and used unfiltered. If given food regularly, the animals usually lived for 2-4 months, and grew from a few millimeters to 12-18 cm. long. This report describes the responses of Aplysia Juliana (hereafter, aplysia) to its food plant, Ulva lactuca (hereafter, Ulva), with data on sensory processes involved.
RESPONSES TO ULVAIf aplysias are without food for a few days, they usually bury in the sand and remain hidden. One may, on looking into an aquarium, be quite unaware that any of these animals are present. If he drops a small piece of Viva into the water, within 10-15 seconds the oral tentacles of the animals appear, followed by the heads, as the sand seems to come alive, and the aplysias crawl out, with tentacles spread (Fig. 1). They climb the sides of the aquarium, holding fast with the posterior sucker that is characteristic of this species, and extend their tentacles as if sniffing the water, as a dog sniffs the air. If an aerator is in action, the animals may seem not to orient well, but if the water is relatively quiet or moving in a specific direction, they go fairly directly toward the Ulva. Upon touching it. they immediately seize it with the mouth and commence feeding.The response to food occurs, therefore, in three major steps arousal, orientation, and feeding, as in many predatory and carrion-feeding gastropods (Kohn. 1961). One is impressed, on first seeing the arousal and orientation, by the rapidity and precision of motion for these animals, and the obvious use of the oral tentacles. On contact with the Ulva, the aplysias react even more rapidly, as if they were using a different sensory modality. Certain questions immediately come to mind: (1) What is given off by the Viva that attracts the animals? (2)
