ORIENTATION AND CURRENT-INDUCED FLOW IN THE STALKED ASCIDIANSTYELA MONTEREYENSIS

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Abstract
1. Ambient water currents enhance internal flow and feeding in Styela montereyensis by forcing water through the branchial basket. Induced flow depends on upstream orientation of the incurrent siphon. 2. In open-coast and protected-outer-coast habitats, most animals occur in surge channels or on upward-facing horizontal surfaces where they sway freely with the surge. Orientation takes place as the flexible stalk bends with the passing of each wave, keeping the posteriorly-recurved incurrent siphon directed upcurrent. 3. In calmer water, animals' growth form is characterized by relatively shorter stalks and ventrally-inclined incurrent siphons. Adults in these habitats are mostly oriented with the mid-saggital plane parallel to the surface of the water. This orientation allows the ascidians to utilize the dynamic force in water surges coming from a single direction. 4. Observed patterns of microdistribution probably result from differential mortality rather than habitat selection. Larvae delay metamorphosis for a short time in complete darkness and show no strong preferences for shaded substrata. Otherwise, they seem almost non-discriminatory with regard to attachment sites.

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