THE BEHAVIOR AND METAMORPHOSIS OF THE LARVA OF BUGULA NERITINA (LINNAEUS): EXPERIMENTAL MODIFICATION OF THE LENGTH OF THE FREE-SWIMMING PERIOD AND THE RESPONSES OF THE LARVAE TO LIGHT AND GRAVITY

Abstract

At 26-30[degree]C the larvae are geonegative and photopositive in diffuse light throughout the larval period, but at 7-10[degree]C they become geopositive and indifferent to light. Cold water prolongs the larval period. Approx. 50% of the larvae can be made geopositive at 26-30[degree]C by illuminating vials containing them only from below. Their geotaxis is not affected by changing the light intensity. Intense light has some effect in driving larvae to the bottom of a vial and makes them photonegative after a short time. Exposing them to darkness does not affect either their previous geotaxis or phototaxis. A 50% reduction in the salt content of sea water greatly prolongs the larval period and renders the larvae geopositive at 26-30[degree]C. Mechanical agitation has a somewhat similar effect. Hypertonic sea water hastens the onset of metamorphosis and causes the organisms to sink to the bottom. Metamorphosed larvae develop well in concentrated sea water, but poorly in a 50% dilution. A slight increase in osmotic pressure by the addition of nonelectrolytes does not cause the same responses as a similar increase in salt content. An excess of either NaCl or KC1 in normal sea water causes a rapid loss of pigment. NaCl greatly hastens the onset of metamorphosis, but KC1 does not. Both cause rapid sedimentation. When the osmotic pressure of diluted sea water (50%) is raised to approx. normal value by adding CaCl2 or MgCl2, metamorphosis is inhibited; the natatory period is greatly prolonged by CaCl2. A tentative explanation of geotropism in this species is offered, and the morphology and metamorphosis of the larvae are described.