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

¹ University College, New York University

1. The distribution, external morphology, swimming movements and metamorphosis of the larvae of B. neritina are discussed. During metamorphosis the eye-spots of the larva function in orientation to light.

2. At room temperature (26-30° C.) the larvae under laboratory conditions are geonegative and photopositive during the larval period. Just before metamorphosis they lose their intense positive reaction to light and become more or less indifferent.

3. A reduction of temperature causes larvae to become geopositive and simultaneously prolongs their free-swimming period. When either ascending or descending gradients of temperature are employed, the greater number of larvae change their responses to gravity between 20 and 23.5° C.

4. In cold water (7 to 10° C.) larvae lose their intense positive response to light and become more or less indifferent. Their behavior at reduced temperatures shows that they may become geopositive without exhibiting a simultaneous negative reaction to light. Their descent to the bottom of a vial is not caused by a negative phototropism. Either of the two tropisms may be independent of the other.

5. Larvae made geopositive by a reduction in temperature do not swim upwards towards a source of illumination, when the rays can enter a vial only from above. Neither do they react negatively to light, when the rays enter only from below. Their positive geotaxis cannot be modified by light. At room temperature, however, approximately half the larvae, originally geonegative, swim downwards towards a source of light placed beneath a vial.

6. At 28-30° C. the geotaxis of the larvae is not affected by rapid changes in light intensity.

7. At room temperature, intense light (sunlight and artificial illumination) has some effect in driving larvae to the bottom of a vial.

8. When larvae are returned to diffuse daylight after a 30-minute period in darkness, they do not change either their phototropism or their geotaxis, when they are maintained at low (7-8° C.) or high (26-30° C.) temperatures. Darkness, however, probably reduces their activity and shortens the larval period.

9. A reduction (40-50 per cent) of the salt content of sea water greatly prolongs the natatory period and causes larvae to become geonegative after a long period of swimming at the surface. Hypertonic sea water, however, greatly reduces the free-swimming period, induces a more rapid onset of metamorphosis and causes the larvae to become geopositive. The development of larvae that metamorphosed in sea water of various salinities is discussed.

10. A slight increase in the osmotic pressure of diluted (50 per cent) sea water by the addition of a non-electrolyte does not cause the same response as a similar increase in salt content.

11. An excess of either sodium or potassium causes a rapid loss of pigment. Potassium has the more pronounced effect. Sodium induces a rapid onset of metamorphosis, but potassium does not. Both sodium and potassium cause rapid sedimentation of larvae.

12. A similar excess of calcium greatly prolongs swimming movements and inhibits metamorphosis. An excess of magnesium inhibits metamorphosis, but does not cause prolonged swimming.

13. Mechanical agitation, hypotonic sea water and calcium apparently bring about a positive geotaxis by abnormally prolonging the free-swimming period of the larvae.

14. A tentative explanation of geotaxis in these organisms and a discussion of the effects of various factors on their metamorphosis are presented.

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