METAMORPHOSIS OF THE VELIGER LARVAE OF NASSARIUS OBSOLETUS (GASTROPODA) IN RESPONSE TO BOTTOM SEDIMENT

¹ Woods Hole Oceanographic Institution, Woods Hole, Massachusetts

1. Experimental evidence shows that the presence of a favorable substratum has a marked effect on the metamorphosis of Nassarius obsoletus larvae. In the presence of a natural substratum the percentage metamorphosis was very significantly greater than in controls without such natural substratum. Larvae postponed metamorphosis for over two weeks when a desirable substratum was not encountered.

2. Certain substrata were preferred over others; a significant difference in the response of N. obsoletus larvae to different sediments was demonstrated experimentally. A correspondence exists between substrata preferred under natural conditions and those favored in experiments in the laboratory.

3. The physical properties of a substratum, such as median grain size and sorting do not directly influence the metamorphosis of N. obsoletus veliger larvae. This was demonstrated both experimentally and by field observations.

4. Certain biological properties are important in making the substratum attractive for the metamorphosis of N. obsoletus veligers. This was shown by the differences in settlement obtained experimentally when the sediment was treated in various ways. When a sediment was rendered completely abiotic by incineration no significant difference existed between the percentage metamorphosis on such a substratum and in controls without substratum. If the biological characteristics of the sediment were drastically altered by heating in sea water, the attractiveness of the substratum to metamorphosis by veliger larvae was significantly reduced, but it remained nevertheless significantly more attractive for metamorphosis than a substratum which had been made completely abiotic.

5. The metamorphosis-inducing properties of a substratum may be transferred to the adjacent water, as was demonstrated experimentally.

6. Experimental evidence showed that the metamorphosis-inducing factor from the substratum is probably a water-soluble substance.

7. Inasmuch as the response to a metamorphosis-inducing factor is possible without contact with the substratum, a rather sensitive chemoreception by N. obsoletus larvae must be involved. Stimulation may be followed by a complex behavior pattern which includes further testing of the substratum and subsequent metamorphosis.

8 Since N. obsoletus is primarily a deposit feeder, the selection of a favorable substratum has considerable adaptive value.

9. The ability to delay metamorphosis and the response to a favorable substratum greatly increase the probability that the larvae of N. obsoletus will terminate their pelagic existence in a favorable habitat.

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