Graduate Thesis Or Dissertation
 

An investigation into the metamorphosis of Dicamptodon ensatus (Eschscholtz)

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  • The investigation was conducted from January 1965 to March 1966. An intensive effort was made to obtain large numbers of animals, as it is believed that unavailability of specimens has been the limiting factor in the amount of research concerning this species. Two hundred and sixty-nine animals were collected. They are listed in the Appendix. It is desired that this list will serve as a source of reference for future investigators. Four methods were used to obtain specimens. The one employed determined the size and form, larval or adult, of the animals taken. Effectiveness of the method used was largely dictated by weather and water conditions. The animals were maintained in water which had been filtered through activated charcoal and crushed oyster shells. Although fair numbers of Dicamptodon were obtained, many died while in the laboratory. Larvae of all ages, when placed in filtered water, emptied the contents of their stomachs and fouled the water. This, coupled with cannibalism, resulted in an excessive loss of specimens. Others died from unknown causes. Thus, the study was restricted by the small numbers of animals available for observation. Nonbreeding larvae were injected with L-thyroxine and observed daily for evidence of transformation. In addition, second-year larvae were observed during normal metamorphosis. The sequence of events and their time relationships were studied. Only external changes, consisting largely of the atrophy of larval structures and pigmentation changes, were considered. Some second-year larvae were maintained at 4° Centigrade, while others were kept in deep water, at room temperature, to determine the effects of low temperature and water depth on transformation. Neither blocked metamorphosis. Data pertinent to size, age, and time of year at transformation were obtained. Second-year larvae, not subjected to thyroxine treatment and ranging from 111.50 to 166.25 millimeters in total length, exhibited metamorphic changes. Three were transforming when collected. One specimen metamorphosed three months earlier than the time formerly proposed for the species. The data of this study indicate that transformation begins in early spring and continues until the end of August. A series of first-year larvae, maintained throughout the investigation, showed no tendency to metamorphose. Neotenes responded positively to intraperitoneal injections of L-thyroxine, their own thyroid homogenate, and pituitary homogenate prepared from the glands of Taricha granulosa and Rana pipiens. One animal received pituitary homogenate prepared from its own gland and those of other neotenes. It exhibited no response over a seven-month period. Metamorphic changes were induced in second-year larvae by treatment with neotenic thyroid homogenate. The data, although tentative, indicate that tissue insensitivity is of minor importance in the metamorphic failure of this species. Pituitary derangement (lack of releasing factor) appears to be a more feasible explanation.
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