| Larvae of Utterbackia imbecillis normally undergo metamorphosis to the juvenile while attached to the gills or fins of a host fish; however, metamorphosis can also be induced in the laboratory in vitro, in a modified cell culture medium. This study examined the metamorphic period and the juveniles that result from the two rearing conditions. Metamorphosis occurs in 2 stages. The first stage is characterized by the degeneration of the larval adductor muscle and high levels of DNA, RNA, and protein synthesis. It is also during this stage that the larval mantle cells form a structure known as the mushroom body. These cells possess a system of vacuoles similar to that seen in the digestive cells of many molluscs. These vesicles contain acid phosphatase, a digestive enzyme, and are presumably involved in both intra- and extracellular digestion. In addition, they appear to be storage sites for lipid and glycogen, especially in juveniles reared on their host fish. The second stage of metamorphosis occurs within the last few days before the animals develop into free-living juveniles. This stage is characterized by high levels of DNA, RNA, and protein synthesis and the development of the juvenile morphology. The present study examined the physiological ecology of juveniles from both rearing techniques. Fish-reared animals possess significantly higher nutrient reserves in the form of triglycerides, cholesterol, and glycogen immediately following metamorphosis, as compared to animals reared in cell culture. In addition, the response of fish-reared juveniles to thermal stress, as measured by protein, RNA, and the RNA:DNA ratio, is similar to that of well-fed animals, while the response of in vitro-reared juveniles more closely resembles starved animals. The results also indicate that not only are fish-reared juveniles more tolerant to thermal and hypoxic stress, but they also grow faster and have higher survivorship than those animals reared in vitro. |
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