Date Awarded


Document Type


Degree Name

Doctor of Philosophy (Ph.D.)


Virginia Institute of Marine Science


In view of questions concerning the digestive and assimilative abilities of larval marine fishes, the changes in these processes with development, and their relationship with starvation-induced mortality; this study has three objectives: (1) to examine the morphological and histological development of the alimentary canal and associated structures of larval spot, Leiostomus xanthurus (Pisces: Sciaenidae); (2) to assess short-term carbon assimilation in discrete age cohorts of larval spot; and (3) to relate any changes in assimilation to development. The alimentary canal and associated structures of larval spot from hatching through transformation (metamorphosis) were examined using light microscopy. These findings were related to results of experimental-radiotracer studies of carbon assimilation in discrete age cohorts of larval spot. Cytological evidence suggested that the alimentary canal and associated structures were functional in first-feeding larvae after the completion of yolk-sac and oil-globule absorption, and changed little prior to transformation. There is no elaboration of tissues during the larval phase. Major development changes associated with transformation were accompanied by changes in habitat and feeding regime. The behavior of carbon-14 indicated that food was quickly digested and carbon assimilated following the ingestion of a uniformly labeled ration. A percentage of newly assimilated carbon entered rapidly into short-term metabolic processes and exited the larva via respiration beginning as early as 1 h after ingestion. Gut evacuation was complete within 6 or 7 h. Uptake, short-term retention, and loss of ('14)C were used to compute two parameters, carbon retention and carbon absorption, that were taken as relative indices of the coefficient of utilization and absorption efficiency. The relationships between these two indices of carbon assimilation and measures of development indicated that assimilative abilities did not improve with larval development. Because there were no major changes in alimentary canal tissues, this might be expected. The indices of assimilation were negatively related to larval condition factor and this relationship has important implications to larval growth and survival. Condition factor is an indicator of larval growth and robustness. Larvae that have been feeding well enough to survive, but less than optimally for maximum growth, apparently assimilate more carbon from a ration than do larvae with a better growth history. These results are plausible in evolutionary terms. Larvae may mitigate the risk of a patchy food distribution by fully utilizing available resources. Such compensatory mechanisms may be of great adaptive significance to pelagic marine larvae.



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