ELEMENTAL (C, N AND P) ANALYSIS OF METAMORPHOSING BONEFISH (ALBULA SP.) LEPTOCEPHALI - RELATIONSHIP TO CATABOLISM OF ENDOGENOUS ORGANIC-COMPOUNDS, TISSUE REMODELING, AND FEEDING ECOLOGY

Whole-body carbon (C), nitrogen (N) and phosphorus (P) cccontent, and stable-isotope composition ((CC)-C-13-C-:12 and N-15:N-14), were follo wed during metamorphosis of bonefish (Albula sp.) larvae (leptocephali ). Metamorphosing larvae depend entirely on endogenous carbon compound s (some of which contain N and P) as an energy source. Two fundamental questions are (1) Do the demands of extensive tissue remodeling durin g metamorphosis require the efficient retention of N and P during the catabolism of carbon compounds? (2) What effect does the lack of feedi ng have on stable-isotope composition? Our results showed that both C and N decreased by similar to 35 to 40%, reflecting the utilization of neutral lipid (triacylglycerols) and N-containing compounds (phosphat idylethanolamine and keratan sulfate glycosaminoglycan) as energy sour ces, and indicating that larvae have little or no capacity to retain N . Evidence suggested that collagen breakdown, measured as a loss of hy droxyproline content, also contributed to N loss. Stable-isotope ratio s, expressed as delta(13)C and delta(15)N, showed no statistically sig nificant differences in early and advanced metamorphosing larvae. In c ontrast to C and N, phosphorus was conserved during metamorphosis and most probably is utilized in the increased bone mineralization occurri ng in advanced larvae. We show, however, that advanced larvae are P-li mited and that normal ossification is dependent upon a supply of exoge nous P obtained after the resumption of feeding. The N:P ratio of 12.3 in early larvae decreased to 8.1 in advanced larvae owing to the cons ervation of P as N was lost. The mean delta(15)N value in early metamo rphic larvae (11.6 parts per thousand,) is consistent with results fro m other studies, and provides further support for the view that premet amorphic leptocephali feed at a very low trophic level.