Abstract
The euryhaline copepodPseudodiaptomus inopinusplay important roles in coastal waters as vectors of docosahexanoic (DHA) and eicosapentaenoic (EPA) acids for larval fish. While DHA and EPA in polar lipids (PLs) are more effective for fish larval development than non-polar lipid forms (NLs), there is little knowledge how much these lipids are accumulated in copepods from microalgae and are effective for early development of fish larvae. We report PLs fatty acid profiles ofP. inopinusfed DHA-poor microalgae and evaluate its significance as a food source for larvae development ofPagrus major, compared with DHA-enriched rotifers. Copepods and rotifers were fed a mixed diet of three algal species (Phaeodactylum tricornutum,Tisochrysis lutea, andPavlova lutheri), in addition of DHA-supplemented Super Fresh Chlorella (SFC) for rotifers. Compared with SFC, the algal mixture had higher EPA but lower DHA. Copepods had higher DHA and EPA in total lipids than rotifers fed each diet. Copepod PLs were specifically enriched with DHA and their contents were higher than both rotifers. On the other hand, PLs EPA contents were comparable between preys, indicating that copepods selectively fortified the PLs. Fish culture experiment showed that larvae fed copepods had higher growth than those fed SFC-enriched rotifers. Principal component analysis for each organism fatty acid composition emphasized trophic modification of DHA by copepods toward larval fish. This study highlighted thatP. inopinuscontribute to enhanced growth of coastal larval fish by efficiently transferring DHA via copepod fatty acid metabolism.
Highlights
Based on the critical period hypothesis (Hjort, 1914), food availability during initial feeding periods is known as a crucial factor for early development of fish larvae and its recruitment
The fatty acid composition of the algal mixture was characterized by peaks of C14:0, C16:1n-7, C16:3n-4, and C18:4n-3 fatty acids, and eicosapentaenoic acid (EPA)
The result of principal component analysis (PCA) for total lipid fatty acid proportions of each trophic level revealed the followings; (1) fatty acid compositions of microalgae were commonly the most different from those for larval fish, and (2) compared with microalgae, P. inopinus and rotifers had closer characteristics of fatty acids toward larvae based on docosahexaenoic acid (DHA) and the others, respectively, (Figure 6)
Summary
Based on the critical period hypothesis (Hjort, 1914), food availability during initial feeding periods is known as a crucial factor for early development of fish larvae and its recruitment. Larval fish reared under sufficient amounts of diets further enhance their growth, survival, and development when the diets are rich in n-3 HUFA (especially DHA; Watanabe et al, 1989; Ncss et al, 1995; Shields et al, 1999; Evjemo et al, 2003; Karlsen et al, 2015). Larvae inhabiting in the coastal areas, exhibit higher growth rate when the ambient zooplankton contain higher n-3 HUFAs (Paulsen et al, 2014). These facts suggest the necessity to investigate how fatty acids are transferred toward larval fish for further understanding of fish recruitment
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