Histological evidence of accumulation of iron in postlarvae of red abalone, Haliotis rufescens

AbstractThe effect of iron on abalone postlarvae (Haliotisrufescens) was investigated in a controlled-cultur-ing system. Three iron concentrations (0.15, 1.5and 15 mg L 1 of Fe) and a control (no ironadded) were used to culture H. rufescens postlar-vae while being fed the diatom Navicula insertaover 10 days. Results indicate that H. rufescenspostlarvae accumulate iron granules in the stom-ach, digestive gland and mantle, but not in thegills or other tissues. The number and diameterof iron granules in tissues increased with increas-ing iron concentration in the culturing environ-ment. The iron accumulation is assumed to havebeen acquired in the digestive system through theiron-enriched diatom feed and in the mantlethrough subcutaneous iron transfer. The lack ofiron granules in the gills suggests that iron is notabsorbed through the respiratory system, as is thecase for many filter feeding bivalves. Exposure tothe highest iron concentration (15 mg L 1 )resulted in tissue abnormalities where granulesaccumulated, and may have significantly affectedthe health of H. rufescens postlarvae. These find-ings provide valuable information for the regula-tion of appropriate iron levels within aquaculturesettings and highlights the importance of moni-toring iron levels within abalone larval culturingenvironments.Keywords: abalone postlarvae, Haliotis rufes-cens, iron granules, histological sections, heavymetal exposure, tissue disintegrationIntroductionFor most marine invertebrates, such as abalone,the larval stage is one of the most vulnerable timesin their life cycle. After the first week in the watercolumn, abalone larvae undergo initial settlementand metamorphosis. Once in their benthonic lifestage, postlarvae begin to feed on microalgae, untilthey complete the development of the radula anddigestive system to consume macroalgae. Thepost-larval period is a critical step in their develop-ment, and requires a good supply of biofilms withdiatoms and bacteria (Searcy-Bernal 1996). How-ever, during the first few weeks after settlement,massive mortalities (>90%) often are observed,especially if the complex of bacteria and diatombiofilms are too dense (Hahn 1989; Searcy-Bernal,Salas-Garza & Flores-Aguilar 1992a; Searcy-Ber-nal, Salas-Garza, Flores-Aguilar & Hinojosa-Rivera1992b). This density-dependent mortality appearsto be related to increased susceptibility to post-larval stresses, such as oxygen saturation andboundary layer conditions (Searcy-Bernal 1996).Postlarvae also may be exposed to high concentra-tions of environmental contaminants (e.g., heavymetals, pathogens), and chemical fluctuations intheir surroundings, which often cause high mor-talities. Within these stressful environments, heavymetals are particularly dangerous for marineinvertebrate larvae and early juveniles, since thesechemicals are easily incorporated and accumulatedin soft tissues (Chang & Reinfelder 2002; Viant,Walton, TenBrook & Tjeerdema 2002; Ferrer, And-rade, Asteasuain & Marcovecchio 2006; Gorski &