Dissolved organic matter as food for black fly larvae (Diptera:Simuliidae)

Black fly larvae occur in many lotic habitats but often dominate the benthos of lake-outlet and brownwater (humic) stream communities. Water in these habitats contains abundant dissolved organic material (DOM). Amorphous material commonly found on black fly labral fans was formerly but erroneously thought to be an endogenously produced glycoconjugate that aided particle capture. We hypothesized that architecture of the labral fans may cause in situ flocculation of DOM, which is then directly ingested. Half-grown Simulium vittatum Zetterstedt and S. piperi Dyer & Shannon larvae were held for 84 h in stir-bar-driven rearing chambers containing filtered water with or without 3.0 mg/L brewer's yeast and with or without 20.0 mg/L DOM (an aqueous extract of moss). Water was partially replaced every 6 h. Scanning electron microscopy (SEM) was used to examine particulate material from test water collected on membrane filters and to study labral fans of selected larvae from parallel experiments. Other larvae were longitudinally sectioned and stained with Aldan Blue, which detects glycoconjugates. Concentrations of DOM (determined by ultraviolet spectroscopy) and fine particulate organic matter (measured by filtration) in chambers did not change over 84 h. Larvae in control and DOM-only trials displayed aggregative behavior typical of severely food-limited simuliids, whereas larvae in yeast-supplemented trials were haphazardly arranged. Control larvae exhibited little or no growth (increase in mean body length). Larvae in both DOM and yeast-supplemented treatments grew significantly more than control larvae, achieving growth rates of 4.0-17.0%/d (DOM-only), 11.3-14.8%/d (yeast-only), or more. Guts of larvae from all treatments stained positively, indicating that they contained glycoconjugates. Filters and labral fans examined by SEM contained yeast cells but not bacterial cells. However, labral fans also lacked evidence of adhering glycoconjugates. Small quantities of amorphous material consistent with flocculated DOM were observed on filters. Simuliid larvae can grow in a medium containing only allochthonously derived DOM. Although evidence is inconclusive, we speculate that DOM is precipitated on labral fans and ingested. Under natural conditions, DOM may be a direct and significant food source for simuliid larvae.