Author index

Abstract

We used the everted-sleeve method (Karasov & Diamond, 1983) and dual radiolabelled substrates to measure carrier-mediated and diffusive fluxes of glucose and four amino acids (AA) across the gut wall of the mobile, surface deposit-feeding holothuroid Parastichopus californicus Stimpson. The relationship between substrate concentration and carrier-mediated uptake rate for d-glucose, l-aspartic acid, l-glutamic acid, l-methionine, and l-tyrosine was adequately described by the classic Michaelis-Menten equation. Passive uptake rates were controlled by substrate interaction with the hydrophobic gut membrane. For the hydrophilic compounds (glucose, methionine and aspartic and glutamic acids) passive uptake contributed 11% or less to total uptake but accounted for 21% of the total for the rare but hydrophobic AA l-tyrosine.Carrier-mediated uptake rates of hydrophilic (and lipophobic) AA were inversely proportional to the relative availability of substrate in sedimentary food. Balance in absorptive delivery was achieved by compensating for rarity in sedimentary food (low luminal concentration) and retarded cross-membrane diffusion with added carriers. Thus the relatively rare (in the food) and lipophobic AA l-methionine possessed the highest carrier-mediated maximal flux (Jmax = 57 pM (mg wet mass of gut tissue)−1 min−1) as well as the highest apparent half-saturation concentration. Glucose, aspartic and glutamic acid, all relatively common sediment organic constituents with intermediate passive permeabilities, had intermediate Jmax values ranging from 6 to 26 pM · mg−1 · min−1. The rare but lipophilic and thus highly permeant AA l-tyrosine had the lowest Jmax of 2.6 pM · mg−1 · min−1, with passive diffusion accounting for substantial uptake.The distribution of nutrient uptake rates along the gut depends upon substrate carrier densities and luminal concentrations of the products of digestion. Rates interpolated at regional (fore-, mid- and hindgut) concentrations were high in the foregut and midgut but decreased in the hindgut for the rare AA methionine and tyrosine; conversely they were low in the foregut and high in both midgut and hindgut for the common AA glutamic and aspartic acid. Despite these varying patterns of AA flux along the gut, when integrated over the whole gut length to estimate total amount of AA absorbed, just as in the sediment food and animal tissue the acidic AA dominated while tyrosine was rare. The total amount of methionine absorbed far surpassed tissue requirements, however, suggesting that it is hyperessential. Our results indicate that AA compositional equality of uptake ratios with food and tissue composition ratios serves as a useful null hypothesis for identifying hyperessential nutrients. Karasov has used AA: sugar uptake ratios to classify vertebrates as herbivores, omnivores or carnivores. By this criterion Parastichopus californicus is clearly a herbivore, showing a very low ratio.