Spatial shifts in food sources for macrozoobenthos in an estuarine ecosystem: carbon and nitrogen stable isotopes.

Abstract Carbon and nitrogen stable isotope (d 13 C and d 15 N, respectively) analyses were made on estuarine macrozoobenthos in order to examine the relationships between their feeding habits (feeding mode and food selectivity) and the spatial shifts in food sources from upstream to downstream in an estuary. The d 13 C values of two ocypodid crabs were similar to those of benthic diatoms, indicating that they use their specialized mouth parts to selectively feed on benthic diatoms. The d 13 C values of a gastropod and another ocypodid crab at the site furthest downstream were higher than values at an upstream site, suggesting that these unselective deposit feeders shift from feeding mainly on benthic diatoms downstream to feeding on sediment organic matter (SOM) upstream. The d 13 C values of deposit feeding polychaetes were not significantly different among sampling sites, indicating that they feed mainly on SOM at all sites. These results show that species-and site-specific feeding habits must be considered when evaluating the roles of macrozoobenthos in regulating estuarine material flows

稲わらトラップ法によるゾウリムシ(Paramecium caudatum)およびアメーバ(Amoeba proteus)の採集と小麦粒を加えたKCM溶液による簡易培養

稲わらトラップ採集法の有効性を検討するために,岩手医科大学の矢巾キャンパス内にある雨水調整池にトラップを設置し,約4週間絞り汁を経時的に採取してゾウリムシの個体数を測定するとともに,アメーバの有無を調べた.その後,これらの絞り汁やKCM溶液に小麦粒1個を加えてゾウリムシとアメーバの培養を行った.その結果,ゾウリムシはトラップ設置2週間後に最大の個体数に達し,その後徐々に減少して最大個体数の1/3～1/4になった.アメーバはトラップ設置1週間後に出現し,4週間後まで存在が確認された.小麦粒による培養ではゾウリムシは培養開始1週間で,アメーバは2週間で実験観察に十分な個体数に増殖し,その後この状態は約1ヶ月間維持された.以上より,稲わらトラップによる採集法を用いると,ゾウリムシやアメーバの採集が秋期にも可能であることが確認された

A low-cost sexual ornament reliably signals male condition in the fiddler crab Uca beebei

Empirical studies of the reliability of sexual signals are needed to test the prediction that females should prefer males in good condition. We examined whether an attractive sexual signal used by male fiddler crabs, Uca beebei, reliably indicates male condition by measuring male blood glucose and lactate levels in the field. The signal is a mud structure, a ‘pillar’, that attracts females for mating. Blood lactate levels (a measure of energy expenditure) of pillar builders (P males) immediately after they built their pillars did not differ from those of nonbuilders (NP males), suggesting that the cost of building a pillar is small. However, P male lactate levels were significantly higher than those of NP males late in the activity cycle after P males had invested more in vigorous courtship using claw ‘waving’. Whereas P males maintained their glucose levels (a measure of energy availability as the major fuel for ATP production) at around 30 mg/dl throughout their daily activity period, NP males showed significantly lower levels in the middle of the period. These results confirm that pillars reliably signal a male’s condition, as measured by his ability to maintain a blood glucose level necessary for costly courtship, even though the construction of a pillar has minimal energetic cost

Assimilation of terrigenous organic matter via bacterial biomass as a food source for a brackish clam, Corbicula japonica (Mollusca: Bivalva)

Corbicula japonica collected from the Kitakami River estuary, northeastern Japan, showed lower δ34S values in soft-body parts (+1.7 to +11.0‰) than the ambient seawater sulfate sulfur (+21‰), and this value gradually decreased at successive sites up to 15.8 km upstream from the river mouth. Previous study using carbon and nitrogen isotopes suggests that the bivalve nonselectively assimilates particulate organic matter of marine and terrestrial origin by filter feeding. This pattern in δ34S values may indicate a considerable contribution of a food source derived from terrigenous organic matter, which has low δ34S values close to 0‰, and the bivalve has been reported to have cellulase and hemicellulase activities. Unique fatty acids (iso 17:0 and anteiso 17:0 acids), both characteristic of sulfate-reducing bacteria, were observed in the salt-free, soft-body parts of the bivalve. The concentration of monounsaturated fatty acids, possibly derived from bacteria, was also high. Trace amounts of a polyunsaturated fatty acid (20:5ω3) specific to dinoflagellates were detected. In Corbicula habitats, reductive sandy layers with ample sulfides that were sporadically intercalated into the oxidative sandy sediment were often observed. The stable isotopic signatures of sediment sulfides (acid-volatile sulfide) and associated pore-water sulfates were −8.9 to +8.6‰ and +22.4 to +26.3‰, respectively, indicating the existence of bacterial sulfate-reducing activity and thiobios biomass. These isotopic signatures of the sediment, with the fatty acid composition of the bivalve, confirm the importance of a food source derived from bacteria belonging to the thiobios in the substrate sediments via pedal feeding, rather than direct digestion of terrigenous organic matter, in this estuarine ecosystem