Institute for Marine Biosciences, National Research Council, 1411 Oxford St., Halifax, NS, B3H 3Z1, Canada.
Aquat Toxicol. 2010 Apr 15;97(2):160-71. doi: 10.1016/j.aquatox.2010.01.009. Epub 2010 Jan 25.
The neurotoxin domoic acid (DA), produced by diatoms Pseudo-nitzschia spp., is transferred to humans via consumption of contaminated bivalves. This study examines feeding mechanisms, namely reduced filtration, pre-ingestive rejection and poor absorption, that might explain the comparatively low DA levels commonly found in oysters during toxic Pseudo-nitzschia blooms. Clearance rate (CR), absorption efficiency (AE) of organic matter and selective rejection in pseudofeces of oysters (Crassostrea virginica) and mussels (Mytilus edulis) were investigated in relation to the DA levels accumulated during 2-wk, simultaneous exposure to toxic Pseudo-nitzschia multiseries. Effects of temperature and P. multiseries cell size were also tested to identify conditions, if any, under which oysters can accumulate unsafe DA levels. Oysters accumulated 3.0-7.5x less DA than mussels from a short-celled P. multiseries clone (length=24microm) at 12 degrees C. This was related to the 7.4-8.5x lower CRs determined for oysters relative to mussels at this temperature. Exposure to a longer-celled P. multiseries clone (81microm) resulted in up to 70x lower toxin levels in oysters compared to mussels, which was attributed to differential feeding selectivity. Mussels were unable to discriminate between long- and short-celled P. multiseries clones from a mixed suspension, whereas oysters were previously shown to preferentially reject long cells (>70microm) in pseudofeces. Both bivalves selectively rejected P. multiseries cells from mixed suspensions containing a flagellate but not another diatom. AE of organics from P. multiseries cells by oysters and mussels was comparably low (42 and 39%, respectively) and thus unlikely to explain their differential DA accumulation. CR and DA uptake by oysters were negligible at <or=4 degrees C but increased with increasing temperature up to 18 degrees C, although mean DA levels barely attained the regulatory limit (20microg g(-1)) when oysters were exposed to long P. multiseries cells. The maximum DA levels accumulated by mussels (320microg g(-1)) and oysters (44microg g(-1)) exposed to short P. multiseries cells in our study support the inter-specific differences in toxicity during Pseudo-nitzschia blooms, which are expected to be exacerbated at lower temperatures and when long cells or chains are dominant. Additionally, when alternate, non-diatom phytoplankton species are present, both bivalves can feed selectively and thus accumulate much lower DA levels than those predicted from their overall CRs. Our results provide support for the evaluation of species-specific management of DA-contaminated shellfish and need to be considered in modeling DA toxin kinetics of the two target species.
神经毒素软骨藻酸（DA）由菱形藻属的物种产生，并通过食用受污染的双壳类动物转移到人类体内。本研究检查了可能解释在有毒菱形藻大量繁殖期间贝类中通常发现的 DA 水平相对较低的摄食机制，即过滤减少、摄食前排斥和吸收不良。在同时暴露于有毒的多列菱形藻 2 周期间，研究了贻贝（Mytilus edulis）和牡蛎（Crassostrea virginica）的清除率（CR）、有机物的吸收效率（AE）和伪粪便中的选择性排斥，以确定是否存在任何条件，使贝类能够积累不安全的 DA 水平。在 12°C 时，从短细胞的多列菱形藻克隆（长度=24μm）中，牡蛎积累的 DA 比贻贝少 3.0-7.5 倍。这与在此温度下确定的牡蛎相对于贻贝的 CR 低 7.4-8.5 倍有关。暴露于较长细胞的多列菱形藻克隆（81μm）导致牡蛎中的毒素水平比贻贝低 70 倍，这归因于不同的摄食选择性。贻贝无法区分来自混合悬浮液中的长细胞和短细胞的多列菱形藻克隆，而牡蛎先前被证明优先在伪粪便中排斥长细胞（>70μm）。两种双壳类动物都选择性地从含有鞭毛的混合悬浮液中排斥多列菱形藻细胞，但不排斥另一种硅藻。牡蛎和贻贝从多列菱形藻细胞中吸收有机物的 AE 相对较低（分别为 42%和 39%），因此不太可能解释它们不同的 DA 积累。当温度低于或等于 4°C 时，牡蛎的 CR 和 DA 吸收可以忽略不计，但随着温度升高到 18°C，CR 和 DA 吸收增加，尽管当牡蛎暴露于长细胞的多列菱形藻时，平均 DA 水平几乎达到监管限值（20μg g(-1)）。在我们的研究中，暴露于短细胞多列菱形藻的贻贝（320μg g(-1)）和牡蛎（44μg g(-1)）积累的最大 DA 水平支持了在菱形藻大量繁殖期间种间毒性的差异，预计在较低温度和长细胞或链占主导地位时，这种差异会加剧。此外，当存在替代的非硅藻浮游植物物种时，两种双壳类动物都可以选择性摄食，因此积累的 DA 水平远低于根据其总 CR 预测的水平。我们的结果为评估受 DA 污染的贝类的特定物种管理提供了支持，并且需要在对两种目标物种的 DA 毒素动力学进行建模时加以考虑。
