Baba Katsuhisa, Miyazono Akira, Matsuyama Keiji, Kohno Shoui, Kubota Shin
Hokkaido Hakodate Fisheries Experiment Station, 1-2-66 Yunokawa, Hakodate, Hokkaido, 042-0932 Japan.
2Hokkaido Tokai University, 5-1-1-1 Minamisawa, Sapporo, Hokkaido, 005-8601 Japan.
Mar Biol. 2007;151(5):1977-1987. doi: 10.1007/s00227-007-0636-x. Epub 2007 Feb 20.
In November 2003, we first observed prevalent occurrence of a hydroid, , on soft body tissues of age zero Japanese scallop () juveniles cultured in large areas of Funka Bay, Hokkaido. The occurrence coincided with massive death of juvenile scallops. A major objective was to clarify ecological relationships between the symbionts, and to infer the relationship between symbiosis and the massive mortality. To do this, we investigated distributions of association rates of hydroids with juvenile scallops at 15-34 sites over 3 years (2003-2005), with age one adult scallops at 24 sites in 2003, and with mussels at 13 sites in 2004. We studied seasonal changes in association rates with juvenile scallops, and numbers of polyps per juvenile scallop at three sites from November 2003 to June 2004. We also quantified the hydroid impacts on juvenile scallop shell length growth and triglyceride accumulation in the digestive gland. The association rate of polyps with juvenile scallops was high in large areas of Funka Bay in 2003, and overlapped the distribution of mussels bearing polyps. Association rates with age one adult scallops were very low in November 2003, even at the sites where polyps were very common on juvenile scallops. Levels of hydroid occurrence in juvenile scallops varies by year. We found that hydroids presence in juvenile scallops declined drastically in 2004 and 2005. The association rates with juvenile scallops, and numbers of polyps per juvenile scallop declined during winter, until they disappeared completely in the following June. Since polyps were rare in adult scallops, we believe that infection of juvenile scallops was probably initiated from the planulae produced by medusae released from polyps growing on spp., especially . Subsequently, the inhabitation spread intraspecifically and interspecifically. In juvenile scallops, inhabitation of polyps reduced shell length growth by 43%, and triglyceride accumulation in digestive glands by 24-47%. Inhabitation of . on juvenile scallop is best regarded as parasitism, rather than inquilinism or commensalism. Occurrence of polyps was probably not a direct lethal factor for juvenile scallops, because there were some sites where association rates were high, but mortalities were low. Massive mortalities in 2003 may have resulted from simultaneous impacts of heavy polyp load and stresses caused by the way in which the animals were handled (transferred from cages for pre-intermediate culture to cages for intermediate culture), because the massive mortality occurred within a month of the transfer. The presence of polyps in juvenile scallops does not affect the quality of the product in Funka Bay, because market size scallops are hydroid-free.
2003年11月，我们首次在北海道封浜大面积养殖的零龄日本扇贝稚贝的软体组织上，观察到一种水螅类生物（ ）普遍出现。这种情况与扇贝稚贝的大量死亡同时发生。一个主要目标是阐明共生生物之间的生态关系，并推断共生与大量死亡之间的关系。为此，我们在3年（2003 - 2005年）内调查了15 - 34个地点的水螅类生物与扇贝稚贝的附着率分布，2003年在24个地点调查了一龄成年扇贝的附着率，2004年在13个地点调查了贻贝的附着率。我们研究了2003年11月至2004年6月期间，三个地点水螅类生物与扇贝稚贝附着率的季节变化以及每个扇贝稚贝上的水螅体数量。我们还量化了水螅类生物对扇贝稚贝壳长生长和消化腺中甘油三酯积累的影响。2003年，封浜大面积区域内水螅体与扇贝稚贝的附着率很高，且与附着有水螅体的贻贝分布重叠。2003年11月，一龄成年扇贝的附着率非常低，即使在扇贝稚贝上水螅体非常常见的地点也是如此。扇贝稚贝上的水螅类生物出现情况随年份变化。我们发现，2004年和2005年扇贝稚贝上的水螅类生物数量急剧下降。水螅类生物与扇贝稚贝的附着率以及每个扇贝稚贝上的水螅体数量在冬季下降，直至次年6月完全消失。由于成年扇贝上很少有水螅体，我们认为扇贝稚贝的感染可能是由生长在 属（特别是 ）上的水螅体释放的水母产生的浮浪幼虫引发的。随后，这种栖息在种内和种间传播。在扇贝稚贝中，水螅体的栖息使壳长生长减少了43%，消化腺中甘油三酯的积累减少了24 - 47%。水螅类生物在扇贝稚贝上的栖息最好被视为寄生，而不是寄居或共生。水螅体的出现可能不是扇贝稚贝直接的致死因素，因为有些地点附着率很高，但死亡率很低。2003年的大量死亡可能是由于水螅体数量过多以及动物处理方式（从预中间培育笼转移到中间培育笼）造成的压力共同影响的结果，因为大量死亡发生在转移后的一个月内。扇贝稚贝上有水螅体并不影响封浜的产品质量，因为上市规格的扇贝没有水螅类生物。
