Long-Term Variation of Polychaete Bioerosion of the Scallop Mizuhopecten yessoensis from the Northwest Part of the Sea of Japan

Abstract

The bioerosion of shells of the Japanese scallop Mizuhopecten yessoensis by endolithic organisms over the last few decades and millenia in areas close to the main sources of organic pollution in Peter the Great Bay was studied. It was established that the occurrence of Japanese scallop shells damaged by borers increased over 2500 years practically from 0 up to 96% in the northern part of the bay at Mys Peschany Cape, which is subject to the impact of the Pazdol'naya River drain. The bioerosion of the Japanese scallop shells has increased considerably over the last two decades in the coastal zone, near the center of the city of Vladivostok, at one of the main sources of the city's wastewater discharge. Thus, in 4-year old individuals 33.0 ± 2.9% of upper valves were eroded on average in 1982; in 1998 it was already 68.3 ± 6.4%. In Reinike strait, that is, at some distance from the mouth of Razdol'naya River and from the main discharge sources of polluted waters of Vladivostok, the shells of the Japanese scallop were eroded to a lesser extent than in the population from the coastal region near the city. However, a significant increase in the degree of bioerosion of the scallop shells was observed over one-and-a-half decades: in 1987 in 4-year scallops 2.3 ± 0.3% of the area of the upper valve was eroded on average, and in 1999 that parameter increased and became 32.6 ± 3.4%. In the western part of Peter the Great Bay near Furugelm Island, which is located 20 km away from the mouth of the Tumannaya River, the degree of bioerosion of scallop shells had also increased significantly over the last 30 years. The increase in the degree of bioerosion of the shell is connected to a gradual increase in the content of organic substances in the bottom sediments. This is a factor favorable for the development of bacteria and phytoplankton, which is a food source for polychaetes—the basic shell boring symbionts of the Japanese scallop.