Swarms of Amphipods

Abstract

ONCE last winter on entering the laboratory here after it had been shut up for a few days, we found the floor, tables, shelves, window-ledges, and even dishes on the highest shelves, covered with great numbers of dead amphipods. These were found to be Orchestia gammarellus (the shore-hopper). About ten days ago an unusually high tide occurred, and the curator and others who were working in the biological station noticed that the steps leading to the beach were swarming with amphipods. On investigating further it was found that the amphipods were coming up in great numbers from high-water mark, that they jumped up the steps, and that they climbed the vertical concrete wall surrounding the station to a height of several feet. Many of them were found about twelve feet above the sea, having come nearly all the way on artificial ground (concrete steps and wall), and they were so abundant on the platform outside the laboratory door that it was impossible to put a foot down without treading on many. Specimens were kept, and Mr. A. O. Walker, who is here with me now, finds that these also are Orchestia gammarellus. This species lives normally at or about high-water mark, and it is abundant here under stones at that line, but Mr. Walker tells me that he has taken it on the one hand nearly at low-water mark, and on the other hand under stones on grass, along with beetles, and we have found it near here far above high-water mark at the side of the road. However, these last are probably exceptional cases, and we are both inclined to think that the two amphipod invasions noticed here have been caused by the Orchestias being driven from their usual haunts by exceptionally high tides. But whether a panic arises on the flooding of their homes, or they lose their way on our concrete, the fact remains that whereas the sea was only a couple of feet higher than an ordinary high tide the amphipods ascended on the one occasion to about twelve and on the other to perhaps twenty feet above their usual level.