PLANKTON OF THE GULF OF MAINE 



369 



Although Pleurobrachia can hardly be classed as an important food supply for 

 other animals, fish do prey on them more or less. In New England waters this applies 

 especially to the spiny dogfish (p. 105). 



Alexander Agassiz, to whom we owe an excellent account of the development 

 of Pleurobrachia, found its eggs in Massachusetts Bay late in July, in August, and 

 in September, when, as he writes (1874, p. 359), "the water round them is filled with 

 eggs floating a few inches below the surface," and when he took the earliest stages 

 after hatching. This, with our own observations, makes it certain that Pleurobrachia 

 is regularly endemic and breeds in large numbers in the Gulf of Maine, of which it is 

 as characteristic an inhabitant as Calamus Jinmarchicus or Sagitta elegans. But how 

 many generations are produced there per year is not known. The older view was 

 that there is only one, and that the product of eggs spawned in late summer and 

 autumn live over winter, to mature and spawn in their own turn the following sum- 

 mer. The presence of large Pleurobrachia in winter and spring as well as in mid- 

 summer and autumn, together with the various sizes of the individuals which go to 

 make up the different schools in different localities at any given season, makes it 

 more probable that one generation succeeds another irregularly throughout the year. 



In spite of conclusive evidence to the contrary, assembled by recent students 

 of ctenophores, Pleurobrachia has often been termed a northern, even an Arctic, 

 form in its occurrence off the New England coast. I must therefore reiterate that 

 this is not the case but that its regular range along the coasts of eastern North America 

 extends southward to Chesapeake Bay; in fact, nearly to Cape Hatteras in the cold 

 season, for I myself have found it plentiful in the waters of Pamlico Sound in 

 winter. 



On both coasts of North America Pleurobrachia grows much larger in cool water 

 (10° or colder) than in warm (Bigelow, 1915, p. 322; Esterly, 1914). Judging from 

 the large size (upwards of 30 millimeters long) and local abundance of Pleurobrachia 

 in the Gulf of Maine, the latter is as favorable an environment for it as are the colder 

 waters off Newfoundland and Labrador; and if numbers of individuals present can 

 be trusted as a criterion this applies equally to the coast water off New York and 

 New Jersey, where rather smaller individuals are so abundant in some summers, for 

 instance 1913, that they have been given a vernacular name ("sago") by local 

 fishermen 



Pleurobrachia is a creature of the upper strata of water. As Alexander Agassiz 

 (1S74, p. 359) remarked long ago, they come to the surface whenever it is smooth, 

 at all times of day; "they are found in the greatest number between the hours of 9 

 and 11 in the morning, and from 4 to 6 in the afternoon in the summer," which is a 

 common habit of this ctenophore in all parts of the gulf during summer and early 

 autumn. In August, 1912, for example, we made our largest catches of Pleurobra- 

 chia at the surface; but they sometimes lie deep throughout the day in midsummer 

 and even in bright calm weather, as was the case on German Bank on August 12, 1913, 

 when we found no Pleurobrachia on the surface at 10 toll a. m., although a haul 

 from 40 meters yielded them in abundance. At other times of year this ctenophore 

 occurs more regularly a few meters (say 20 to 30) down than shallower, as exemplified 



