46 
BULLETIN OP THE BUREAU OF FISHERIES 
hydroid medusa Mitrocoma cruciata reaches maturity during this same month, 
when it may appear near shore in numbers sufficient to give a distinctive aspect to 
the tow, as was the case at the mouth of Penobscot Bay on June 14, 1915 (station 
10287 p. 348). For the sake of clarity I should point out, at the risk of repetition 
(p. 389), that diatoms still swarm along a narrow coastwise belt east of Penobscot 
Bay in June. 
The advance of summer (from June on) sees an actual decrease in the number of 
copepods, owing, no doubt, to the destruction wrought among them by fishes and 
other enemies (p. 97). In part this decrease is made good by constant reproduction, 
evidence of which was afforded by an abundance of copepod nauplii near Cape Cod 
on July 8, 1913 (station 10057, surface), on July 7, 1915 (station 10300), and on 
August 29, 1916 (station 10398) ; likewise by the presence of large numbers of juvenile 
Cal anus 23 between Cape Ann and the Isles of Shoals in July, 1912. The offshore 
banks also serve as a copepod nursery in July— at least locally — for copepod eggs, 
nauplii, and juveniles abounded on the surface near Nantucket Lightship on the 
25th of that month in 1916 (station 10355), while the presence of young Calanus 
at various stages in development in most of the summer towings proves that this 
copepod breeds more or less regularly throughout the summer. Our experience, 
however, does not suggest that sufficient reproduction takes place during the warm 
months to maintain the local stock of calanoid copepods against depletion by the 
many dangers to which it is subjected. 
As copepods dwindle in numbers the other groups of common boreal animals 
increase, lending an increasing diversity to the plankton of the offshore parts of the 
gulf during the summer, most noticeably in the western side, where the plankton 
is most monotonously calanoid in May and June, thus producing the midsummer 
state already described (p. 17). Events notable in this gradual alteration are a 
great production of Euthemisto, resulting from local centers of reproduction such 
as I have just mentioned (p. 20); the active propagation of euphausiids (p. 20); a 
general penetration toward the western and northwestern shores of the Gulf on the 
part of the pteropod Limacina retroversa (p. 119); the appearance of shoals of the white 
and red jellyfishes (Aurelia and Cyanea) in the coastal belt as they disperse and 
drift seaward from their estuarine nurseries (pp. 360, 362) ; the presence of large Stauro- 
phora, often in abundance (p. 342); and the offshore swarming of the hydroid medusa 
Phiaiidium languidum (p. 350). It is during the summer, too, that the large and 
conspicuous arrow-worm Sagitta serratodentata first appears in any number in the 
gulf as a visitor from warmer waters to the south and east outside the edge of the 
continent, and spreads its range northward and westward as described elsewhere 
(p. 322). The copepod population, also, becomes diversified as the summer advance 
by increasing numbers of Anomalocera and Centropages, not only within the gulf 
but also on Georges Bank, where the former (which we did not find in spring) is 
practically universal and comparatively abundant in August. 24 The ctenophore 
Pleurobrachia pileus reaches its maximum abundance on the German Bank ground 
13 Identified by Dr. C. O. Esterly. 
24 The “green copepod” of Doctor Kendall’s field notes. 
