68 
BULLETIN OF THE BUREAU OF FISHERIES 
short-lived incursions by the more delicate Arctic forms, to the more successful 
though equally temporary immigrations by animals that are able to survive under 
the physical conditions which they encounter in the gulf and even to grow there, 
but not to breed; such, for example, as Sagitta serratodentata and Eukrohnia. The 
next step toward successful colonization would be the ability to breed in the gulf in 
small numbers or during especially favorable years, which would still leave the species 
concerned dependent on immigration from prolific centers elsewhere for the main- 
tenance of the local stock. In the nature of the case instances of this sort are difficult 
to demonstrate without intensive and long-continued studies of the plankton, but it 
is evident that the copepods Calanus hyperboreus and Metridia longa both fall in 
this class (p. 61) ; also the curious pelagic worm Tomopteris catharina, the continuous 
and rather common occurrence of which in the gulf and its wide dispersal there 
depend chiefly on immigrants of northern origin (it is a north-boreal form), for 
while it breeds in the gulf in some summers it fails to do so in others (p. 338). It is 
probable, also, that the large naked pteropod Clione limacina has this same faunal 
status, breeding in sufficient numbers for the local production, coupled with individual 
longevity, to give it a uniform distribution over the gulf and so to obscure the routes 
followed by the immigrants from colder waters east and north of Cape Sable, on 
whose visits its continuous presence in the gulf equally depends (p. 127). 
The amphipod genus Euthemisto stands a rung higher on the ladder of pro- 
gressive colonization, for it neither breeds so abundantly (though it does so regularly) 
in the gulf nor grows to so large a size there as it does over the outer edge of the 
offshore banks — Georges and Browns (p. 158). Local fluctuations in the abundance 
of animals of this status throw no direct light on their waves of immigration, being 
due, as often as not, to local centers of reproduction within the gulf itself and even 
close up to the land, such as we have occasionally encountered for Euthemisto 
(p. 160) ; but greater abundance in the eastern part of the gulf than in the western, 
especially if coupled with prolific centers of reproduction in the zone of mixed water 
over the outer part of the continental shelf abreast of it (and this is true of Euthe- 
misto), shows that the stock produced within the gulf receives frequent accessions 
to its numbers from outside. 
No doubt one or other member of the plankton might be found to represent 
every conceivable intergradation from utter failure to perfect success in colonizing the 
waters of the Gulf of Maine (for all members of the plankton are colonists in the last 
analysis) were the known record sufficiently complete. The copepod genus Euchaeta, 
for example, may be taken as representative of animals that breed indifferently and 
grow equally large along the continental slope, in the Eastern Channel, and in 
the gulf wherever the depth is sufficient, as proven by the occurrence of sexually 
adult males, of females with large egg clusters, and of juveniles. For this copepod 
the gulf basin is simply a diverticulum from its general geographic range. Most 
successful of all are those that find a more favorable environment in the inner 
parts of the gulf than in the waters immediately tributary to it, and it is to this 
group that such members of the local zooplankton as the copepods Calanus ftn- 
marchicus and Pseudocalanus elongatus and the chaetognath Sagitta elegans belong. It 
is true that most, if not all, the animals of this category have equally prolific centers of 
