N 



Figure 2. — The siphonophore Nanomia cara photographed 

 from a viewport of the submersible: p, pneumatophore; n, nec- 

 tophore; g, gastrozooid. An excellent schematic drawing of this 

 species can be found in Mackie (1964). 



colonies of A^. cara were extremely fragile and 

 isolated pieces of stem were not uncommon. When 

 siphonophores came into contact with the submer- 

 sible, their tentacles frequently adhered while 

 stem and nectophores fragmented and floated 

 away. 



Most colonies were negatively phototactic and 

 contracted their stem and tentacles as they drifted 

 into the radius of the submersible's lights. Con- 

 traction usually initiated an escape swimming re- 

 sponse. The siphonophores could move rapidly 

 away at any orientation, and some were observed 

 swimming with pneumatophore and nectosome 

 pointed directly downward. We estimated that es- 

 cape speeds exceeded 20 to 30 cm/s. 



The most numerous invertebrates among or ad- 

 jacent to the densest localizations of A'^. cara were 

 the euphausiids, Meganyctiphanes norvegica and 

 Thysanoessa inermis; mysids, principally 



Neomysis americanus; and hyperiid amphipods, 

 principally Parathemisto gaudichaudii and 

 Hyperia galba. We observed one siphonophore 

 which had recently ingested an euphausiid; the 

 others had no prey of this size in their feeding 

 polyps. 



Calanoid copepods, among them the large 

 species Calanus finmarchicus and Euchaeta nor- 

 vegica, were also locally abundant among the 

 aggregations of Nanomia cara. Plankton samples 

 taken in June 1976 showed that these calanoids 

 were rich in lipids, as a heavy slick of oil droplets 

 formed after they were preserved in 4% Formalin.^ 

 The copepods were apparently being eaten by N. 

 cara as fragments of siphonophores removed from 

 the same plankton samples were distended by 

 lipids droplets inside feeding polyps and palpons, 

 where lipids would concentrate during digestion of 

 prey. 



Discussion 



The density of siphonophore colonies in the Gulf 

 of Maine was considerably greater than Barham's 

 (1963) estimate of the abundance (300 colonies/ 

 1,000 m^) of a congeneric species (N. bijuga) in the 

 San Diego Trough. Barham concluded that the 

 gas-filled floats of A'^. bijuga were of adequate di- 

 mensions to act as strong sound scatterers and 

 that at these densities this siphonophore could 

 contribute significantly to scattering layer forma- 

 tion. Thepneumatophores of A^. bijuga arvdN. cara 

 are similar in dimension, and aggregations of N. 

 cara should be equally effective sound scatterers. 

 In fact, in fall and winter 1975-76, fishermen in 

 the Gulf of Maine reported near-bottom, dense 

 layers of sound-reflecting organisms in areas 

 where trawl nets were being clogged with A^. cara 

 (F. E. Lux pers. commun.). 



The cause of the aggregation of N. cara in the 

 Gulf of Maine has not been determined. It is con- 

 ceivable that widespread reproduction of N. cara 

 occurred in fall and winter 1975-76 and that local 

 patterns of circulation aided in concentrating and 

 maintaining the aggregation and prey items. It is 

 clear, however, that localization of siphonophores 

 like A'^. cara at densities exceeding 1 colony/m^ will 

 interfere with commercial fishing efforts by clog- 

 ging the meshes of nets trawled for shrimp, silver 

 hake, and redfish. Aggregations of siphonophores 



•Reference to trade names does not imply endorsement by the 

 National Marine Fisheries Service, NOAA. 



283 



