358 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



16.7° C. to a high of 23.5° C. The new species 

 occurred only in surface or near-surface night 

 samples taken over the relatively shallow depths of 

 the continental shelf. However, it seems reason- 

 able to suspect that arnoldi is more typically a 

 bathypelagic species, as are its congenitors. Con- 

 sidering the nature of the tows containing aimoldi. 

 vertical migration could readily account for the 

 temporary presence of the species in surface wa- 

 ters. Regarding subsurface conditions at the 

 time and place of these tows, temperatures were 

 found to be virtually identical at all depths. 

 Therefore, from the available data arnoldi occurs 

 at temperatures within the subtropical range 

 whereas its congenitors appear to have been found 

 only in temperatures confined to the cold tem- 

 perate-boreal range, irrespective of latitude. 



Remarks. — The female of the new species is not 

 readily separated from its congenitors, arnmtns, 

 simiUs, and pacifcit.s. It appears to be charac- 

 terized by (1) flaring of the posterior margins of 

 the thoracic segments, (2) the more dorsal origin 

 and the dorsal tilt of the spiniform processes ex- 

 tending from thoracic fusion segment IV- V, and 

 (8) details of the first pair of legs, including a 

 reduced external spine on Eci, a thick spine on 

 Res, and a broad truncate shoulder with recurved 

 spinules on Ri. Other structures may ultimately 

 prove more valuable for identification, such as the 



mandibular gnathal lobe and the length relation 

 of the fourth leg and the abdomen. 



The male is easily distinguished from the other 

 species. The most outstanding character is the 

 relatively great length of the fifth legs, which are 

 approximately twice the length of the abdomen. 

 Furthermore, in the right fifth leg the length of 

 the first segment of the exopodite is considerably 

 greater than that of the endopodite. In ai'noldi 

 the ratio of these segments is approximately 3.6 : 1, 

 whereas in pae/fieu,^ and sii)uli.>i this ratio is about 

 2.0: 1, as calculated from Brodski (1950: text fig. 

 65), Vanhoffen (1907: pi. 22, fig. 30), and Sars 

 (1902: pi. 21). In armatus the fifth legs lack 

 endopodites. 



Despite the limited degree of morphological 

 characterization and the lack of data on variabil- 

 ity, it is proposed that this Gulf of Mexico form be 

 given specific status because (1) the most out- 

 standing character occurs in a sexual structure 

 (i. e., fifth legs of the male) and because (2) it 

 appears to be geographically as well as ecologi- 

 cally separated (i. e., temperature range) from 

 other forms of Bradyidiu.H. 



This species is named in honor of Edgar L. Ar- 

 nold (I^. S. Fish and Wildlife Service), Fishery 

 Researcli Biologist, in charge of plankton collect- 

 ing operations aboard the Alaska during its sur- 

 vey of the Gulf of Mexico. 



Family EUCHAETIDAE 



EUCHAETA Philippi 

 Euchaeta paraconcinna, new species 



Plate 2, figs. 1-16 



Localities, Material. — Gulf of Mexico : material obtained 

 from 16 localities within lats. 22°21'-30''00' N., longs. 

 82°19'-96°00' W. at depths of 1-10 m. Plankton tows 

 made during period from .Januar.v-.June, in years 19.")l-i53. 

 ( .ila.'ika : cruise 1, station ."JO; cruise 4, stations 2S, .36; 

 cruise 8, stations 5, 6, 30, 33, 34, 36 ; cruise 10, stations 17, 

 18, 25, 30; cruise 11, stations 16, 23, 24.) Material 

 abundant, in excess of 100 individuals, mostl.v females. 



Onslow Bay, N. t\, M/V .itlaiitin: station 178 (May 

 1!».')3, oblique tow 30-1.5 m., one female) ; station 189 (.Tune 

 lit.'JS, oblique tow 20-0 m., one female). 



Measurements. — All measurements from dorsal 

 view at 32 X magnification; cephalothorax meas- 

 ured along midsagittal plane from apex of frontal 

 organ to posterior margin of intersegmental fold 



between thoracic fusion segment IV-V and genital 

 segment ; length of abdomen from anterior margin 

 of genital segment to articulation between fourth 

 innermost seta and right furcal ramus. Other- 

 wise, methods are as presented under preceding 

 species. 



Measurements given include total length (TL) 

 and cephalothorax-abdomen length ratio (CAR). 

 Measurements grouped since variation similar 

 in TL and CAR of specimens from various 

 localities. 



1. Adult female: 20 specimens selected at random, TL 

 range 2.79-3.23 nun., mean with standard error 3.04 ±.0.34 

 mm., standard deviation 0.146 mm., CAR range 2.3-2.6 : 1, 

 mean 2.4: 1. 



2. Adult male: live specimens selected at random, TL 

 2.63 mm., CAR 2.7 : 1 : TL 2.49 mm., CAR 2.8 : 1 ; TL. 2.56 

 mm., CAR 2.7 : 1 ; TL 2.64 mm.. CAR 2.8 : 1 ; TL 2.63 mm., 

 CAR 2.8: 1. 



