FISHERY BULLETIN: VOL. 69. NO. 4 



C^ 



NEW 

 YORK 



Hysis mixta 



Body Size 



Body lengths range from 5.0 to 25.0 mm. Im- 

 mature specimens are a common comi^onent, 

 accounting for 69'' r of all specimens caught. 

 Many of these immature specimens are large 

 (15 to 20 mm). Also noteworthy is that many 

 large males and females do not possess fully 

 developed secondary sex characteristics. A 

 summary of body length measurements by 

 months and stage of development is listed in 

 Table 7. 



Table 7. — Means and ranges of body length of Mysis 

 mixta by months and stages of development. 



Figure 14. — Geographic distribution of Mysis mixta 

 based on specimens in the collection at the NMFS Bio- 

 logical Laboratory, Woods Hole. 



The occurrence of specimens at the surface 

 (Albatross IV, cruise 66-14, station 24) and in 

 several samples between middepths and surface 

 (Albatross IV, cruise 66-14, stations 35, 36, 39, 

 and 40), as well as those taken on the sea bot- 

 tom, indicates a diurnal vertical migration. Spec- 

 imens at the surface were collected at 0300 hr 

 and midwater samples were taken between 2100 

 and 0900 hr. This imijlies inhabitance of upper 

 waters during hours of darkness and a bottom 

 habitat in the daytime. 



Sex Ratio 



The NMFS collection contains 119 females 

 but only 2 males, which is a ratio of 0.017 male 

 to 1 female. Whether males occur in a habitat 

 different from females, or are actually so much 

 less abundant than females, is unknown. This 

 is the only species in the collection having such 

 a grossly unbalanced sex ratio. 



Length of Life and Spawning 



Although Smith (1879) suggested this spe- 

 cies might be an annual that spawns during the 

 winter, our analysis of the maturity status and 

 size frequency data of the NMFS specimens 

 leads us to believe that M. mixta has a 2-year 

 life span and spawns in winter or early spring. 



Our material is scanty for purposes of deter- 

 mining length of life, but the data do indicate 

 a clear trend in growth and development (Table 

 7). The specimens reveal two definite age 

 groups in both spring and fall. In May the im- 

 matures average 6.9 mm and the adults 20.6 mm. 

 In Octol)er the immatures average 16.7 mm and 

 the adults 23.4 mm. Juveniles increase in length 

 at a rate of about 2 mm per month until winter 

 when the growth rate slows apjireciably. They 

 mature and spawn in the winter or early spring. 



None of the NMFS specimens are ovigerous. 

 The only indication of spawning season is the 

 presence of small (5.3 to 6.3 mm) specimens in 

 Ma.v, which suggests a late winter or early 

 spring spawning. 



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