FISHERY BULLETIN: VOL. 69. NO. I 



MYSIS I 



(Fig. 10) 

 Mean TL = 3.3 mm (3.2-3.5 mm) 

 Mean CL = 1.2 mm (1.1-1.3 mm) 



N = 18 



With the molt from the third protozoeal to the 

 first mysis substage, the larvae undergo another 

 radical change and assume a more shrimplike 

 appearance. The most apparent change is the 

 development of functional pereiopods with long 

 brushlike exopods. The antennae also undergo 

 considerable change, with the exopods of the 

 second antennae becoming modified to form 

 flattened antennal blades. 



The carapace fits the body more closely than 

 in preceding stages and covers all but the last 

 two thoracic somites. The rostrum is not de- 

 pressed as in the protozoeal substages, but pro- 

 trudes forward on a horizontal plane. Supra- 

 orbital spines are still present although reduced 

 in size. A small spine now occurs on the antero- 

 ventral corners of the carapace. In addition, 

 a pair of hepatic spines have been added to the 

 carapace, one spine on each side originating 

 from a point located approximately one-sixth the 

 carapace length from the anterior margin. 



An ocellus is present in this and subsequent 

 mysis substages. 



The first antenna is composed of three seg- 

 ments, the first being I'/j times the length of 

 the second and third combined. Numerous 

 setae now occur along the apjiendage, and sev- 

 eral arise at the apex of each segment. In ad- 

 dition, a prominent ventromedian spine is pre- 

 sent on the first segment. The distal segment 

 gives rise to two unsegmented branches, the ex- 

 ternal one bearing six or seven setae and being 

 twice as long as the internal one, which bears 

 two terminal setae. 



The second antenna consists of a protopod of 

 two segments, an unsegmented endopod with 

 three lateral and three terminal setae, and a 

 flattened, unsegmented, bladelike exopod with 

 a single lateral seta and 11 setae along the 

 medial and terminal margins. 



The mandible and maxillae remain essentially 

 the same as in the preceding substage except 



that the exopod of the second maxillae has be- 

 come enlarged and bears 10 setae. The first 

 and second segments of the endopod of the first 

 and second maxillipeds have acquired an addi- 

 tional seta. The exopod of the first maxilliped 

 has gained a seta and that of the second has lost 

 three. The third maxilliped has evolved further 

 and is now longer than the first two. It has 

 a two-segmented protopod that bears two setae. 

 The first segment of the five making up the 

 endopod possesses two setae; the second, one; 

 the third, none; the fourth, three; and the fifth, 

 five. The unsegmented e.xopod has five or six 

 terminal and subterminal setae. 



The five pairs of pereiopods have undergone 

 considerable enlargement and their exopods 

 serve as the principal swimming organs during 

 the mysis stage. The endopods of the first three 

 pairs are modified into rudimentary chelae that 

 have four or five terminal setae. The first 

 pereiopod consists of a protopod of two seg- 

 ments, an unsegmented endopod, and an exopod 

 which bears five or six terminal and subterminal 

 setae. The last four pereiopods were not ex- 

 amined in detail. 



The dorsomedian spines of the first two ab- 

 dominal segments have been lost while those on 

 the third, fourth, and fifth segments are still 

 prominent. The fifth segment retains a pair 

 of midlateral spines. A dorsomedian and a 

 ventromedian sjiine are present on the sixth 

 segment in addition to the paired midlateral and 

 ventrolateral spines found in the preceding sub- 

 stage. Anlages of the pleopods can he seen on 

 the ventral surface of the first five abdominal 

 segments. 



The uropod has developed an unsegmented 

 protopod that possesses a large posteroventral 

 spine and a smaller postei'olateral spine. The 

 endopod carries 11 setae on its medial and termi- 

 nal borders, while the exopod, which has about 

 13 setae on its medial and terminal margins, 

 has, in addition, a prominent spine on its postero- 

 lateral edge. 



The telson is cleft terminally and bears seven 

 pairs of terminal and one pair of lateral spines. 



232 



