YOUNG: VERTICAL DISTRIBUTION AND PHOTOSENSITIVE VESICLES 



parts (length 155 /xm, inner diameter 3 to 6 ju,m). 

 The processes are long and slender and organized 

 in a straight, parallel alignment. 



Each organ is small in larvae and lacks screen- 

 ing pigment. At 35 mm ML, the vesicles form a 

 narrow strip along the posterior surface of the 

 peduncle lobe (Figure 35A). The largest larvae 

 have relatively small organs without screening 

 pigment; the youngest juveniles have large organs 

 that are heavily pigmented. In the juvenile and 

 adult stages, the organ exhibits positive allomet- 

 ric growth (compare Figure 35B and C). In the 

 adult stages, the organ exhibits positive allomet- 

 ric growth (compare Figure 35B and C). In the 

 brain. The ventral half of the organ is particularly 

 enlarged and the organs on each side of the brain 

 contact broadly (but do not fuse) below the ventral 

 midline of the brain. 



Taonius pavo (LeSueur 1821) 



Vertical Distribution (Figure 36) 



The vertical distribution of T. pavo has been 

 described by Young ( 1975d). Larvae probably live 

 in the upper 400 m, although only one capture was 

 made. Juveniles were found primarily between 

 600 and 650 m, and adults were captured between 

 725 and 970 m. Diel vertical migration does not 

 occur. 



40 60 80 100 IJO HO 160 180 200 220 



WANTIE LENGTH, mm 



Figure 36. — Vertical distribution ofTaoniuspavo. From Young 

 U975d.). Symbols as in Figure 1. 



F1GL!RE 37. — Photosensitive vesicles of Taonius pavo . A. Lar- 

 va. B. Juvenile, 140 mm ML. C. Adult, 220 mm ML. 



lumen is unoccupied. Sensory processes occupy 

 about V5 (i.e., about 230 ^im) of the lumen diame- 

 ter on the anterior, posterior, and dorsal sides and 

 slightly more (about 300^t.m) on the ventral sides. 

 The processes are loosely packed and intertwined 

 to a large extent dorsally and more tightly packed 

 ventrally. Inner diameters of the processes vary 

 greatly from about 3 to 30 ^im. The wall on the 

 dorsal half of the organ contains about two layers 

 of sensory-cell bodies compared with about three 

 layers ventrally. In a 140-mm ML juvenile, the 

 dissected vesicle was also hollow, with an even 

 thinner region of the lumen occupied by sensory 

 processes. The organs exhibit positive allometric 

 growth (Figure 37). 



Sandalops melancholicus Chun 1906 



Vertical Distribution (Figure 38) 



The vertical distribution in this species has been 

 reported by Young ( 1975d). Larvae were found in 

 the upper 400 m. Juveniles were captured between 

 450 and 674 m, and two adults were captured near 

 800 and 1,075 m. Diel vertical migration does not 

 occur. 



Photosensitive Vesicles (Figure 37) 



Taonius pavo has a single set of organs located 

 on the posteroventral side of the peduncle com- 

 plex. Each organ consists of a single oval vesicle. 

 No screening pigment is present. The large size of 

 the vesicle in a 220-mm ML specimen belies its 

 internal structure. The large central region of the 



Photosensitive Vesicles (Figure 33B) 



Sandalops melancholicus has a set of organs 

 located along the ventral surface of the peduncle 

 complex. Each organ consists of a single bilobed 

 vesicle (R. E. Young 1977). Slight positive allo- 

 metric growth of the vesicles occurs between the 

 juvenile and adult stages. 



603 



