YOUNG: VERTICAL DISTRIBUTION AND PHOTOSENSITIVE VESICLES 



dalops melancholicus andBathothauma lyromma) 

 had large organs. Leachia pacifiva, which had 

 small organs, spent most of its life in epipelagic 

 waters and then descended to depths > 1,000 m. 

 Onychoteuthis compacta seemed to range widely 

 during the day and had rather large organs (its 

 habitat, however, is poorly known). 

 Brachioteuthis had similar organs but probably 

 occurred below 800 m during the day. The ommas- 

 trephids had a complex arrangement of organs, 

 yet these animals were primarily epipelagic. In 

 juveniles of many species (e.g., enoploteuthids), 

 the size of the organs (relative to the size of the 

 brain) may be large; yet their absolute size was 

 small when compared with adults occupying the 

 same depths. 



Compared with squid, all octopods had small 

 organs. With the probable exception of the tubular 

 eyed Amphifretus pelagicus, octopods probably do 

 not occupy depths between 400 and 700 m during 

 the day except as juveniles in transit to greater 

 depths. Amphifretus pelagicus is the only pelagic 

 octopod that exhibited clear modification of its or- 

 gans. In contrast to the small organs, each consist- 

 ing of a single vesicle, of other octopods, this ani- 

 mal has a larger organ composed of many separate 

 vesicles. 



Presumably the general trends with depth were 

 related to depth gradients in both downwelling 

 daylight and bioluminescent light. Downwelling 

 daylight decreases exponentially with depth. 

 Bioluminescent activity should increase from 400 

 to 600-800 m then decline rapidly if numbers of 

 midwater fishes at various depths (see Amesbury 

 1975) provide an index to bioluminescent activity 

 during the day. While many vesicles may detect 

 both downwelling skylight and bioluminescent 

 light, we will examine evidence for these two func- 

 tions separately. 



The eyes of some mesopelagic animals can prob- 

 ably detect silhouettes at depths of 750 to 775 m 

 (Young and Roper 1977). Presumably some photo- 

 sensitive vesicles are at least as sensitive as the 

 eyes, especially when we consider the large size 

 and apparent high pigment density of some (see 

 Young 1972a). The large dorsal organs of squid 

 were positioned so they are exposed to downwel- 

 ling daylight. Large central organs appeared to be 

 exposed to this light in species lacking dorsal or- 

 gans. 



Some experimental evidence indicates that 

 midwater cephalopods detect downwelling light 

 with these vesicular organs. A number of 



cephalopods have been seen to conceal themselves 

 with bioluminescent light (Young and 

 Roper 1977). This counterillumination requires 

 that the intensity of downwelling light is precisely 

 determined by the animal, and the photosensitive 

 vesicles seem the likely photoreceptor (Young 

 1973, 1977). Recently R. E. Young, C. F. E. Roper, 

 and J. Walters (in manuscr.) covered the dorsal 

 organs oi Abraliopsis sp. B while it was counteril- 

 luminating and recorded a 909f drop in its 

 luminescence. They concluded that the dorsal or- 

 gans detect downwelling light. Since animals can 

 detect downwelling light with these organs for 

 counterillumination, they may use this photic in- 

 formation for other purposes as well. 



Vertical migration in many midwater animals 

 is closely associated with changing light levels 

 (Boden and Kampa 1967). Since cephalopods mi- 

 grate during twilight periods, light cues received 

 by the vesicular organs may serve to trigger or 

 regulate their migrations. This view is supported 

 by three sources of evidence. First, nerves from the 

 vesicles pass into the peduncle complex of the 

 brain. Messenger (1967b) suggested, on the basis 

 of experimental evidence in Octopus, that this 

 complex is part of a visuomotor system: visual 

 information from the eyes enables this complex to 

 exercise control over locomotion. Secondly, ex- 

 perimental evidence on the function of the photo- 

 sensitive vesicles in neritic Octopus strongly 

 suggests that these organs regulate diurnal activ- 

 ity patterns (R. Houck pers. commun.). Finally, 

 most migrating cephalopods have large vesicular 

 organs positioned to detect downwelling light. The 

 only exceptions are species of Mastigoteuthis and 

 Chiroteuthis , whose migration patterns are not as 

 distinct as in other species. 



If dorsal and central organs function primarily 

 in the detection of downwelling light, we may have 

 a clue to the peculiar arrangement of vesicular 

 organs in ommastrephids. The ommastrephids 

 were the only squids that had central organs on 

 the dorsal surface of the optic stalk as well as 

 dorsal organs. In Nototodarus hawaiiensis, these 

 two organs differ morphologically (the small cen- 

 tral organ has large component vesicles and the 

 large dorsal organ has small vesicles), but the 

 organs are adjacent to one another. The structural 

 differences suggest separate functions for the or- 

 gans, yet their close proximity indicates that both 

 will be exposed to the same source of light. The 

 same argument holds for these organs in other 

 ommastrephids, although the two organs are 



613 



