58 DISCOVERY REPORTS 



and this, as we have seen, is supported by the similarities between their swimbladders. But there is 

 little resemblance, other than that implied by the general label ' euphysoclist ', between the miri- 

 pinnatous and myctophoid type of swimbladder. Again, it would seem that the two groups are not 

 closely related and that each is the result of a distinct evolutionary trend within the iniomous complex. 

 But morphological knowledge of the various groups is by no means comprehensive. In time it may 

 be possible to regard the Chondrobrachii, Cetunculi, Miripinnati and perhaps the Giganturoidea and 

 Lyomeri as suborders of the Iniomi. At present their differences are more apparent than their 

 similarities. 



Whereas the Isospondyli are the basal group of soft-rayed fishes, the Berycomorphi occupy a 

 similar position within the spiny-finned groups. These fishes have ' advanced ' beyond the iniomous 

 level of organization in the development of spinous rays in the vertical and pelvic fins. But like many 

 of the soft-rayed fishes with a fully developed caudal fin, nearly all Berycomorphi have nineteen 

 principal rays in this fin. 



After their origin, the berycomorphs seem to have split up into two series of lineages, now repre- 

 sented by the suborders Berycoidea and Anoplogastroidea. The former group mainly consists of 

 shallow and deep water fish, whereas in the latter the fishes are predominantly bathypelagic in habit. 

 It is curious that like the stomiatoids among the Isospondyli, the purely deep-water group of beryco- 

 morphs acquired a swimbladder with a reversed polarity, the gas-secreting complex coming to a focus, 

 as it were, at the posterior end of the sac. The differences between the stomiatoid and anoplogastroid 

 swimbladder, to which some reference has already been made, are summarized in Table 2 : 



Table 2. Szvimbladder structure: Berycomorphi mid Isospondyli 



Isospondyli Berycomorphi 



Swimbladder Stomiatoidea Anoplogastroidea 



Type Paraphysoclist Euphysoclist (with oval) 



Type of rete Bipolar Unipolar 



Position of rete Posterior Posterior 



Number of retia One One or two 



As in the earlier comparison between the stomiatoids and myctophids, there is little reason to 

 suppose that the second group might have been derived from the first. However, the berycomorphs 

 may well have come from a primitive type of iniomous fish (in that its premaxillae alone formed the 

 biting part of the upper jaw). The present-day representatives are all marine fishes and their evolu- 

 tionary radiations, with the notable exception of the Holocentridae, have been mostly directed to- 

 wards the deep ocean. Considering only marine teleosts, it looks as though some of the groups, that 

 evolved during the early 'explosive' radiations of the soft-rayed and spiny-rayed teleosts, were 

 ' edged out ' from the more productive shelf-waters into the deeper, less favourable waters of the ocean. 



As the common ancestor of both assemblages is likely to have had an open swimbladder, and as 

 a closed one seems to be essential for an oceanic fish, it may well be that the physoclistous condition 

 was independently evolved in the deep-sea Isospondyli and Berycomorphi. Furthermore, the same 

 is probably true within the Isospondyli themselves, for it seems clear that the deep-sea salmonoids 

 were derived from physostomatous stock (p. 55) and it is also evident that the stomiatoids were not 

 part of this evolutionary line (p. 50). Again, the type of closed swimbladder possessed by the Mycto- 

 phidae could well have been independently acquired. While the common ancestor of the soft-rayed 

 and spiny-rayed groups must be sought at an early stage in teleost evolutionary history, the common 

 factor in these developments of a closed swimbladder seems to be no more than residence in a living 

 space in the deeper reaches of the ocean. 



