328 DISCOVERY REPORTS 



It would appear therefore that in the Isospondyli and the Iniomi there is some correlation between 

 the possession of a closed swim-bladder and pectorals having angles greater than 45 . 1 But the 

 specialization of pectoral fins as brakes is best seen in the higher groups such as the Percomorphi. It 

 must also be remembered that many of the isospondylous and iniomous fishes with closed swim- 

 bladders undertake considerable diurnal vertical migrations. At the end of a descent, compression of 

 the swim-bladder may leave the fish somewhat heavier than the sea-water, and, until the volume can 

 be restored by gas secretion, the fish will need to make compensating movements. Perhaps this is why 

 the pectorals are not so highly specialized as in the Percomorphi and are set so as to allow some lift- 

 force. 



While a closed swim-bladder means that the fish is independent of the surface for replenishing the 

 gases and is able to achieve a finer control of specific gravity, there are certain limitations. Moreover, 

 this discussion does not imply that all fishes with open swim-bladders have only a rough control of 

 their specific gravity. Measurements of their specific gravity (Jones and Marshall, 1953) and observa- 

 tions of their behaviour indicate that cyprinid and esocid fishes are perfectly able to reduce their 

 weight in water to vanishing point. Such fishes are able to manoeuvre more easily than, say, clupeids. 



Lastly, the pectorals tend to be lateral and ' brake-set ' not only in the Myctophidae but also in other 

 myctophoid fishes, the Aulopidae, Chlorophthalmidae, Synodontidae, Harpadontidae, Ipnopidae and 

 Bathypteroidae. As already mentioned these are predominantly bottom-dwelling fishes and have no 

 swim-bladders, and it is hardly surprising that the pectoral fins should have this setting, for it is a 

 common feature in benthic fishes. Considering only those without a swim-bladder, pectorals with an 

 upright or nearly upright axis are found in the scorpaenids, cottids, nototheniids, gobiids and gobie- 

 socids. Certainly the pectorals in these fishes are used in locomotion and, being so placed, may allow 

 of better control of body movements as the fish grubs for food. The similarity in the setting of these 

 fins in the pelagic Myctophidae on the one hand and the remaining benthic myctophoids on the other 

 is therefore probably an instance of an unusual type of convergence (the independent acquisition of 

 like characters). 



The dorsal fin. Except for Alepisaurus, the dorsal fin of alepisauroids has 6-15 rays and is set about 

 half-way down the body length. In Anotopterus there is no trace of a dorsal fin, while Alepisaurus has 

 a long sail-like dorsal, very like that of the sail-fishes {Istiophorus). This convergence is, indeed, so 

 striking that it led Dollo (1909) to conclude — on very slender evidence — that Alepisaurus uses its 

 dorsal fin as a sail, much in the manner sometimes seen in Istiophorus. 



In Alepisaurus ferox the dorsal fin, which has about 40 rays, originates just behind the head and 

 ends above the origin of the anal fin, i.e. the base of the dorsal extends over more than two-thirds of 

 the standard length. The longest of the anterior rays may measure at least twice the greatest depth 

 of the body. 



The muscles which move the dorsal fin rays are shown in Text-fig. 8 a, the upper figure being of the 

 first three rays, the lower of the 14th and 15th rays. It will be seen that the erector muscles are power- 

 fully developed, while the depressors and particularly the inclinators are a good deal smaller. 



The basal supports for the dorsal fin are also of interest. At the base of each ray is a distal radial, 

 but it would appear that the intermediate and proximal elements have fused to form a long continuous 

 rod, running the entire length of the dorsal fin, an unusual feature in fishes (Text-fig. 8 a, b). But such 

 a development is hardly surprising in view of the strains imposed on the high dorsal as it is hauled up 



L Two myctophids listed on p. 326, Gonichthys cocco and Ctenobranchus nigro-ocellatus have no swim-bladder yet have pec- 

 toral angles of 70-75 . But both species have a more stream-lined, slender body than other myctophids and both may well 

 keep to a particular level by vigorous swimming movements (rather like Scomber). Beebe and Vander Pyl (1944) have remarked 

 on the vigorous unceasing swimming movements of Gonichthys cocco as compared with other lantern-fishes. 



