ALEPISAUROID AND MYCTOPHOID FISHES 327 



It is significant that the little available evidence suggests that certain isospondylous fishes are some- 

 what heavier than the environment. This is borne out by the observations of Breder and Krumholz 

 (1943) on Harengula pensacolae and Anchoa mitchilli, and by measurements of the specific gravity of 

 Clupea harengus (Magnan, 1929). Moreover, the studies of the first two authors on Megalops atlanticus 

 suggest that the simple, unspecialized type of open swim-bladder, characteristic of most Isospondyli, 

 has a limited function in regulating the specific gravity. After remarking that the pectoral movements 

 of tarpon are not unlike those of Anchoa mitchilli, Breder and Krumholz (1943) continue thus: 

 ' Observations in an aquarium of tarpon ranging from 6 cm. to 100 cm. indicate that they seem to be- 

 come heavier as the time for them to rise for breath approaches. Their pectorals typically work harder 

 and finally with a burst of tail effort they rush to the surface and gulp .... After the ingestion of air 

 they are usually lighter than water and frequently have difficulty descending, until they emit small 

 bubbles by way of the gill clefts, after which they reach a state of approximate balance and from then 

 on become heavier again.' 



It would appear therefore, that as far as present knowledge goes, the development of a capacious 

 swim-bladder in the Isospondyli has not always led to a close correspondence between the specific 

 gravity of the body and that of the environment. Correlated with a slight tendency of the fish — at least 

 for part of the time — to sink, the pectorals are set in such a way that they can act as hydroplanes 

 and beat downwards to raise the head. The angle between the axis of the pectoral fin and the horizontal 

 axis of the body in a number of isospondylous fishes is given below. 



Elops saurus (40 ) Megalops atlanticus (45 ) 



Chanos chanos (15-20 ) Clupea harengus (25 °) 



Sardinops sagax (30 ) Harengula pensacolae (20°) 



Ilisha filigera (45°) Pristigaster cayanus (30-35°) 



Odontognathus compressus (45-50°) Salmo salar (40°) 



Coregonus sp. (45°) Salvelinus alpinus (45°) 



Osmerus eperlanus (40-45°) Retropinna oameroides (40°) 



Plecoglossus altirelis (25°) Galaxias fasciatus (55°) 



But not all Isospondyli have an open swim-bladder (see Jones and Marshall (1953)). A closed swim- 

 bladder is commonly found in the Gonostomatidae and Sternoptychidae (Marshall, 1951) and it is of 

 interest to measure the pectoral angle in representatives of these families. 



Maurolicus muelleri (6o°) Vinciguerria attenuata (40°) 



Gonostoma denudatum (45-50°) Ichthyococcus ovatus (45°) 



Argyropelecus aculeatus (65°) Sternoptyx diaphana (60°) 

 Polyipnus nuttingi (50°) 



A comparison of the pectoral angle in Gonostoma denudatum with that in G. elongatum and 

 G. bathyphilum (20-25°) is revealing, when considered in the light of the foregoing discussion. 

 G. denudatum has a well-developed gas-filled swim-bladder with a powerful gas-secreting complex; 

 G. elongatum has a fat-filled swim-bladder with a degenerate complex; G. bathyphilum has no swim- 

 bladder. Since the two latter species are probably heavier than their environment, the presence of 

 pectoral fins with a hydroplane-like setting is not surprising. And as G. denudatum can presumably 

 bring its weight in water to vanishing point, the possession of pectorals which are beginning to look 

 more like brakes than hydroplanes is more comprehensible. 



From these considerations we may return to the Myctophidae, most of which have a swim-bladder. 

 In all investigated species (Marshall, 1951), this swim-bladder is closed and is usually provided with a 

 highly developed gas-secreting complex. Reference to the list of pectoral angles of myctophids (see 

 p. 326) shows that most species have pectorals with a setting more efficient for braking (or paddling) 

 than for hydroplaning. 



