HORN: SWIM-BLADDER STATE AND STRUCTURE 



1960), although decreased bore is an adaptation 

 Hmited in most fish species by the size of the eryth- 

 rocytes. 



Swim-Bladder Regression in Relation to 

 Behavior and Mode of Life 



Data from the present study and other sources 

 on depth distribution, association with floating 

 objects, and locomotion and buoyancy make it pos- 

 sible to formulate a general outline of the changes 

 in behavior and mode of life that accompany the 

 regression of the swim bladder and which are part 

 of the transition from the juvenile to the adult 

 state. 



Depth Distribution 



Adult stromateoids generally occupy a wide 

 range of depths either over the continental shelf or 

 in the open ocean, whereas larvae and juveniles of 

 all or nearly all species occur in the surface layers 

 (mainly the upper 100 m) (Haedrich 1967, 1969; 

 Horn 1970a). Larval nomeids (Psenes and 

 Cubiceps) are important constituents of the 

 epipelagic fauna; this is known especially for the 

 eastern tropical Pacific (Ahlstrom 1971, 1972). 

 The Centrolophidae and Tetragonuridae were 

 listed by Ahlstrom (1969) as two of the principal 

 families of deep-sea fishes which had larvae in the 

 surface layers of the California Current region. 

 The stromateid, Peprilus simillimus, occurs 

 mainly in the upper 50 m of coastal waters off 

 California and Baja California (Ahlstrom 1959), 

 and ariommid larvae and juveniles apparently 

 live in the surface layers although the adults are 

 benthopelagic (Horn 1972). Thus, swim-bladder 

 loss occurs as the fishes increase their range of 

 vertical distribution. 



Association with Floating Objects 



Beginning at a small size (^ 10 mm SL) and 

 usually ending before maturity is reached (^200 

 mm SL), stromateoids commonly associate with a 

 wide variety of animate and inanimate floating 

 objects (Mansueti 1963; Haedrich 1967; Horn 

 1970a) and during the period that the swim blad- 

 der is functional (Table 4). The associations are 

 not obligatory but rather, as Mansueti (1963) de- 

 scribed them, temporary ecological phenomena in 

 which the objects (e.g., jellyfishes or fiotsam) are 

 essentially passive hosts and the fishes active op- 



portunists. Scyphozoan medusae of several genera 

 form a major group of associates particularly of 

 stromateids and to some extent of centrolophids, 

 nomeids, and tetragonurids (Mansueti 1963). The 

 nomeid, Nomeus gronovii, and the Portuguese 

 man-of-war, Physalia, form the apparently most 

 intimate and enduring of "fish-jellyfish" associa- 

 tions. Certain stromateoids have also been found 

 inside pelagic ascidians (Grey 1955), beneath the 

 ocean sunfish, Mola (Mackay 1972) and beneath 

 floating plants such as Sargassum (Haedrich 

 1967). Several species occur beneath flotsam, and 

 the nomeid, Psenes cyanophrys , is one of the more 

 abundant fishes under drifting objects (Hunter 

 and Mitchell 1967, 1968; Gooding and Magnuson 

 1967). Drift associations are not well understood 

 but probably provide one or more ecological ad- 

 vantages such as food, protection, or visual 

 stimuli. 



Juvenile stromateoids in their coloration and 

 maneuverability are well adapted for life beneath 

 floating objects, especially coelenterates. Young 

 fish typically have a banded, mottled, or blotched 

 pattern whereas adults are generally uniform in 

 color or are dark above and pale below. The dura- 

 tion of the juvenile color pattern is similar to the 

 period when the fishes are associated with floating 

 objects, and the patterns according to Haedrich 

 (1967) serve as protective coloration beneath the 

 shifting shadows of objects, especially jellyfishes. 

 Nomeus which retains its association with 

 floating objects longer than most or all other 

 stromateoids also retains its mottled color pattern 

 in the largest specimens known. 



Maneuverability and avoidance by the fish ap- 

 pear to be of primary importance in all or most 

 stromateoid-coelenterate associations (Mansueti 

 1963; Horn 1970a). Peprilus triacanthus placed in 

 tanks with a medusa, Chrysaora quinquecirrha, 

 gradually increased the amount of time spent near 

 the jellyfish and after 72 h remained within a 4-cm 

 distance of the bell at least 75% of the time (Horn 

 unpubl. data). Hovering and rapid turning were 

 significant parts of the locomotor behavior of the 

 fish in avoiding the tentacles of the medusa. Con- 

 tact of the skin of the fish by the tentacles resulted 

 in nematocyst firing as evidenced by the clinging 

 of the tentacles to the fish's body causing the fish to 

 rapidly swim away. In a two-way feeding relation- 

 ship, P. triacanthus frequently nibbled at the 

 manubrium and tentacles of the medusa, while 

 weakened or otherwise slow-moving fish were 

 captured and ingested by the medusa. 



105 



