FISHERY BULLETIN: VOL. 73, NO. 1 



Although Lane (1960) reported that Nomeus 

 can survive doses ofPhysalia toxin as much as 10 

 times the amount that would kill other fishes of 

 the same size and type, Nomeus is stung if forced 

 into contact with the tentacles (Lane 1960) and 

 can be killed if touched by the tentacles according 

 to Zahl (1952). Maul (1964) found that 

 Schedophilus (= Mupus) ovalis also suffered 

 large weals on the body from nematocysts when 

 in contact with Physalia and that safety for the 

 fish must be due in part to its ability to avoid con- 

 tact with the tentacles. Mansueti (1963) concluded 

 that in all fish-jellyfish associations the former 

 skillfully maneuver between tentacles and gen- 

 erally avoid being stung but that contacts are 

 inevitable. 



Locomotion and buoyancy 



The differences in locomotor behavior found be- 

 tween juvenile and adult stromateoids that have 

 been observed illustrate the importance of ma- 

 neuverability for juveniles and correspond to 

 swim-bladder loss and increased independence of 

 floating objects as maturity is reached. The paired 

 fins are important locomotor devices among 

 stromateoids. The pectoral fins are moved in a 

 rotary manner for maintaining position in juve- 

 niles of Peprilus triacanthus and Schedophilus 

 medusophagus when hovering beneath floating 

 objects (pers. obs.) and sculled for effecting con- 

 tinuous swimming at less than maximum speeds 

 in these species (Horn 1970b, unpubl. obs.) and in 

 other stromateoids such as Cubiceps gracilis (Fig- 

 ure 8). I have observed adults of both P. triacan- 

 thus and P. simillimus in public aquaria and 

 calculated that the pectorals are used at least 

 80-909?^ of the time as a main propulsive force at 

 cruising speeds. The pelvic fins which may be 

 absent (all stromateid species except one) or 

 small (as in certain centrolophids) are well devel- 

 oped in juveniles of certain species. Pelvics are 

 large in Nomeus and apparently important for 

 increasing maneuverability and enhancing pro- 

 tective coloration for a fish living among the tenta- 

 cles of Physalia. 



The relative length of the paired fins changes 

 with age and varies among the species (Haedrich 

 1967; Horn 1970b). Extremes are represented by 

 P. triacanthus and S. medusophagus (Figures 

 9, 10). In P. triacanthus (which lacks pelvic fins) 

 the relative length of the pectoral fin increases 

 rapidly until the fish reaches about 75-80 mm 



Figure 8. — Cubiceps gracilis, 68 mm SL, swimming in plastic 

 container and using pectoral fins as principal locomotor force. 

 Swim bladder of this fish partially regressed (see Table 4). 

 Specimen captured at the surface in the North Atlantic. 



SL beyond which the fin length ceases to increase 

 (Figure 9). This fish size is in the range of that 

 when the swim bladder regresses and the fish 

 deserts its coelenterate host (Table 4). Individuals 

 of P. triacanthus greater than 75-80 mm SL are 

 negatively buoyant (see below) and swim continu- 

 ously using mainly the long pectorals which also 

 generate dynamic lift. In S. medusophagus the 

 relative length of the paired fins decreases with 

 age (Figure 10), a pattern opposite that of P. 

 triacanthus. The swim bladder regresses in a size 

 range of about 40-60 mm SL corresponding closely 

 to the size interval during which the marked 

 change in paired fin length occurs and apparently 

 during which the fish deserts its coelenterate host 



106 



