FISHERY BULLETIN: VOL. 74, NO. 4 



length of third maxillipeds. Finally, nearly all 

 species cooccur with at least one other species that 

 is much more closely related than any of the other 

 Hawaiian sergestids. Interspecific competition 

 should be strongest between members of a species 

 pair; the ways in which two closely related serges- 

 tids divide up the mid-water environment should 

 suggest the kinds of competition that occur in the 

 mid-water environment and how competition is 

 minimized. 



Table 8 shows some observed parameters of 

 Hawaiian sergestids. The dendrogramlike pattern 

 at the left is a subjective representation of the 

 affinities among the species, based on morpho- 

 logical features. Some differences will be noted in 

 the vertical distribution patterns of the species 

 pairs; for example, Sergestes vigilax is more 

 broadly distributed than S. armatus and tends to 

 live shallower at night. However, most species 

 pairs are commonly found together over much of 

 their vertical ranges. The most striking difference 

 among closely related species is adult size. In every 

 case the most closely related species show little or 

 no overlap in the adult size range. For example, 

 Sergia scintillans appears nearly identical to S. 

 fulgens, differing chiefly in the number of photo- 

 phores on the antennal scale and exopod of the 

 uropod. However, adult S. scintillans vary from 

 5.5 to 10.5 mm CL, while adult S. fulgens vary from 

 11 to 16.5 mm CL. The only exception to this rule, 

 the species triplet Sergestes orientalis-S. tantil- 

 lus-S. consobrinus, is a revealing case. Sergestes 



orientalis is well separated in size from S. conso- 

 brinus, the largest females of S. consobrinus 

 overlapping only slightly with the smallest males 

 of S. orientalis. However, S. tantillus, while 

 somewhat smaller in average size than S. orien- 

 talis, still overlaps considerably in size with the 

 larger species. In this case it turns out that S. 

 tantillus is primarily an equatorial species 

 (Judkins 1972), occurring only rarely in Hawaiian 

 waters. Mac Arthur (1972) has shown on theoretical 

 grounds that when three similar species differ in 

 only one parameter, such as body size, the compe- 

 tition pressures are strongest on the middle 

 species. One of the factors determining the north- 

 ern limit of S. tantillus may be this competition 

 from both a larger and a smaller species. 



Specialization solely by adult size could still 

 result in competition if adults of the small species 

 cooccur with similar-sized juveniles of the large 

 species. In this case, other specializations appear 

 to become important. When the species have 

 similar vertical ranges, the juveniles may live 

 shallower than the adults. For example, adult 

 Sergia bigemmea and adult 5. inequalis both occur 

 at about 150 to 225 m at night. Juvenile S. in- 

 equalis in the 10- to 13-mm CL range, the size of 

 adult 5. bigemmea, are mostly found between 50 

 and 150 m, so that similar-sized individuals of the 

 two species seldom occur together. 



Competition could occur if the large species lives 

 somewhat deeper than the small species, so that 

 juveniles of the large species live at about the 



Table 8.— Characteristics of Hawaiian sergestid species. Dendrogram shows estimated phylogenetic affinities among 



species. 



Species 



Adult size Day depth Night depth Population size 



(CL, mm) (m) (m) (no./100 m') 



I — Sergestes orientalis 

 _n — Sergestes tantillus 

 ' Sergestes consobrinus 



Sergestes armatus 

 Sergestes vigilax 



Sergestes atlanticus 



Sergestes cornutus 



Sergestes erectus 



Sergestes sargassi 



Sergestes pectinatus 



J Sergia fulgens 



' Sergia scintillans 



Sergia gardineri 

 Sergia bigemmea 

 Sergia inequalis 



I Sergia bisulcata 



' Sergia maxima 



Sergia tenuiremis 



Sergia laminata 



Petalidium suspiriosum 



832 



