HELMINTH PARASITISM OF 



THREE LARVAL FISHES IN 



THE NORTHERN GULF OF MEXICO 1 



Helminth infections of the pelagic larvae of marine 

 fishes are ecologically germane to fisheries biology 

 for two important reasons. First, endoparasites, 

 although not likely to actively kill host larvae, may 

 passively contribute to larval mortality by competing 

 for nutrients and space in the alimentary canal 

 (Rosenthal 1967) or by causing pathological lesions 

 (Yamashita 1979), thereby compromising growth 

 (May 1983). Because growth abets both feeding suc- 

 cess and predator avoidance, it is linked with survival 

 and therefore is an important factor in determining 

 cohort size (Hunter 1981). Second, helminth infec- 

 tions can be useful indicators of trophic relationships 

 because the life stages of cestodes and trematodes 

 are transmitted through intermediate hosts before 

 infection of the definitive host (see review in 

 Campbell et al. 1980). 



Given that larval fishes eat copepods, the vectors of 

 many marine helminth infections (Cheng 1964; Gib- 

 son and Bray 1979), it should not be surprising that 

 they are infected with cestodes and trematodes. Yet, 

 despite extensive laboratory and field studies of lar- 

 val fish feeding (see review in Hunter 1981), there are 

 only incidental reports, none comprehensive, of 

 helminth infections (Lebour 1918; Ogilvie 1927; 

 Hentschel 1950; Bowers and Williamson 1951; 

 Rosenthal 1967; Marak 1974; Mackenzie 1974; 

 Yamashita 1979). Herein I report the prevalence and 

 temporal variation of cestode and trematode infec- 

 tions in three species of larval fishes collected in the 

 northern Gulf of Mexico: gulf menhaden, Brevoortia 

 patronus Goode; spot, Leiostomus xanthurus Lace- 

 pede; and Atlantic croaker, Micropogonias undu- 

 latus (Linnaeus). 



Methods 



Larvae of gulf menhaden, spot, and Atlantic croaker 

 were collected on four cruises in the northern Gulf of 

 Mexico in December 1979, February 1980, Decem- 

 ber 1980, and February 1981. Cruises generally 

 occupied three stations (at the 5.5, 27, and 55 m 

 isobaths) along each of three transects (off of Galves- 

 ton Bay, the Mississippi Delta, and Cape San Bias). 

 Collections from three discrete depths taken atOOOl, 

 0600, 1200, and 1800 h (CST) at each station with a 



Multiple Opening/Closing Net and Environmental 

 Sensing System 2 (MOCNESS) (Wiebe et al. 1976) 

 were fixed in 57c Formalin buffered with sodium 

 borate. Nominal depths of the MOCNESS samples 

 corresponded to strata just above the thermocline, in 

 the middle of the upper mixed layer, and just below 

 the surface. The MOCNESS was equipped with nine 

 1.0 by 1.4 m, 505 jum mesh nitex nets and with 0.2 5 by 

 0.35 m, 67 /xm mesh nets "nested" inside. 



Larvae of all three species were removed from 

 MOCNESS samples, measured (notochord or stan- 

 dard length), and dissected, except when the total 

 number of a species exceeded 30 larvae. In these 

 cases, 30 larvae were chosen randomly from a num- 

 bered grid by consulting a table of random numbers. 

 Contents of the entire alimentary canal, including 

 parasites, were identified and enumerated. Hel- 

 minths were stained with Mayer's paracarmine and 

 mounted to aid in identification. 



The cooccurrence of cestodes and trematodes was 

 assessed by the index of affinity (Fager 1957; Fager 

 and McGowan 1963). Independence in the pre- 

 valence of helminth infections was assessed by con- 

 structing four- way contingency tables (species offish 

 larvae X cohort X month X prevalence of infection) 

 and by referring G 2 to a chi- square distribution (Fien- 

 bergl970). In this log likelihood ratio test, IX 10 " 7 

 was added to each observed value in order to allow 

 the use of natural logarithms with observed zero 

 incidences. 



Results and Discussion 



Parasites 



Two taxa of helminths were identified, a tetraphyl- 

 lidean cestode of the Scolex pleuronectis complex and 

 a digenean hemiurid trematode Aphanurus sp. All 

 specimens of S. pleuronectis were plerocercoids with 

 the exception of one juvenile that had undergone 

 strobilization. Aphanurus sp. were late metacer- 

 cariae or adults; gonads were developed but ova were 

 never visible. 



Helminths occurred primarily in the midgut; only 4 

 of 64 (6.2%) gulf menhaden and none of the spot and 

 Atlantic croaker larvae had helminths in the hindgut 

 (sensu Iwai 1969). Usually a single larva was infected 

 by only one cestode and/or one trematode; 8 of 64 

 (12.5%) gulf menhaden larvae were infected 

 simultaneously by as many as three helminths. 



Cestode plerocercoids of the S. pleuronectis com- 



'Contribution No. 83-31B, Southeast Fisheries Center Beaufort 

 Laboratory, National Marine Fisheries Service, NOAA, Beaufort, 

 N.C. 



: Reference to trade names does not imply endorsement by the 

 National Marine Fisheries Service, NOAA. 



FISHERY BULLETIN: VOL. 81, NO. 4. 1983. 



895 



