FISHERY BULLETIN: VOL. 71, NO. 2 



Table 1. — Incidence of distal fusion of pmx (pre- 

 maxillary) and mx (maxillary) bones in specimens of 

 Stenella graffmani. 



Age 

 class 



Examined 



Pmx and 

 mx fused 



Error in 

 classification 



Subadults 

 Adult 



31 

 21 



19.4% 

 14.3% 



' As determined by criterion of total length, color pattern, 

 and sexual maturity. 



Table 2. — Lesions of the ventral side of the skull in 

 Stenella graffnumi, presumably due to infection by a 

 Crassicaiida-\iV.t nematode. 



calves than in subadults or adults. What is 

 more, the most severely damaged skulls were 

 those of calves. Such extensive damage is cer- 

 tainly irreversible, in that if repaired, traces 

 would still be apparent in the form of bone 

 callouses, depressions, or altered shape of bone 

 elements. No such traces were found in any 

 of the adult skulls examined, although we 

 searched for them carefully. We conclude, there- 

 fore, that heavy damage is indicative of a 

 probably morbid condition and that parasitism 

 or a correlated condition is a significant factor 

 in natural mortality of the spotted porpoise in 

 the eastern tropical Pacific. 



The patterns of infection relative to age by 

 cysts of the cestodes Monorygma and PhijUo- 

 bothrium (Figure 15) closely resemble each 

 other, with little or no occurrence before sub- 

 adulthood and 100% occurrence and greatest 

 numbers in adults. As these are larval forms, 

 the cysts presumably would have to be ingested 

 by predators of the porpoise in order for the 

 life cycles of the parasites to progress. Killer 

 whales, Orcijnis orca, have been observed by 

 Perrin to prey on spotted porpoise, and sharks 

 of several species have been observed to feed 

 on dead and injured spotted porpoise during 

 tuna fishing operations (Leatherwood, Perrin, 



Garvie, and La Grange, manuscript).^ Killer 

 whales and/or sharks are therefore possible 

 candidates for terminal hosts of these two 

 cestodes, although adult Tetraphyllidea have 

 only been found in elasmobranchs. However, 

 a life cycle involving a warm blooded inter- 

 mediate host ser-ving a cold blooded definitive 

 host has not been previously demonstrated. 

 The two nonlarval cestodes, Tetrabothrium 

 forsteri (Figure 16) and Strobilocephalus tri- 

 angularis (Table 3), exhibit different patterns 

 of occurrence, more like those of the nematodes, 

 occurring with greatest numbers and fre- 

 quencies in calves in the case of T. forsteri and 

 in calves and subadults in the case of S. tri- 

 angularis. This was especially striking for 

 S. triangularis. In the two calves and two 

 subadults with extremely heavy infections, the 

 last meter of the intestine was all but blocked 

 and swollen to three or four times its normal 

 diameter. No adult examined was found to be 

 infected with S. triangularis. 



Spinner Porpoise, Stenella cf. 

 S. lofigirostris 



The smaller sample of spinner porpoise was 

 divided into three age classes on the basis of 

 length and sexual maturity, with subadults 

 again defined as immature animals of adult 

 size. Despite the small number of data for the 

 spinner (Appendix Table 2), some differences 

 are apparent between the patterns of infection 

 for this species and for S. graffma)ii. For 

 Aiiisakis, the specimen with the greatest num- 

 ber of worms (CV285) was a large adult female, 

 which contrasts with the finding for S. graff- 

 ma}ii, in which heaviest infections occurred in 

 subadults. Bone erosion suggesting severe infec- 

 tion of the ventral air sinuses by a nematode 

 was encountered in only 1 skull (of a subadult) 

 of 53 examined. The parasite specimen was 

 not recovered, precluding identification, but the 

 damage was similar to that caused by the 

 Crassicauda-\ike nematode in S. graffmani. 



* Leatherwood, J. S., W. F. Perrin, R. L. Garvie, 

 and J. C. La Grange. Observations of sharks attacking 

 porpoise (Stenella spp.). (Unpubl. manuscr.) 



466 



