ONTOGENIC HABITAT SELECTION BY HADWENIUS-PONTOPORIAE (DIGENEA, CAMPULIDAE) IN THE INTESTINE OF FRANCISCANAS (CETACEA)

The linear habitat selection of 4 sequential maturity stages (1, 2, 3, and 4) of the trematode Hadwenius pontoporiae in the intestines of 26 South American dolphins Pontoporia blainvillei was investigated. The franciscana is a suitable host for H. pontoporiae because all 26 hosts were infected, the infrapopulations being composed mostly of gravid ( stage 4) worms. Most trematodes were found in the first third of the i ntestine. The niches of the maturity stages decreased from stage 1 to 3. Gravid worms favored the most anterior part of the duodenum, wherea s stages 1, 2, and 3 occurred more posteriorly in every host. The dist ributions of the maturity stages showed a narrow site fidelity and wer e ordered following the developmental sequence. The positions of stage s 2, 3, and 4 covaried positively when the effect of intensity was con trolled. All this evidence suggests that H. pontoporiae undergoes an a nteriad ontogenetic migration from the posterior to the most anterior duodenum. At high intensities, the distribution of stages 2, 3, and 4 expanded and shifted posteriorly. According to the maturity structure of H. pontoporiae at the infrapopulation level, it is proposed that th ere might be more developing worms at high intensities, and, therefore , the expansion and shift might result, at least partly, from the pres ence of more migrating worms along the fundamental niche of H. pontopo riae. Other Hadwenius species show a similar general pattern of distri bution to H. pontoporiae. This raises the possibility that the habitat selection behavior of H. pontoporiae may be due to the inheritance of prior phylogenetic constraints.