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 blanvillei 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 intestine. The niches of the maturity stages decreased from stage 1 to 4. Gravid worms favored the most anterior part of the duodenum, whereas stages 1, 2, and 3 occurred more posteriorly in every host. The distributions of the maturity stages showed a narrow site fidelity and were ordered following the developmental sequence. The positions of stages 2, 3, and 4 covaried positively when the effect of intensity was controlled. All this evidence suggests that H. pontoporiae undergoes an anteriad 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 there might be more developing worms at high intensities, and, therefore, the expansion and shift might result, at least partly, from the presence of more migrating worms along the fundamental niche of H. pontoporiae. Other Hadwenius species show a similar general pattern of distribution 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.

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