Parasitic Helminth (Acanthocephalan) Infection as a Cause of Mortality in the California Sea Otter (Enhydra lutris)
IAAAM 1999
Murray Dailey1; Karl Mayer2
1The Marine Mammal Center, Marin Headlands, Sausalito, CA, USA; 2Moss Landing Marine Laboratory, Moss Landing, CA, USA


The growth of the California sea otter population has been steady but slow and range expansion in the southern animals has faltered. This slower growth is occurring in California due to mortality primarily caused by infections of acanthocephalan parasites.

From October, 1997 to August, 1998, 39 beach-cast sea otter carcasses were collected from Monterey Bay to Pismo Beach California and examined for parasites. Acanthocephalans of the genera Polymorphus spp. (19 of 39, 48.7%) and Corynosoma (38 of 39, 97.4%) were found with mean counts of 310 and 646 worms/otter respectively. Mortality due to heavy acanthocephalan infections leading to intestinal perforation and peritonitis was found in 10 (25.6%) of the animals examined. This is substantially higher than previous studies by Hennessy and Morejohn in 1977 (1%)1 , or Thomas and Cole in 1996 (14%)2. Thomas and Cole found that 67% of mortality due to acanthocephalan peritonitis in southern sea otters occurred in pups and juveniles.2 In the present study, similar trends in mortality were not found, although mortality rate due to acanthocephalan peritonitis was higher among immature sea otters than other age categories (1 pup, 2 immatures, 3 subadults, 4 adults), and higher in males than females (8 males, 2 females). The increased mortality appears to be directly linked to greater infection rate and intensity of Polymorphus spp. consumed in the food chain of the sea otters.

Polymorphus spp. are commonly parasites of marine birds where they reach sexual maturity. No sexually mature female worms were found in otter infections. The intermediate hosts of Polymorphus spp. are reported to be crabs of the genera Emerita and Blepharipoda, which inhabit sandy habitats.1 A comparison of prey selection, foraging habitat selection and age of infected otters indicate the following: that pups and juveniles are at a competitive disadvantage relative to older animals due to lack of size and experience, and are forced to forage on less desirable sandy bottom habitat where crabs are abundant, rather than on a rocky substrate. Parasite loads may be greater among immatures than pups due to food supplies and loss of immunity from mothers support (nursing). The frequency of occurrence of heavy Polymorphus infection by carcass recovery location indicates a north to south-sandy habitat gradient.


1.  Hennessy SL, GV Morejohn. 1977. Acanthocephalan parasites of the sea otter, Enhydra lutris, off coastal California. California Department of Fish and Game. 63:268-272.

2.  Thomas NJ, RA Cole. 1966. The risk of disease and threats to the wild population. Endangered Species Update. 13 (12): 23-27.

Speaker Information
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Karl Mayer
Moss Landing Marine Laboratory
Moss Landing, CA, USA

Murray D. Dailey, PhD
The Marine Mammal Center, Marin Headlands
Sausalito, CA

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