FISHERY BULLETIN: VOL. 83, NO. 4 



of microsporidan, muscles, ovaries, connective 

 tissues, and gut epithelia are infected. One of the 

 microsporidans, a parasite of Gammarus pulex L., 

 infects the longitudinal abdominal muscles of its host 

 in the same manner as does the A. agassizi parasite, 

 but is knovv^n to have a variable number of spores 

 per envelope It was named Glugea muelleri (Pfeif- 

 fer, 1895, in van Ryckeghem 1930), later named 

 Thelohania giraudi (Leger and Hesse, 1917), and has 

 been called Stempellia muelleri (Pfeiffer) by 

 Bulnheim (1975) and Microsporidium giraudi 

 (Leger and Hesse) by Sprague (1977). The relation- 

 ship of "Glugea muelleri Pfeiffer" and the micro- 

 sporidan from A. agassizi remains to be determined. 

 Although "Glugea muelleri Pfeiffer" and the A. 

 agassizi parasite are remarkably similar in being 

 restricted to the longitudinal abdominal muscles of 

 their hosts, the latter is probably significantly 

 smaller. Fixed spores of the A. agassizi parasite are 

 about 1.5 X 3 ^m, and fresh spores of "Glugea 

 muelleri" are 2.2 x 4.5 jum. 



The method of transmission of the A. agassizi 

 parasite is not known. Microsporidans are usually 

 transmitted orally, but transovarial transmission also 

 occurs in amphipods. Transovarially transmitted 

 microsporidans of Gammarus duebeni Lilljeborg in- 

 fect the ovary, and, depending on the species, cause 

 complete or partial feminization of males (Bulnheim 

 1975, 1977). The parasite of A. agassizi did not in- 

 fect the ovary, and because it was regularly found 

 in normal males, it apparently does not cause 

 feminization of males. Bulnheim (1971) successful- 

 ly transmitted "G. muelleri Pfeiffer" to several 

 species of Gam/marus, including euryhaline ones, by 

 feeding of infected muscle 



Prevalence of the muscle parasite of A. agassizi 

 apparently increases with age of the host, and it 

 could be hypothesized that this microsporidan is 

 transmitted orally, that the older the host the more 

 chances it has had to become infected, and that the 

 parasite does not contribute to increased mortality 

 in the population. Adult males are active swimmers 

 and might prove an exception because impaired mus- 

 cle function could lead to increased predation. If this 

 occurred, one would expect infected males to be 

 preferentially removed from the population, leading 

 to a lower prevalence of infection in this stage In- 

 deed, prevalence in adult males was only 2%. 

 However, relatively few males were collected during 

 the survey, and the low prevalence could prove to be 

 sampling artifact. Note that in the sample from 

 cruise E, station 47 (Tkble 3), both postovigerous 

 females and adult males were uninfected, but 2 mo 

 later, at the same station, prevalence in post- 



ovigerous females was 21% and the only infected 

 adult male found during the survey was also collected 

 at that time The discrepancy in prevalence might 

 be due to sampling of slightly different populations. 

 As discussed below, there is no assurance that the 

 same population vras sampled spatially, and temporal 

 differences conceivably might also have complicated 

 the results. 



Relationships of the microsporidans seen in the 

 various species of amphipods could not be decided 

 on the basis of material fixed and prepared as it was. 

 It would be interesting to determine whether the 

 parasites of Unciola spp. and Ampelisca vadorum 

 are the same or different species, and what their 

 relationship is to the A. agassizi parasite There were 

 some differences in the habits and the developmen- 

 tal stages present in the three amphipods. Vegetative 

 stages were common in the case of the A. vadorum 

 parasite and fairly common in Unciola spp., but 

 usually rare or absent in A. agassizi. Several dif- 

 ferent tissues were infected in A. vadorum, but ex- 

 cepting a few infections in nervous tissue, only ab- 

 dominal muscle was infected in Unciola spp. and A. 

 agassizi. Previous investigators have found that 

 microsporidan infection is well tolerated by am- 

 phipod hosts, and that defense reactions against 

 these parasites generally are limited and may come 

 into play mainly when host tissue becomes necrotic 

 (reviewed by Bulnheim 1975). The muscle-inhabiting 

 microsporidan of A. agassizi is obviously a primary 

 parasite of that species and is seldom attacked by 

 the host. However, the similar parasite of Unciola 

 spp. often either provokes attack merely by its pres- 

 ence or damages the muscle so that a response oc- 

 curs to the necrotic tissue In either event, it is possi- 

 ble that this parasite is not fully adapted to Unciola 

 spp., because arthropods are known to be less 

 tolerant of non-adapted parasites (Salt 1970; 

 Unestam and Weiss 1970) 



With exception of the muscle-infecting species 

 from A. agassizi, microsporidans are not common 

 parasites of benthic amphipods in the areas sampled, 

 even considering that some infections must have 

 been missed because not all would be seen in the 

 limited number of sections examined from each 

 amphipod. 



Sampling methods used in the survey do not lend 

 themselves to precise studies on progression of 

 parasitic infections through particular populations. 

 Sampling cannot be done often enough to show if 

 and when additions to populations (with perhaps dif- 

 ferent prevalences of parasites) are provided by im- 

 migrating juveniles or other stages of these short- 

 lived animals. Further, populations may not be 



504 



