Corynosoma cetaceum in the stomach of franciscanas, Pontoporia blainvillei (Cetacea): an exceptional case of habitat selection by an acanthocephalan.

Adult acanthocephalans are typically found in the intestine of vertebrates, where they can readily absorb nutrients. However, Corynosoma cetaceum has been frequently reported in the stomach of cetaceans from the Southern Hemisphere. The ecological significance of this habitat was investigated by examining data on number, sex ratio, maturity status, biomass, and fecundity of C. cetaceum in different parts of the digestive tract of 44 franciscanas Pontoporia blainvillei. Individual C. cetaceum occurred in the pyloric stomach (PS) and, to lesser degrees, in the duodenal ampulla (DA) and the main stomach (MS). Females outnumbered males in all chambers, although the sex ratio was closer to 1:1 in the MS; there also was a higher proportion of nongravid females, with a smaller biomass in the MS than in the PS and the DA. This evidence suggests that cystacanths are released from prey tissues in the MS, where entire prey are reduced to semi-fluid chyme. The 3 chambers harbored gravid females that did not differ significantly in mean biomass or fecundity. The maturity status of females was nearly identical between the PS and the DA. In the MS, the higher proportion of non-gravid females is probably due to the occurrence of newly recruited females to this site. Mean biomass and fecundity of gravid females covaried strongly and positively among chambers within hosts. These results suggest that there are no major differences between the 3 chambers with respect to the suitability for reproduction by C. cetaceum. However, although the MS is the largest chamber, it harbored the smallest number of gravid females. Interestingly, worms were largely restricted to the aboral portion of the MS, a sheltered region where a concentration of chyme, and thus nutrient availability, likely occurs. Linear distribution differences of gravid female C. cetaceum at increasing intensities suggest that reproductive females occupy chambers according to available space. In summary, the stomach should be considered the main habitat for C. cetaceum. The choice of this habitat is puzzling because other Corynosoma species occur in the intestine, and because the stomach of cetaceans is not an absorptive site.

Constructional morphology and mode of attachment of the trunk of Corynosoma cetaceum (Acanthocephala: Polymorphidae).

Dead specimens of Corynosoma cetaceum were used to describe the trunk musculature of this species and to infer the use of the trunk as a secondary holdfast. Inferences were based on trunk muscle arrangement, changes in trunk shape, size and distribution of spines, and geometry of tegument thickness. The foretrunk of C. cetaceum is swollen and forms a spiny disk that is bent ventrally. The disk is flattened by several groups of muscles not described previously, which seem able to finely adjust the disk surface over the substratum. Disk attachment appears to be accomplished by two dorsal neck retractor muscles specialized in pulling the anchored proboscis into the foretrunk. This mechanism has been described in other acanthocephalans, becoming surprisingly efficient when used with a flattened, armed foretrunk. The ventrally spined hindtrunk requires force to move downwards in order to attach. A single ventral neck retractor muscle seems specialized in pulling the posterior trunk forward, inducing a downward force due to the muscle's precise points of insertion. This mechanism necessarily generates ventral wrinkling that needs to be eliminated for the spiny surface to be functional. The trunk ventral muscles are apparently arranged so as to concentrate the "excess" of the tegument into a single fold, optimizing the use of the remaining surface for attachment. The size and distribution of spines, as well as the geometry of tegumental thickness, conform to these observations. Morphological changes, seemingly simple, such as structural bending, may have triggered a cascade of subtle modifications and new functions during acanthocephalan evolution, reflecting how morphological integration and novelty interact.

Polymorphus arctocephali Smales, 1986, a synonym of Corynosoma cetaceum Johnston & Best, 1942 (Acanthocephala: Polymorphidae).

Polymorphus arctocephali (Acanthocephala: Polymorphidae) was differentiated from P. cetaceum based on patterns in the trunk spine distribution and slight morphometric differences. The comparison of both species involved samples from South Australia and did not include P. cetaceum from South America. In this paper we re-examine the systematic position of P. arctocephali based on a more detailed morphological and geographical analysis. Results indicate that P. arctocephali does not differ in trunk spine distribution with respect to P. cetaceum, and that its morphometric differences can be subsumed under the natural variation found within P. cetaceum populations. Therefore, P. arctocephali becomes a junior synonym of P. cetaceum. P. cetacueum was transferred from Corynosoma to Polymorphus due to the absence of genital spines in both sexes. However, adopting the less restrictive definition of genital spines used by several authors, females of P. cetaceum could be considered as bearing genital spines. Species of the genus Andracantha also have genital spines, but specimens of P. cetaceum possess a continuous field of trunk spines, which precludes the assignment of this species to Andracantha. Other generic level characters, as well as ecological data, support clearly the transference of P. cetaceum back to Corynosoma. Therefore, this species becomes Corynmosoma cetaceum Johnston & Best, 1942.

Parasitology meets ecology on its own terms: Margolis et al. revisited.

We consider 27 population and community terms used frequently by parasitologists when describing the ecology of parasites. We provide suggestions for various terms in an attempt to foster consistent use and to make terms used in parasite ecology easier to interpret for those who study free-living organisms. We suggest strongly that authors, whether they agree or disagree with us, provide complete and unambiguous definitions for all parameters of their studies.

Ontogenetic 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 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.

Parasitology year 2000.

We predict that in order for parasitology to thrive by the year 2000 the various subdisciplines of evolution, ecology, biosystematics, and genetics must develop holistic approaches and use parasite models to answer basic biological questions. The students of tomorrow must work as part of a multidisciplinary team; and their questions and answers must be conceptually integrated into the broader biological framework of evolution and ecology.

Recruitment-driven, spatially discontinuous communities: a null model for transferred patterns in target communities of intestinal helminths.

Populations and therefore communities of intestinal helminths of vertebrates are fueled by recruitment of new individuals from outside the host. The source of new individuals is often an intermediate host that harbors several infective propagules of 1 or more species. Hence these source communities are transmitted in packets of infective propagules to target communities in definitive hosts. Packets not only provide recruits to target communities, but, because a packet of propagules possesses its own structure, it may also transmit structure to the target community. We use this system to examine the contribution that structure in the source pool of propagules makes to the structure of recruitment-driven target populations and communities. By treating the dynamics of such target populations and communities as immigration-death processes, we conclude: (1) Unlike a birth-driven population a recruitment-driven target population will grow to an asymptotic limit even in the absence of density-dependent processes or reaching carrying capacity; (2) the frequency distribution of the number of recruits entering target populations will determine the frequency distribution of adults in target populations; (3) interspecific associations among species in the source community will be transmitted to target communities, but the magnitude of the transmitted associations will depend upon the relative survival rates of the species; and (4) for associations of equal magnitude in a source community, the magnitude of a transferred negative association will be less than the magnitude of a positive association in a target community. Two examples of source communities in salt marsh crabs reveal that source infracommunities exist with the hypothesized structure. Further, the source helminth communities display a greater number of positive than negative interspecific associations. The inequity in transfer and the existence of a greater proportion of positive associations in source communities may explain the widespread occurrence of excess positive associations that has been noted in recruitment-driven communities.

Host fragmentation and helminth parasites: hedging your bets against extinction.

We consider the probability of parasite extinction due to anthropogenic fragmentation of host populations and in the absence of host extinction. We conclude that extinction at infrapopulation and infracommunity levels is both common and trivial. Extinction may occur in communities at higher levels but only if metapopulations or suprapopulations become extinct. Suprapopulations are highly complex and unlikely to become extinct in the face of simple host fragmentation. We acknowledge parasite metapopulations as being the most likely to become extinct, but only locally. Our reasoning for this is that, in the absence of complete host extinction, populations of the parasite in other fragments are likely to serve as sources for reinvasion (e.g. a rescue effect). We identify a number of features that may act as hedges against extinction for many parasites and conclude by attempting to identify what form an extinction might take.

The relationship between pattern and scale in parasite communities: a stranger in a strange land.

Problems of pattern and scale are considered in relation to helminth communities of freshwater fish by examining them at different hierarchical taxonomic and spatial scales, with a view to seeking generalizations of heuristic value, assessing the importance of phylogenetic and ecological determinants of community structure and improving understanding of unpredictable communities. Initially, communities were analysed at the level of salmonid genera, focusing on Oncorhynchus, in its heartland in Canada: then in O. mykiss throughout its global range and finally in individual localities to which it has been introduced in Britain. In the heartland, communities are dominated by salmonid specialist helminths, forming a phylogenetic element: the minority ecological element comprises broad generalists and non-salmonid specialists. Most species except generic specialists are shared between host genera. As the distance to which O. mykiss was translocated from its heartland increases, so generic specialists disappear first and then salmonid specialists decline. The community is thus increasingly composed of generalists and it also becomes increasingly poor. Helminths may be acquired from native salmonids and/or unrelated hosts, depending on availability. This same pattern is paralleled in individual localities in a restricted region: the phylogenetic element reflects the native salmonid species present and the ecological element the presence of other genera of fish; i.e., a supply-side situation. The change of scale in analysis has thus enabled the recognition of generalizations and patterns of heuristic value and improved the understanding of unpredictable communities by interpreting local variation as ecological 'noise' that often obscures fundamental patterns. In this and other taxa of fish, phylogenetic elements dominate helminth communities in the heartlands, but ecological elements dominate as the host increasingly becomes a stranger in a strange land.

Species richness in helminth communities: the importance of multiple congeners.

Using data sets derived from published literature, the contribution of congeneric species to helminth component community richness is evaluated. Consideration of the frequency distribution of congeners in relation to host and parasite groups reveals that the distributions are unimodal, that singletons are the commonest class and that the frequency of occurrence of congeners decreases with increasing number of species per genus. Congeners may be found in any group of hosts or parasites, but are more common amongst cestodes of aquatic birds. Two patterns of occurrence of congeneric species are recognized: one in which from a few to multiple congeners are found within a single helminth genus, exemplified by dactylogyrid monogeneans and cloacinid nematodes, and the other in which there is a simultaneous occurrence of congenerics and confamilials such that there are several genera involved, but each represented by fewer species, exemplified by strongyles in horses. The question of whether these patterns can be considered examples of species flocks is discussed. We conclude that multiple congeners and species flocks are interesting phenomena but, except in isolated cases, they make insignificant contributions to community richness.

Helminth communities in avocets: importance of the compound community.

This paper reports patterns of similarity and overlap in species presence and patterns of linear distribution of intestinal helminths in 22 avocets from 4 populations. Avocets collected from ephemeral bodies of water in Alberta and Manitoba had communities composed largely of species that are avocet specialists plus some that are host generalists. The composition of helminth communities in these hosts was similar to that reported in earlier surveys of avocet helminths. There was little evidence for competition between helminth species in these communities. In contrast, avocets collected from permanent bodies of water in Alberta had communities composed largely of species that are specialists in various duck species, particularly lesser scaup. These helminths were superimposed on the normal community, fitting into linear gaps along the intestine but also overlapping the distributions of avocet specialists. These lesser scaup specialists exhibit interactive patterns amongst themselves and, to some extent, with avocet specialists. Helminth communities in avocets from ephemeral bodies of water have vacant niches and are largely isolationist in nature. Those in avocets from permanent bodies of water are saturated and are more interactive in nature.

Patterns in helminth communities in freshwater fish in Great Britain: alternative strategies for colonization.

Examples of the apparently stochastic nature of freshwater fish helminth communities illustrating the erratic and unpredictable occurrence and distribution of many species are provided for six species of fish from several localities throughout Britain. By focussing on parasite colonization strategies two categories of helminths are recognized: autogenic species which mature in fish and allogenic species which mature in vertebrates other than fish and have a greater colonization potential and ability. Three groups of fish are distinguished: salmonids, in which helminth communities are generally dominated by autogenic species which are also responsible for most of the similarity within and between localities; cyprinids, in which they are dominated by allogenic species which are also responsible for most of the similarity within and between localities; and anguillids, whose helminth communities exhibit intermediate features with neither category consistently dominating nor providing a clear pattern of similarity. Recognition and appreciation of the different colonization strategies of autogenic and allogenic helminths in respect of host vagility and ability to cross land or sea barriers and break down habitat isolation, and their period of residence in a locality, whether transient or permanent, provides an understanding of, and explanation for, the observed patchy spatial distribution of many helminths. Comparison with other parts of the world indicates that colonization is a major determinant of helminth community structure.

Unusual finding of encapsulated nematode larvae (Spiruroidea) in Bartramia longicauda and Numenius americanus (Charadriiformes) in western Canada.

Third-stage spiruroid larvae were found encapsulated on the serosa of the small and large intestines and in the mesentery of one of 15 adult upland sandpipers (Bartramia longicauda) from Manitoba, Canada, and three of 18 adult long-billed curlews (Numenius americanus) from Alberta, Canada. The larvae resemble third-stage larvae of Physocephalus sexalatus and birds may serve as a paratenic host of this unidentified spiruroid species.