Abstract
BackgroundFunctional inference on the attachment of acanthocephalans has generally been drawn directly from morphology. However, performance of structures is often non-intuitive and context-dependent, thus performance analysis should be included whenever possible to improve functional interpretation. In acanthocephalans, performance analysis of attachment is available only for Acanthocephalus ranae, a species that solely relies on the proboscis to attach. Here we compare body morphology and muscle arrangement in 13 species of Corynosoma, which use their spiny body as a fundamental holdfast. A basic performance analysis using live cystacanths of two representative species is also provided.MethodsAdults of 13 Corynosoma spp. were obtained from 11 marine mammal species. Specimens were cut and carefully cleaned to examine muscle arrangement through light and scanning electron microscopy. Live cystacanths of C. australe and C. cetaceum were selected for performance analysis. Video records of evagination-invagination cycles of the proboscis were obtained and analysed with a video editor.ResultsThe basic arrangement of proboscis retractors, trunk circular and longitudinal muscles, neck retractors and receptacle retractors, was conserved in all Corynosoma species. Interspecific variability was found in the relative development of disk muscles: minimum in C. enhydri, maximum in C. cetaceum; the distal insertion of the ventral neck retractor: ventro-lateral in C. cetaceum, C. hamannni and C. pseudohamanni and ventral in the other species; and the distal insertion of the receptacle retractors: more proximal in species with a longer hindtrunk. Performance analysis indicated striking similarities to that described for A. ranae except that (i) the foretrunk bends ventrally during the evagination-invagination cycles of the proboscis; (ii) disk muscles can flatten the tip of the foretrunk regardless of these cycles; and (iii) the receptacle bends ventrally and is driven to the hindtrunk by coordinated action of receptacle retractors.ConclusionsSpecies of Corynosoma are able to use up to six holfast mechanisms. Attachment relies on a similar performance to that described for A. ranae. However, structural ventral bending of an inflated, spiny foretrunk, with a parallel re-arrangement of foretrunk muscles, have generated unexpected novel functions that make attachment extremely effective in species of Corynosoma. Interspecific variability in trunk shape and muscle arrangement grossly correlates with the rheological conditions each species experiences in their microhabitats within the gut of marine mammals.
Highlights
Functional inference on the attachment of acanthocephalans has generally been drawn directly from morphology
Acanthocephalus ranae is the only acanthocephalan species for which the attachment mechanism was hitherto described based on detailed observations of muscle arrangement and performance of live worms
The new evidence obtained in this study for 13 Corynosoma spp. indicates that their pipe-shaped body, which results from the ventral bending of an inflated foretrunk, brings about significant re-arrangements of foretrunk muscles
Summary
Functional inference on the attachment of acanthocephalans has generally been drawn directly from morphology. Hammond [9,10,11] used live detached specimens of Acanthocephalus ranae (Schrank, 1788) to describe cycles of evagination-invagination of the proboscis as well as the mechanisms that worms use to anchor to the intestinal wall of toads (see Additional file 1: Data S1 and Additional file 2: Figure S1 for a brief description of the morphology, performance and attachment function in A. ranae). This approach allowed this author to unveil details of the attachment function that could have been overlooked from examination of morphology alone
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