Abstract
Dicyemids are enigmatic parasites found within the excretory systems of benthic cephalopods. The phylogenetic position and overall diversity of dicyemids remains poorly understood, in part because current species delimitation criteria are based solely on morphological traits. Understanding the diversity of parasite species is particularly problematic because they tend to be devoid of consistent (informative) morphological traits while simultaneously rich in morphological variation associated with developmental stages and environmental conditions. In this study, we tested the boundaries of currently described morphospecies of dicyemids using molecular phylogenetic data. Variation within sequences of the small subunit (18S) rRNA gene was explored because this marker (1) is known to be fast-evolving in parasitic eukaryotes, (2) is one of the few molecular markers to have been previously sequenced in some dicyemids, and (3) has been used successfully as a DNA barcode in other groups of parasites. Three species of cephalopods were collected, each hosting several different morphospecies of dicyemid parasites. Thirty-four individual dicyemids encompassing eight different morphospecies were isolated and their 18S rDNA sequenced. Molecular phylogenetic analyses of these data were incongruent with current morphology-based species descriptions. The 18S rDNA sequences suggest that each host species of cephalopod harbors one species of dicyemid encompassing a great deal of morphological variation. The addition of DNA sequences to understanding dicyemid diversity clarifies species boundaries in a lineage that is difficult to define in nearly every aspect.
Published Version
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