Abstract
The jaw apparatus in several annelid families represents a powerful tool for systematic approaches and evolutionary investigations. Nevertheless, for several taxa, this character complex has scarcely been investigated, and complete comparative analyses of all annelid jaws are lacking. In our comprehensive study, we described the fine structure of the jaw apparatus and the ventral pharyngeal organ (VPO) in Histriobdella homari – a minute ectocommensal of lobsters putatively belonging to the Eunicida – using different comparative morphological approaches, including SEM, TEM, CLSM and subsequent 3D reconstruction. The H. homari jaw apparatus is composed of ventral paired mandibles and dorsal symmetrical maxillae consisting of numerous dental plates, ventral carriers and an unpaired dorsal rod, and the general assemblage and arrangement of the different parts are highly comparable to those of other eunicid families. The jaw ultrastructure of histriobdellids resembles that of the families Dorvilleidae and (juvenile) Onuphidae. Furthermore, our data reveal that in the process of development of the jaw apparatus, the mandibles, maxillae II and unpaired dorsal rod are formed first, and the remaining maxillae and ventral carriers appear later. Notably, the muscular apparatus differs from that in Dorvilleidae and Onuphidae in terms of the number and arrangement of muscle fibers encompassing the jaws – not only because of the very small size of Histriobdella but also because histriobdellid maxillary protraction occurs due to straightening of the dorsal rod and thus requires a different muscular scaffold. Based on our investigations, the arrangement of the muscular apparatus of the jaws, the presence of paired ventral carriers and the dorsal rod, and the morphology of the ventral pharyngeal organ represent a histriobdellid autapomorphy. Our datasets form a basis for further comparative analyses to elucidate the evolution of Eunicida and jaw-bearing Annelida.
Highlights
Annelids, one of the largest groups of lophotrochozoan animals [1], exhibit a wide variety of lifestyles and body forms [1, 2]
Using a comparative set of morphological methods, we investigated the fine morphology and ultrastructure of the ventral pharyngeal organ and jaw apparatus in Histriobdella homari to place the histriobdellid jaw apparatus within the accepted classification of eunicidan jaws
Our comparative and comprehensive investigations using a set of different morphological methods highly support the phylogenetic placement of Histriobdellidae within the Eunicida
Summary
One of the largest groups of lophotrochozoan animals [1], exhibit a wide variety of lifestyles and body forms [1, 2]. One reason might be that the correct reconstruction of fossilized complete jaw apparatuses requires detailed knowledge of the morphology, diversity, fine structure and organogenesis of extant annelid jaws, which are still very incomplete for the majority of jawbearing taxa. The maxillary apparatuses in extant Eunicida can be of four types: ctenognath (comb-like jaws), prionognath (saw-like jaws), symmetrognath (symmetrical jaws), and eulabidognath (pincer-like jaws) (see Table 1 for further details). Even for those taxa, comprehensive data on ultrastructure, jaw formation and replacement are still very scarce [12, 18]. The jaws of the minute Charlie Chaplin worms – Histriobdellidae Claus & Moquin-Tandon, 1884 – are among the poorly understood and understudied examples
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