Phylogeny of Myzostomida (Annelida) and their relationships with echinoderm hosts.

Mindi M Summers,Greg W Rouse

doi:10.1186/s12862-014-0170-7

Abstract

BackgroundMyzostomids are marine annelids, nearly all of which live symbiotically on or inside echinoderms, chiefly crinoids, and to a lesser extent asteroids and ophiuroids. These symbionts possess a variety of adult body plans and lifestyles. Most described species live freely on the exterior of their hosts as adults (though starting life on the host inside cysts), while other taxa permanently reside in galls, cysts, or within the host’s mouth, digestive system, coelom, or gonads. Myzostomid lifestyles range from stealing incoming food from the host’s food grooves to consuming the host’s tissue directly. Previous molecular studies of myzostomids have had limited sampling with respect to assessing the evolutionary relationships within the group; therefore molecular data from 75 myzostomid taxa were analyzed using maximum likelihood and maximum parsimony methods. To compare relationships of myzostomids with their hosts, a phylogeny was inferred for 53 hosts and a tanglegram constructed with 88 associations.ResultsGall- and some cyst-dwellers were recovered as a clade, while cyst-to-free-living forms were found as a grade including two clades of internal host-eaters (one infecting crinoids and the other asteroids and ophiuroids), mouth/digestive system inhabitants, and other cyst-dwellers. Clades of myzostomids were recovered that associated with asteroids, ophiuroids, and stalked or feather star crinoids. Co-phylogenetic analyses rejected a null-hypothesis of random associations at the global level, but not for individual associations. Event-based analyses relied most upon host-switching and duplication events to reconcile the association history.ConclusionHypotheses were revised concerning the systematics and evolution of Myzostomida, as well their relationships to their hosts. We found two or three transitions between food-stealing and host-eating. Taxa that dwell within the mouth or digestive system and some cyst forms are arguably derived from cyst-to-free-living ancestors – possibly the result of a free-living form moving to the mouth and paedomorphic retention of the juvenile cyst. Phylogenetic conservatism in host use was observed among related myzostomid taxa. This finding suggests that myzostomids (which have a free-living planktonic stage) are limited to one or a few closely related hosts, despite most hosts co-occurring on the same reefs, many within physical contact of each other.Electronic supplementary materialThe online version of this article (doi:10.1186/s12862-014-0170-7) contains supplementary material, which is available to authorized users.

Highlights

Myzostomids are marine annelids, most of which live symbiotically on or inside echinoderms, crinoids, and to a lesser extent asteroids and ophiuroids
The maximum parsimony (MP) and maximum likelihood (ML) results for each gene partition are summarized in Additional file 1: Table S2
Asteromyzostomum Jägersten, 1940 was found with high support as sister to Protomyzostomum Fedetov, 1912 and this clade was sister to the remaining myzostomids found on crinoids

Summary

Introduction

Myzostomids are marine annelids, most of which live symbiotically on or inside echinoderms, crinoids, and to a lesser extent asteroids and ophiuroids These symbionts possess a variety of adult body plans and lifestyles. Myzostomida Graff, 1877 is a clade of marine annelid worms with around 150 described species Most spend their adult lives living on or inside crinoid echinoderms; about a dozen species are associated with ophiuroids and asteroids; two occur on black coral (Antipatharia) [1,2]; and one has been recorded from a sponge [3]. Myzostomids possess a variety of body plans and lifestyles in which they steal food from or directly consume the host (Figure 1) Those that live on the surface of the animal are mainly disk-shaped or elongated; they use their chaetae to hold onto the host while inserting their proboscis into the host’s food groove to steal food [4,6]. Those living on the outside of the animal and within galls, cysts, the mouth, and digestive track are presumed to be stealing the host’s food, while those within the coelom and gonads are believed to be eating the host directly [4,6]

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