Category: Dicyemids (Mesozoans)
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Dicyemids are millimetric, worm-like ('vermiform') animals that are endoparasitic, typically inhabiting the renal organs of cephalopods such as octopus and cuttlefish. Their bodies are made up of an elongated inner axial cell surrounded by a layer of 8-40 ciliated 'somatoderm' cells, with the cells of the anterior region modified to form a head or 'calotte' for attachment to host tissue. Their specialised body shape and unusual two-phase life cycle both have remarkable parallels with the chromidinid ciliates, vermiform protistans that also inhabit cephalopod renal organs (more usually pelagic species, rather than benthic ones as in dicyemids). Although the exact affinity of dicyemids is unclear, most evidence points to them being higher metazoans, belonging in a group with platyhelminthes, molluscs and annelids. This places them at vast evolutionary distance from Chromidina, which belongs with other ciliates as part of a larger group of protists called Chromalveolata and therefore their shared adaptations represent a stunning example of convergence.
Dicyemids reproduce asexually within their host, but have a sexual phase resulting in a free-living larva that leaves the host's body to seek a new cephalopod for infection. In the asexual phase of the life cycle, the polyploid axial cell produces diploid 'axoblasts' that develop into new vermiform individuals. The sexual phase is triggered once population density in the host organ exceeds a certain threshold, and involves some vermiform individuals producing gonads and haploid gametes, which upon fertilisation develop into free-living 'infusorifom' embryos. Chromidina also has a biphasic life-cycle, budding either asexual 'aptomites' or sexual 'tomites' that conjugate and leave the host to seek out the appendages or setae of their primary hosts, crustaceans (which are consumed by cephalopods, completing the life cycle).
The dicyemid head region or calotte varies in shape between species, each adapted for attachment to a different region of the renal organs, thus allowing niche segregation when more than one dicyemid species is present in the same host. Conical calottes attach deep within the infoldings of the renal epithelium whereas discoidal and cap-shaped calottes attach most effectively to distinct regions of the smooth outer epithelium. Chromidina species have apparently converged on a similar mechanism for preventing direct competition for resources between species in the same host, as C. elegans has a conical attachment region whereas C. coronata occupies a different area and has a disc-shaped 'head'. The incredibly similar ecologies of dicyemids and Chromidina is emphasised by the observation that Chromidina typically parasitizes pelagic cephalopods, while dicyemids prefer to infect benthic species, reducing the likelihood of direct encounter and competition between the unrelated but equally adapted organisms.