So far in our investigation of Spiralia we have covered the phyla Platyhelminthes
and Gastrotricha. Let’s now move on to their close relatives in phylum Cycliophora. The
fascinating creatures which inhabit this clade were first discovered only very recently in 1995,
by Dr. Peter Funch, who has graciously assisted in the writing of this tutorial,
as well as Dr. Reinhardt Møbjerg Kristensen, whose assistance we will enjoy when we cover the
phylum Micrognathozoa a bit later in the series. Now, we’ve covered some strange animals so far,
but the microscopic sac-like cycliophorans, also known as Symbions or “ring-bearers”
just might be the strangest. They, like the flatworms and Gastrotrichs, are bilaterally
symmetrical, meaning they can be split into left and right halves which are rough mirror images.
They are triploblastic, meaning they have three unique germ cell layers. They are unsegmented,
meaning they do not demonstrate metamerism. They are generally noncolonial, meaning they
do not form interconnected colonies, though their unique feeding stage with a developing
buccal funnel inside can be regarded as two individuals. And they are acoelomates,
meaning they do not have a body cavity, and lack respiratory organs. The fluid inside
their brood chambers is circulated by cilia. They comprise a phylum of incredibly tiny animals
that measure less than a half a millimeter wide at their largest, and live their lives attached
to the mouthparts of cold-water lobsters in the North Atlantic Ocean and the Mediterranean Sea.
They are considered to have a commensalist relationship with their lobster hosts
since they feed by driving uneaten food particles from the lobster into their
digestive systems though ciliary action. They do not appear to harm their host lobster in any way.
Cycliophorans are deeply connected to their host. Their unique life cycle is due, in part,
to the fact that they live primarily upon the cuticle attached to the setae,
or the bristle-like extensions, of an animal that continually molts. They have six basic
life stages: the asexual feeding stage, the sexual male stage, the sexual female stage,
the pandora stage, the Prometheus larva, and the chordoid larva.
Let’s follow the life journey of a single cycliophoran starting with a settled larva.
This larva will develop into a feeding individual with an inner bud. At this point a few different
things can happen. First, let’s follow the asexual stage. A small organism, known as a pandora larva,
can escape from the feeding individual and colonize the same lobster host.
These larvae are produced in the intermolt period of the host. In this way, the pandora larva can
colonize the same lobster. This larva will then settle and develop back into a feeding stage.
A feeding stage individual can potentially develop to hold a male larva released as a
Prometheus larva that settles onto another feeding individual. The settled juvenile
male will then further develop one to three sexually mature males inside its own body,
each of which is only about one third of its size, meaning the sexually mature males are
smaller than the juvenile males and are one of the smallest known animals in the world. Meanwhile,
other feeding stages develop to hold developing females. Each female contains a single oocyte.
Once fertilized, the pregnant female escapes and settles into a sheltered area of the same host.
It then encysts and produces a single motile chordoid larva that hatches from the cyst and
settles onto a new host, where it develops into a feeding stage and initiates its asexual cycle.
Due to their unique life cycle and cryptic nature, the exact phylogeny of the cycliophorans
has been debated for quite some time. Proposed homologies have placed the cycliophorans as
cousins to Rotifera, kin to Bryozoans, and closely related to Entoprocta. Though they have been
accepted as members of Spiralia, it is possible their precise placement may change in the future.
Regardless of where they end up, their relatively recent discovery and their abundance on species
of lobster widely consumed by humans means there is likely much more to learn about them,
and potentially many other species of cycliophorans waiting to be discovered.
That covers the symbions, so let’s move forward and look at some other phyla.