As we move along in our investigation of Spiralia, we arrive at two interesting phyla, Orthonectida
and Dicyemida. Animals in these phyla are invertebrate parasites in the sense that they
are invertebrates themselves, and they parasitize other invertebrates. Animals in these phyla are
also referred to as mesozoans. The name Mesozoa comes from the Greek for mesos, or middle, and
zōion, or animal. They were originally considered to be a missing link between the unicellular
eukaryotes and all other animals, and as such they were considered to be part protozoa and
part metazoa. The mesozoans were once proposed as a subkingdom closely related to the animals, but
not quite animals. In the all-animal phylogeny, they are considered to be part of spiralia,
but their relationship to all other spiralian phyla is unclear. Hence, they form a polytomy.
Currently, all members of Orthonectida and Dicyemida are considered to be highly
degenerate metazoans, meaning they are animals that have lost one or more of the characteristics
shared by all other animals. However, genomic analyses of dicyemids have revealed that they
contain spiralian peptides, and orthonectids were found to contain HOX genes, which are
found only in animals and no other organisms. The Dicyemids, also known as rhombozoans,
live exclusively within the excretory systems, more specifically the renal sac, of benthic
cephalopods, that is, organisms such as the common octopus that are found living only on the
sea floor. These organisms have both a sexual and asexual mode of reproduction. But unlike all other
animals, they lack tissues, organs, symmetry, gastrulation, and even an extracellular matrix,
which is a three-dimensional network of macromolecules, a characteristic shared by nearly
all multicellular organisms. They do, however, demonstrate spiral development, a characteristic
found only in the spiralians. In short, it’s not hard to see why they’re such a mystery.
Adults in this phylum, sometimes called vermiforms, or nematogens, are long and worm-like.
They are composed of a singular axial cell surrounded by just a handful of somatic cells.
Some individuals can grow to be up to seven millimeters in length,
though most are a fraction of a millimeter. Within their bodies, reproductive cells develop
into vermiform embryos which can exit the parent into the cephalopod renal duct, and then develop
into adults. Vermiforms will reproduce asexually in conditions of low crowding, but if the area
becomes too crowded the reproductive cells of some adults develop into hermaphroditic
gonads that produce fertilized eggs. These eggs develop into an infusoriform embryo that in turn
develops into an infusoriform larva. These larvae contain more cells and are more complex
in body organization than adults. They have been documented being expelled as waste from the hosts,
but their method of development into adults and their colonizing of a new host is still unknown.
Orthonectids spend most of their lives as trophic plasmodium within the tissues of
their host. In this stage they are little more than a multinucleated mass of naked protoplasm.
They essentially resemble slime molds, but instead of living on the forest floor, they live within
the tissues of their host. In this diagram we can see the host tissues as well as the blob-like mass
that is the orthonectid. The nuclei are the black dots. Within the body of the organism, a few cells
develop into worm-like male and female adults that leave the host and swim free searching for mates.
These short-lived adults lack digestive, circulatory, respiratory, and excretory systems.
They are little more than gametes and a reduced nervous system surrounded by contractile muscle
cells and a layer of cilia. If a one millimeter long female meets a much smaller dwarf male of
the same species, they reproduce by uniting their genital pores. The fertilized eggs develop into
ciliated larvae that exit the female and search for a host. If a larva makes it into a suitable
host’s tissue, it disintegrates and scatters its cells within the host’s tissue in order to become
new infective cells known as agametes that develop into plasmodium and restart their life cycle.
So, the orthonectids, like the dicyemids break almost all the rules of Kingdom
Animalia. Their unique life cycle, cryptic nature, and morphology have both fascinated
and confounded researchers since their discovery. There is still much to learn about them,
but we have many other phyla to cover, so let’s continue onwards in our investigation of Spiralia.