B & B Chapter 7 pp 209-217.
Phyla of Uncertain Affinity
• P: Placozoa
• Mesozoa
– P: Rhombozoa
– P: Orthonectida
Phylum: Placozoa
• Trichoplax adhaerens was discovered in
1883 in a seawater aquarium at the Graz
Zoological Institute in Austria.
A Quote
• “Placozoa are perplexing beasts to say the least.
Denizens of marine environments, these tiny, extremely
simple animals have no fossil record and nothing is
known about how they interact with the other
organisms of the marine realm since they have never
been observed in their natural habitat.
• That sounds surprising, but really, they are so small
and delicate that they are easy to miss. They’ve also
bamboozled generations of zoologists who have tried to
figure out how they are related to other animals.“
Form and Function
• Size range 1-3 mm across, but only 0.025
mm thick.
• Superficially, Placozoa look like the
single-celled organisms.
• They are an extremely thin gelatinous
disc composed of around 1000-3000 cells
of four distinct types.
Form and Function
• (1) cilia-bearing cell
that forms the upper
surface of their body
• and (2) cilia-bearing
cells and (3) gland
cells on the lower
surface.
• Sandwiched between
the upper and lower
surface are (4) fibre
cells that form a
central, connective
layer in these animals.
Revised Body Plan
How Trichoplax feeds.
Reproduction
• Two types of reproduction
– Asexual
– Sexual
Asexual Reproduction
• Vegetative reproduction
by binary fission.
• One larger individual
divides into two or more
smaller individuals.
Sexual Reproduction
• Placozoans reproduce sexually, i.e. by producing female and
male gametes.
Sexual Reproduction
• Placozoans reproduce sexually, i.e. by producing female and
male gametes.
• However, sperm cells have not been unequivocally identified
yet.
Sexual Reproduction
• Placozoans reproduce sexually, i.e. by producing female and
male gametes.
• However, sperm cells have not been unequivocally identified
yet.
• Oocytes appear in small numbers in cultured placozoans
when triggered by environmental factors.
Sexual Reproduction
• Placozoans reproduce sexually, i.e. by producing female and
male gametes.
• However, sperm cells have not been unequivocally identified
yet.
• Oocytes appear in small numbers in cultured placozoans
when triggered by environmental factors.
• Beyond early cleavage stages up to 128 cells, no embryonic
development has been observed.
Sexual Reproduction
• Placozoans reproduce sexually, i.e. by producing female and
male gametes.
• However, sperm cells have not been unequivocally identified
yet.
• Oocytes appear in small numbers in cultured placozoans
when triggered by environmental factors.
• Beyond early cleavage stages up to 128 cells, no embryonic
development has been observed.
• Field specimens have never shown signs of sexual
reproduction, but genetic evidence suggests the presence of
recent events of sexual reproduction in nature.
Sexual Reproduction
Developing Oocyte
Fertilized Egg
Developing Embryo Membrane and Sperm (Sc)
Diversity and Relationship to
Other Metazoans
• Historically placozoans have been placed at the base of the
metazoan Tree of Life because of their simple morphology.
Diversity and Relationship to
Other Metazoans
• Historically placozoans have been placed at the base of the
metazoan Tree of Life because of their simple morphology.
• From a morphological point of view their bauplan is by far
the closest surrogate of the hypothetical first metazoan.
Diversity and Relationship to
Other Metazoans
• Historically placozoans have been placed at the base of the
metazoan Tree of Life because of their simple morphology.
• From a morphological point of view their bauplan is by far
the closest surrogate of the hypothetical first metazoan.
• Early molecular phylogenetic studies based on small and
large ribosomal subunits (18S and 28S) have placed
Trichoplax at all different positions in the metazoan Tree of
Life.
Diversity and Relationship to
Other Metazoans
• Historically placozoans have been placed at the base of the
metazoan Tree of Life because of their simple morphology.
• From a morphological point of view their bauplan is by far
the closest surrogate of the hypothetical first metazoan.
• Early molecular phylogenetic studies based on small and
large ribosomal subunits (18S and 28S) have placed
Trichoplax at all different positions in the metazoan Tree of
Life.
• Recent studies with very large datasets also failed to resolve
the phylogenetic position. Nonetheless, a very basal position
seems to become corroborated.
Relationship to Other Metazoans
Phylogeny based
on total evidence
Diversity
• Historically Trichoplax adhaerens was the
only species known in this phylum.
• However, more recently genetic and
morphological data suggest that there are
multiple species within this phylum.
Diversity
Phylogeny based on the 16S large
mitochondrial ribosomal RNA
Distinct morphological feature
the so-called ‘concave disc’
Phylum Rhombozoa (VAN
BENEDEN 1876)
• Rhombozoa (rom-bo-ZO-a) is made of
two Greek roots that mean a rhomboid
animal [rhomboid –rhomboeides and
animal -zoo
Phylum Rhombozoa AKA
Mesozoa
• The Mesozoa are tiny, ciliated animals
that parasitize marine invertebrates.
• Their affinities with other phyla are
obscure chiefly because of the simplicity
of their structure and their unusual
biology.
Characteristics
• A mesozoan’s body is made up of two layers of cells but
these are not homologous with the endoderm and
ectoderm of diploblastic animals.
• Twenty to thirty somatic cells enclosing a long,
cylindrical axial cell with one to several hundred
axoblasts and/or embryos.
• However, because they are parasites of cephalopod
kidneys, their structural simplicity likely is a
consequence of their parasitic lifestyle.
Dycemids in the kidneys of an
Octopus
Morphology of Dicyemid Stages
from the Octopus Kidney
Somatic
Cells
Life Cycle of Dicyemida
• Asexual and Sexual Reproduction
Asex and
autoinfection
Sexual
reproduction
and free living
stage
Recent Phylogenetic Hypothesis
Phylum Orthonectida
• Orthonectozoa (OR-tho-nek-to-ZO-a) is
made of two Greek roots meaning
"straight swimming" [straight -ortho;
swimming -nekto]; -ida is a standard new
Latin ending for a higher taxon.
Phylum Orthonectida
• A small (20 spp.) phylum of poorly researched
parasites of marine invertebrates.
• Orthonectids are microscopic animals that
consist of a single layer of ciliated cells that
surround a mass of sex cells.
• They swim freely inside their
flatworm/polychaete/echinoderm or bivalve
hosts and have two separate sexes.
Phylum Orthonectida
• Once they are ready to reproduce they leave their host.
• The male will penetrate directly into the female’s body.
• The zygote will develop into a ciliated larva that will
escape from its mother to seek out a new host.
• Once it finds a host it will lose its cilia and develops into
a plasmodium larva. Which will break up into
numerous cells that will become the next generation.
Life Cycle
Plasmodia
Sexual Adults from
Plasmodia Cells
Adult Male and
Female free living
Big Picture
• These are very problematic taxa, not only
because they are poorly known, but also
because they are so simple in form.
• However, at the same time they are very
exciting in the fact that they challenge
our understanding of metazoan
relationships.