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.

Sexual Adults from Plasmodia Cells Adult Male and Female free living 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.