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Placozoa: An Evolutionary Leftover? 

Phylum Placozoa gets only brief mention in Darwin’s Doubt. That’s understandable, because it has no fossil record, and has almost no resemblance to any other animal phylum. Latin taxonomic names often lose their pomp when translated, and this is a classic: placozoa means “flat animal.” Meyer only includes it in Figure 2.5 (page 32) as one of nine phyla without a fossil record. What is it? Here are some clues to consider:

  • It is small, just about a millimeter in size.
  • It is the only phylum with just one recognized species, Trichoplax adhaerens.
  • Like an amoeba, it has no well-defined shape, but is flat.
  • Unlike an amoeba, it is multicellular.
  • It has no organs or systems.
  • It has only about six cell types. The outer ones have cilia that enable it to move.
  • It lives on the ocean floor, consuming detritus.
  • Individuals show a surprising amount of genetic diversity but little phenotypic diversity.
  • There are suggestions of sexual reproduction, but no demonstration of it in the wild.
  • It has worldwide distribution.

Evolution News asked a few years ago, “The ‘Flat Animal’: Is It a Cambrian Ancestor?” Answer: It has more complexity under the hood than appearance suggests, but is not a Cambrian ancestor. In fact, it may be a stripped-down organism, although it is free living (not parasitic). Now a new paper in PLOS Biology is out, devoted to Placozoa. Has anything new been learned in the meantime?

Hong Kong Sea Dragon

The main finding by Michael Eitel et al. is a possible new species found in a mangrove-lined river near Hong Kong. They identify it as H13, and give it the name “Hoilungia hongkongensis” meaning “Hong Kong sea dragon.” (That’s an improvement on “flat animal.”) Under a microscope, though, it would be nearly impossible to see any difference from Trichoplax.

Placozoans are a phylum of tiny (approximately 1 mm) marine animals that are found worldwide in temperate and tropical waters. They are characterized by morphological simplicity, with only a handful of cell types, no neurons, no tissue organization, and even no axial polarity. Since the original description of Trichoplax adhaerens 135 years ago, no additional accepted species has been established, leaving the Placozoa as the only animal phylum with only a single formally described species. While classical morphological species identification has failed to reveal further species, single-gene DNA sequence analyses have identified a broad and deep genetic diversity within the Placozoa. To address the significance of this deep genetic diversity in this morphologically uniform phylum, and to better understand its consequences for speciation processes, general biology, and species delimitation in the Placozoa, we sequenced the genome of the placozoan isolate “H13,” a lineage distantly genetically related to T. adhaerens. Our multilevel genomic comparisons with the T. adhaerens genome show considerable differences in the general structure of the genome and the makeup and history of various gene families of biological relevance to habitat adaptation. Based on comparative genomics, we here describe the second placozoan species and show that it belongs to a new genus.

Does the new species show any sign of evolution? Out of thousands of compared sequences, very few showed any possible signs of positive evolution:

Only 3 of the 6,554 one-to-one orthologs had dN/dS ratios slightly >1, indicating positive selection… One of these seems placozoan specific, since it could not be annotated because of missing UniProt BLAST hits and InterPro domains, respectively. For the second, GO annotation and InterPro IDs indicate a role in telomere maintenance. The third positively selected gene (CYP11A1) is putatively a cholesterol side-chain cleavage enzyme acting in the mitochondrion.

This does not mean that the Hong Kong placozoans evolved anything new. It just indicates fine-tuning of existing functions for the particular habitat. If this phylum has been around since the Ediacaran (as some believe), that’s a depressingly low amount of progress. There’s been no dramatic diversification within the body plan, as seen within arthropods and chordates. Evolutionists have a name for this: bradytely, a low rate of evolution. Evolution is fast, in other words, except when it is slow. Are they saying that the genetic diversity in placozoans never produced any change in phenotype? These poor creatures never got eyes or wings in 600 million years? They didn’t even get the new-fangled ion channels.

Ion Channels Appear Earlier

An unrelated paper now claims that ion channels appeared much earlier than the Cambrian explosion. Lynagh et al., writing in PNAS, say that “Acid-sensing ion channels emerged over 600 Mya and are conserved throughout the deuterostomes.” We are deuterostomes; we have two (deutero-) holes (-stomes) in our digestive tract. (Enough said.) Previously, it was thought only vertebrates had these particular types of ion channels, which are essential for nervous systems. By discovering and comparing acid-sensing ion channels (ASICs) in various phyla, they conclude that the channels had to appear long before the phyla themselves — in the Ediacaran period.

The conversion of extracellular chemical signals into electrical current across the cell membrane is a defining characteristic of the nervous system. This is mediated by proteins, such as acid-sensing ion channels (ASICs), membrane-bound receptors whose activation by decreased extracellular pH opens an intrinsic membrane-spanning sodium channel. Curiously, ASICs had only been reported in vertebrates, despite the homology of many other ion channels in vertebrates and invertebrates. Using molecular phylogenetics and electrophysiological recordings, we discover ASICs from tunicates, lancelets, sea urchins, starfish, and acorn worms. This shows that ASICs evolved much earlier than previously thought and suggests that their role in the nervous system is conserved across numerous animal phyla.

If they “evolved,” how did that happen? They “evolved or were already present in ancestral deuterostomes,” the authors answer. They “emerged.” 

In any case, the presence of ASICs in ancestral deuterostomes means that ASICs emerged at least 600 Mya, well before the Cambrian explosion. Tentative phylogenetic evidence suggests that ASICs evolved after the protostome/deuterostome split.


This work shifts the estimate of the origin of ASICs from after to before the Cambrian explosion, and it shows that the loss of proton sensitivity in ASIC4 occurred after vertebrates moved onto land. The occurrence of ASICs in invertebrates points toward additional homology between invertebrate and vertebrate nervous systems regarding excitatory neurotransmission.

They “emerged,” and they “evolved much earlier than previously thought,” we learn. Then they were “conserved” forever after. 

Simple Leaps?

Did they identify some mutation that caused the emergence? No; they only mention ones that were detrimental, neutral, or altered the sensitivity of some channels to protons. Surely these evolutionists found some positive natural selection, didn’t they? No; if they did, they never mention it. The channels just “emerged.” Nervous systems “emerged” once the channels emerged. Lest we think these were simple leaps, listen to their description:

Neurons are set apart from other cells by their rapid conduction and exchange of information. This is in large part due to ligand-gated ion channels, membrane-bound receptors that convert chemical information, such as neurotransmitter release, into electrical current. Not surprisingly, the evolution of nervous systems is intricately linked to the emergence and expansion of ligand-gated ion channels in the genome. For example, certain neurotransmitter receptor families seem to have expanded uniquely in those basal animals with nervous systems, and these include the glutamate receptors typically associated with fast synaptic transmission and plasticity.

Ion channels are so important and sophisticated, two scientists won the Nobel Prize in 2003 for elucidating their modes of action and the elegant mechanisms of their selectivity filters.

Emergence and conservation sound more like design was responsible, not evolution. But these evolutionists never deal with another curious ramification of their conclusion: if these channels emerged 600 million years ago, and the Cambrian explosion occurred about 540 million years ago, why did these complex channels “evolve” 60 million years before an animal “emerged” that could use them? And why did the lowly placozoans miss out? 

Evolution works in mysterious ways.

Photo: Hoilungia hongkongensis, by Hans-Jürgen Osigus, Stiftung Tierärztliche Hochschule Hannover, via EurekAlert!