If I didn’t study Volvox, I would probably study placozoa. Placozoa are animals, but you wouldn’t know it to look at them. They look and behave very much like giant amoebae, big enough to be visible to the naked eye.
Isn’t it gorgeous? The placozoa seem ready-made for studying the evolution of multicellularity in animals: they have a simple body plan with just a few differentiated cell types (sort of like Volvox, come to think of it). They are also vastly understudied. There’s just one described species, even though we know they are genetically diverse:
We’re pretty sure they have sex, but no one has successfully bred them. Usually they reproduce asexually, either by binary fission or budding. They have a worldwide distribution in tropical and temperate latitudes:
Bernd Schierwater’s World Placozoa Database has a nice summary of what we know about placozoa. Professor Scheirwater is one of my scientific heroes for getting a paper through peer review with the title “My favorite animal, Trichoplax adhaerens.” I do wish he’d quit calling them ‘basal’, though. Extant taxa cannot be basal. The database website includes this gem:
This is the smallest of all animal databases yet; it contains a single nominal species, Trichoplax adhaerens.
Well, it’s about to get bigger. Michael Eitel and colleagues have posted a preprint to bioRxiv describing not only a new species, but a new genus of placozoa: “A taxogenomics approach uncovers a new genus in the phylum Placozoa.” The new taxon was called Hoilungia hongkongensis in the original preprint, but this was withdrawn in the revised version because,
According to the International Code of Zoological Nomenclature preprint publication of taxonomic names is discouraged. Consequently, the Xxxxxxxxx yyyyyyyyyyyyy / X. yyyyyyyyyyyyy given here is a dummy only. The valid name will be available upon formal journal publication.
Whole genome comparisons between Trichoplax adhaerens and the new taxon showed large-scale genomic rearrangements, comparable to those between mice and humans:
Genetic distances between Trichoplax adhaerens and the new taxon, based on homologous proteins, are comparable to those between genera in other animal groups:
The new taxon corresponds to H13 in Figure 4 above, and the sequenced haplotype of Trichoplax adhaerens to H1. As the authors point out, these are two of the most distantly related haplotypes. Still, the depth of the divergences within ‘Group A’, at least based on 16S sequences, suggests that there are likely to be several other placozoan taxa yet to be described.
Just as in the volvocine algae, placozoan taxonomy has until recently been based on morphology. And just as in the volvocine algae, morphology is clearly insufficient to parse deep divergences within this group, though for different reasons. With cheap whole genome sequencing, I expect we’ll see more species, and maybe even higher taxa, described in the next few years.
There is an immense amount we don’t know about the placozoa: what are their relationships to other animals, how do they have sex, what is their basic ecology…The landscape of placozoan biology is largely unexplored, and there are likely to be wonders as yet undiscovered.
Eitel M, Francis WR, Osigus H-J, Krebs S, Vargas S, Blum H, Williams GA, Schierwater B, Wörheide G. 2017. A taxogenomics approach uncovers a new genus in the phylum Placozoa. bioRxiv , 0–47. doi: 10.1101/202119
Eitel M, Osigus H-J, Desalle R, Schierwater B. 2013. Global Diversity of the Placozoa. PLoS One 8, 1–12. doi: 10.1371/journal.pone.0057131
Schierwater B. 2005. My favorite animal, Trichoplax adhaerens. BioEssays 27, 1294–1302. doi: 10.1002/bies.20320