Tanadrin

Are you the sort who loves mysteries? Well why not go down the rabbit hole and curl up with one of the most fascinating biological mysteries of our time!

Facetotecta is an extremely mysterious subclass of thecostracan crustaceans, and is so mysterious in fact that we have NEVER found an adult specimen of these organisms! Though the larval forms of facetotecta are found fairly regularly, we have no idea what they grow into, and pretty much no clue as to why we don’t know. Though in 2008 we did discover a potentially significant clue.

When larval facetotecta were inoculated with crustacean molting hormone, they successfully molted into what is presumed to be the next natural life phase. These animals were limbless, unsegmented, organisms that appeared outwardly slug-like. Based on this experiment, some biologists theorize that adult facetotecta are endoparasites that make their home within the body of another animal, which might explain why they’re so damn hard to find.

Crustacean y-larvae (subclass Facetotecta) were first described from marine plankton in the late 1800s, and they have since been recorded from the arctic to the tropical waters of all oceans. The adult organisms have never been identified, and the Facetotecta is the only crustacean group with a formal taxonomy based solely on larval stages.

Ontogenetic, morphological and molecular evidence all point to a taxonomic relationship with the crustacean class Thecostraca. These comprise the well-known barnacles (Cirripedia) and the Ascothoracida, but the Facetotecta forms a separate subclass and is not an ingroup in any of these two taxa. [Like Thecostraca], the developmental sequence hitherto known for y-larvae... comprises a series of (five) naupliar instars and a succeeding stage, the y-cyprid, which is obviously adapted for settlement... the Ascothoracida are parasites in anthozoans and echinoderms while the Cirripedia... also include the Rhizocephala, one of the most specialized groups of parasites within Crustacea. This has prompted speculations that the Facetotecta are also parasitic.

Exposure to [crustacean molting hormone 20-hydroxyecdysone]... induced y-cyprids to metamorphose into a new and unexpected instar. This 300–400 μm long juvenile has a greatly simplified morphology and is surrounded by an extremely thin (<5 nm) cuticle... Following exposure to the hormone, metamorphosis begins after 12–24 hours by a retraction of all, or nearly all, cyprid tissues into a compact body anteriorly, which is then surrounded by the newly secreted, thin cuticle. While still inside the y-cyprid, the body of the juvenile begins to exhibit contractions and bending motions... The ypsigon is unsegmented, slug-like and lacks appendages.

I guess the revision is "we hit optimal complexity for life pretty quickly"?

Szathmáry 2015 is a pretty good treatise of this sort of major complexity bumps. I also summarized the basic mechanisms here. McShea 2016 notices three parallel trends over time: sometimes, a new level of organization is added at the top (e.g. cells combine to form multicellular bodies); diversity increases one level down (e.g. differentiation of cells in more complex multicellular organisms); and diversity reduces two levels down (individual cells within multicellular bodies have fewer subparts than independent unicellular organisms, since they don't have to survive on their own).

McShea 2016, the "evolutionary syndrome". Left could be a unicellular alga like Chlamydmonas, middle a simple colonial alga like Volvox, and right a complex plant with specialized cell types.

AIUI even Stephen Jay Gould, famous for arguing that evolution has no large-scale tendencies and is largely driven by contingency, acknowledged that the complexity of the most complex organism at any given time increased since he origin of life, if for no other reason that increasing is all it can do when you start from a maximally simple organism. Others could say that there are sensible evolutionary reasons for complexity to increase. Certainly the idea that there is a universal innate pro-complexity tendency (technical term is orthogenesis) in all organisms is discredited for a good reason.

Gould, 1994

Nevertheless, many lineages become simpler. Yeasts are fungi that returned to unicellularity, and many parasitic animals lost most of their internal structures (shout out to Orthonectida, which reverted to having only two kinds of cells). Complexity certainly isn't the same as fitness or evolutionary success. After all, the most numerous known species on Earth is a marine bacterium, Pelagibacter ubique, with as many as a billion billion billion individuals, and the most common autotroph is another bacterium, Prochlorococcus.

Complexification is useful in many cases, to take advantage of scale economy and comparative advantage, but so is simplification. Depends on circumstances. There certainly have been many instances throughout the history of life in which the maximum complexity available to organisms increased, even though not all lineages necessarily became more complex.

Your SECURITY GNOMES are raising an alarm.

> define 'balmy'

Balmy means "pleasantly warm."

Your stock of DICTIONARIES has decreased to 31.

Your SECURITY GNOMES report BANDIT ARTILLERY!

> observe night

The night is cool and balmy.

Your SECURITY GNOMES have entered COMBAT DELIRIUM.

> resolve temperature discrepancy

You haven't acquired enough THERMOMETERS to notice the discrepancy.