biology zoology blog benno meyer rochow appendicularians (1)


Certainly more than just an appendix to vertebrate evolution

The subject of this week’s science blog takes me back several decades when I was a fisheries and marine sciences student spending time on a research vessel with two female student colleagues. My professor had given me the task to measure sperm motility in a number of commercially important fish species. However, that man seemed to have had no idea whatsoever of life and research on a ship and that the ship’s engines produce a constant vibration that makes it impossible to distinguish under the microscope actively moving or passively jittering sperm. That ship-based project was nonsense and I did something very different from what I was meant to do: I ditched that incompetent academic supervisor and followed my own interest. (caught trigger fish and lit a cigarette with a jellyfish)

Anyway, one of the female students not only liked one of the crew (who fathered her child), but also fancied a group of little known, but fantastically interesting planktonic critters: the appendicularians. These rather transparent and usually only a centimeter or less long, delicate animals resemble tadpoles. And indeed that is what most zoologists believe they have evolved from: tadpoles, but not those of frogs but those of tunicates, sessile sea squirts. This group of animals is termed “Urochordates”, because its members’ larvae (the tunicate tadpoles) for the first time in evolution sport a stiff dorsal rod in their bodies, known as the notochord (or chorda), which later in evolution is thought to have given rise to the vertebrate column in the vertebrates. These unimposing animals are thus placed at the beginning of vertebrate evolution. Retaining larval features into adulthood, known as neoteny, has commonly been linked with novel lines of evolution and appendicularians in particular are such examples, for even as adult individuals they retain the larval characteristics of a tunicate tadpole, i.e. a trunk (= the “body section”), and a tail with a dorsal nerve cord and chorda plus an ability to whip the tail to and fro and swim around.

All approximately 100 species are marine, planktonic filter feeders and probably mostly hermaphroditic (but self-fertilization is apparently uncommon). What is, however, certain is that they are capable of filtering the smallest particles of the plankton that other filter feeders are either not interested in or simply cannot trap with their filter apparatuses. Appendicularians secrete a proteinaceous “house” in which they reside. The house has two openings for in-currents and ex-currents created by the undulations of the appendicularian’s flat muscular tail. Across the in-current opening the animal secretes a coarse screen to prevent larger particles from getting into the house. The really important trap of the nanoplankton (the smallest of the plankton) is a very fine mesh screen inside the house. Because the coarse screen at the entrance of the house gets clogged up quickly, appendicularians frequently discard their houses and construct new ones. The old houses drift around in the ocean water as non-living plankton and serve as food (just like the appendicularians’ faeces) for other planktonic organisms. To move around, the animals use a mini-jet propulsion system, releasing water from the ex-current opening of their house.

I think these remarks have shown that our female student had exhibited excellent taste and chosen a fantastically interesting group of animals to work with. Whether one could say the same regarding her choice of sailor who became her child’s father I am unable (or unwilling) to say.

© Dr V.B. Meyer-Rochow and, 2020.
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