The Evolution of Floating

 

Scientists love their terminology—too much, I think. The terminology at play today includes plankton (floating critters mostly at the mercy of the current), benthos (critters hanging on the bottom), neuston (hanging in the air-water interface), and maybe some others like nekton (swimmers), but mostly neuston.

 

Neuston critters are also called pleuston, just to keep things messy. Sometimes, there is a subtle difference: Pleuston is used for critters that live within the air-water interface, and neuston is used for those hanging below (via surface tension) or standing above (also using surface tension). However, generally, pleuston is not used, so we can safely just use neuston. After all, don’t we have too many terms anyway?

 

Wikipedia, of course, has a great summation image. I’m going to steal it:




Classifications of the living zones

 of aquatic and marine animals.



Water striders; a familiar air-water interface denizen.


A recent paper by Anthony et al. (2024) tests a hypothesis by A.I. Savilov, from 1958, that floating animals (neuston and plankton) evolved from bottom-dwelling (benthic) ancestors. Since many animal groups have benthic and floating members, for example, barnacles, nudibranchs, shelled snails, and cnidarians, the researchers analyzed molecular phylogenies to see if they supported this hypothesis. 



A floating snail. Floating on the underside of the water’s surface. 


Their conclusions? The molecular data supported the hypothesis in 4 out of 5 groups analyzed. So, for example, the floating (and amazingly beautiful) nudibranch Blue Sea Dragon, Glaucus atlanticus, is fully encompassed by a bevy of benthic relations. This suggests that that floater is not as closely related to other nudibranch floaters as it is to benthic nudibranch in the mostly benthic Facelinidae family. If this is born out, some taxonomic adjustments may be needed.

 

Similar situations were found in the barnacles and shelled snails examined. The only group examined that did not support this hypothesis was Physalia, the Portuguese man o’ war, which they conclude likely evolved from free-floating ancestors. This begs the question: Did jellies come before corals? 


.

Physalia, the Man o’ War, a beautiful and deadly floater.


It turns out that Jellyfish-like forms appear earlier in the fossil record, with soft-bodied medusozoans dating to the Ediacaran period (~565 million years ago). Molecular analyses suggest Medusozoa (the group that includes true jellyfish) may have diverged earlier than Anthozoa (the group that includes corals and sea anemones).

 

Features of floaters, feeding behavior, and prey are similar to their benthic cousins. This makes sense if they are closely related. 

 

One of the floaters, the barnacle, Dosima fascicularis, is fascinating. It produces its own float, which is described as a gas filled cement structure. Its referred to as cement in several papers, but a paper in Biological Bulletin in 2012 states, “The morphology of the cement apparatus and of the polymerized cement of this species is almost unknown.” Why is it a “cement”? The building material cement is made of limestone (mostly calcium carbonate), that is used to stick and hold things together. 

 

The ”cement” of the barnacle is not calcium carbonate but primary protein. I guess they call it a cement because it binds things together, what is it binding together? The rest of the barnacles, I guess. The float structure is elastic fibers (proteins) that form a foam that traps gases. The gases allow the floating of course, the density of the structure would cause it to sink without the trapped gases. The gases are thought to be carbon dioxide. The makeup of this float material is very similar to adhesives used by other barnacles. Some researchers refer to this structure as a foam-like adhesive. I like that, it produces a better image in my mind than the word cement. The barnacle is commonly called the Buoy Barnacle.

 

The foam has a low pH, 3 to 4, which promotes keeping the CO2 in a gaseous state. Ponder the evolution of this structure. How did this start? A barnacle got dislodged from its solid base and survived in the wilds of open water? If its adhesive trapped some gases, then it stayed afloat longer, allowing it to feed more. The carbon dioxide stayed in gaseous form at lower pH values. This would be a challenge surrounded as it is by ocean water with a pH above 8. Fascinating. 

 

FYI, the gases are thought to be carbon dioxide but it’s actually unknown. The gas float of the Man o War can contain 10% carbon monoxide (more in youngsters). The gas is variable among individuals. Other than CO the float gas is very similar to air, oxygen close to 15% and nitrogen the rest (75% or more). This makes me question the guess that the gas used by barnacles is carbon dioxide. I think you should go study floating barnacles.

 

 

We’d better finish with a few floating barnacle pictures:


Ryan and Branch (2012) give us this detailed image of a Buoy Barnacle, Dosima fascicularis. The scuta are the major plates (only one side shown here), while the capitulum includes all the plates of the shell (in barnacles with a peduncle).

Two Buoy Barnacles on a float (left). There can be whole clusters attached.

  



Sources and Further Readings:

 

Anthony CJ, Bentlage B, and Helm RR. 2024. Animal evolution at the ocean’s water-air interface. Current Biology 34: 196–203.

 

Boetius J. 1952–1953. Some notes on the relation to the substratum of Lepas anatifera L. and Lepas fascicularis E. et S. Oikos 4: II: 1952–1953.

 

Church SH, Abedon RB, Ahuja N, Anthony, CJ, Ramirez DA, Rojas LM, Albinsson ME, Álvarez Trasobares I, Bergemann RE, Bogdanovic O. et al. 2024. Global genomics of the man-o’-war (Physalia) reveal biodiversity at the ocean surface. bioRxiv, preprint.

 

Helm RR, 2021. Natural history of neustonic animals in the Sargasso Sea: reproduction, predation, and behavior of Glaucus atlanticus, Velella velella, and Janthina spp. Marine Biodiversity 51(99).

 

Lane CE. 1964. Biology of Physalia. Final Report. Office of Naval Research. Department of the Navy. Contract Nonr 840 (17).

 

Munro C, Zapata F, Howison M, Siebert S. & Dunn CW. 2022. Evolution of gene expression across species and specialized zooids in Siphonophora. Molecular Biology and Evolution 39(2): msac027. https://doi.org/10.1093/molbev/msac027.

 

Savilov AI. 1958. Pleuston of the Western Pacific. Dokl. Akad. Nauk SSSR 122: 1014–1017

 

Zheden V, Byern JV, Kerbl A, Leisch, Staedler Y, Gunwald I, Power AM, and Klepal W. 2016. Morphology of the cement apparatus and the cement of the buoy barnacle Dosimaa fascicularis (Crustacea, Cirripedia, Thoracica, Lepadidae). Biological Bulletin 223: 192–204. 



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