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Writer's pictureLuke Fletch
Oct 2, 20204 min read

A Deep Marine Dive into the Phylogeny and Speciation of Sipohonophores

Watch out! Here comes the Flying Spaghetti Monster! Bathyphysa conifera, a species closely related to Physalia physalis in the family Rhizophysidae, was nicknamed after the satirical internet deity by the oil workers who first saw it. Despite siphonophores sharing similar overall characteristics (such as jellyfish-like bodies and projections), the 175 different species of Siphonophorae grace the ocean depths and surfaces with unique colorations, morphologies, and light shows. Physalia physalis’ family, Physaliidae, and the flying spaghetti monster’s family, Rhizophysidae, comprise the suborder of Cystonecta within the order Siphonophorae.


Bathyphysa conifera, the infamous Flying Spaghetti Monster.

The flying spaghetti monster itself. The resemblance is uncanny.



Using ribosomal RNA, researchers have created phylogenetic trees to understand at least part of the evolutionary history of siphonophores. The closest outgroup sister to the siphonophores is the well-known Hydra, which is a genus of freshwater organisms that we studied freshman year in our biology labs. Hydra is a member of the order Anthoathecata, which is a part of the class Hydrozoa. Toward the base of the tree, the ancestral siphonophore was proposed to be a dioecious individual with a pneumatophore, siphosome (feeding and reproductive structures), and the likely absence of a nectosome, which would contain nectophores for jet propulsion. The ancestral siphonophore likely led to early cystonects and an ancestral codonophoran. Cystonects are a basal group toward the root of the siphonophore phylogenetic tree. Our lovely Portuguese Man o’ War and flying spaghetti monster are two of the five species within the cystonect group. A key difference between the ancestral codonophoran and cystonects is that the codonophoran had a nectosome. Nestled within the codonophorans are the paraphyletic group of physonects and monophyletic group of calcycophorans.


Current phylogenetic tree of Siphonophorae, with outgroups at the top. Genetic data is missing for many siphonophore species due to lack of research within the field.



I would like to interrupt the discussion of the phylogeny to extrapolate on the importance of the nectosome for speciation. There is a controversial theory, and please note controversial and not completely accepted, that gene flow between siphonophores without nectosomes (e.g. cystonects) was maintained between metapopulations of cystonects because oceanic gyral centers inhibited breaks in gene flow for hundreds of millions of years. Although the oceans are enormous, Physalia physalis and other cystonects would not have had certain breaks in gene flow that other metapopulations might have. Allopatric speciation would be much less likely to occur, at least on the surface. “But author,” you ask, “why couldn’t there be speciation outside of the gyral centers and below the surface?” Although I could not find anything explicitly stated about speciation events from ancestral siphonophores to cystonects and ancestral codonophorans, I assume that the evolution of the nectosome allowed metapopulations to create allopatric speciation events by escaping gyral centers and having an active role in determining the height of the water column in which the individual resided. Furthermore, cystonects have pneumatophores, and have less opportunities to influence their location. That balloon-structure keeps them on the water’s surface and decreases the period of their life history in which they could have an allopatric speciation event along a vertical space. The same reasoning applies to why parapatric speciation has not really occurred with cystonects; hybrid zones and environmental gradients did not have an opportunity to form. There are only five cystonect species and 170 codonophorans. At least part of the reason why codonophorans form a larger proportion of siphonophores is due to nectosomes.

The codonophorans, consisting of physonects and calcycophorans, was determined by genetic analysis. From this analysis, key evolutionary events between families and the two phyletic groups became clear. All the calcycophorans and some of the physonects evolved monoecy, meaning that monoecy evolved twice within siphonophores. Furthermore, calcycophorans lost their pneumatophores, whereas all physonects retained their pneumatophores. Nearly all the codonophorans have nematocyst “batteries,” which refer to the feeding nematocysts being contained on the side branches. These are called tentilla. The appearance of tentilla coincided with the loss of large polyps from nectosomes and changes in diet from soft-bodied prey to hard-bodied crustacean prey. There is a beautiful biodiversity of tentilla, from tipped to non-tipped, hollow or solid, spiny or non-spiny, and so on.


A fantastic diagram to help visualize the key traits of each phyletic group. Please note the presence of nectosomes in the ancestral codonophoran.



Physalia physalis does not have nematocyst batteries, but only regular nematocysts. Physalia has two kinds of nematocysts, atrichous isorhiza and stenoteles, whereas the flying spaghetti monster and its family only has atrichous isorhiza. Physalia nematocysts are held by complex fibrillar baskets anchored to a gluey substance below the cell. During development, the nematocysts form and then slide down the tentacle with important migrations to the reproductive structures to defend the offspring once it is released from the colony. Atrichous isorhiza look like party-blowouts once fully extended and stenoteles expand outward with a nasty shaft filled with spines. In both cases, the tubule and general structure of the nematocysts face inward until stimulated to release and puncture the target.


Atrichous isorhiza, found on both the flying spaghetti monster’s family and the Portuguese Man o’ War.

Stenotele, found on the Portuguese Man o’ War. A nematocyst with that many spines looks very painful! Notice how the stenotele transitions from a compact structure to discharging with great force and speed.



Because this blog post has connections crossing all over the passage, I would like to provide a summary listing key details. Current phylogenetic trees place Siphonophorae sister to Anthoathecata within the subclass Hydroidolina, which is within the class of Hydrozoa. The tree first branches off to cystonects, where the Portuguese Man o’ War is placed, and codonophorans. Codonophorans branch off to the paraphyletic group of physonects and the monophyletic group of calcycophorans. Cystonects are comprised of Physalia and five species of Rhizophysidae that share the absence of nectosomes, which is possibly a key trait for the evolution of physonects and calcycophorans and occurred slightly before the proliferation of numerous kinds of tentilla and nematocysts within codonophorans.


Thanks for reading! Untentilla next time!

 

Works Consulted

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