Cyclostomi

Cyclostomi Temporal range: Ma PreꞒ Ꞓ O S D C P T J K Pg N
Sea lamprey from Sweden
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Infraphylum:	Agnatha
Superclass:	Cyclostomi Duméril, 1806
Classes
Myxini (hagfishes) Hyperoartia (lampreys)

Cyclostomi, often referred to as Cyclostomata

Most studies based on anatomy have supported the vertebrate hypothesis,^[10] while most molecular phylogenies have supported the cyclostome hypothesis.^{[2][8][11][12]}

There are exceptions in both cases, however. Similarities in the cartilage and muscles of the tongue apparatus also provide evidence of sister-group relationship between lampreys and hagfishes.^[13] And at least one molecular phylogeny has supported the vertebrate hypothesis.^[14] The embryonic development of hagfishes was once held to be drastically different from that of lampreys and gnathostomes, but recent evidence suggests that it is more similar than previously thought, which may remove an obstacle to the cyclostome hypothesis.^[15]

Several groups of Paleozoic jawless fish have been suggested to be more closely related to cyclostomes than to jawed fish, including conodonts and anaspids. The presence of mineralised elements in these jawless fish, like the oral conodont elements and the armoured body covering of anaspids and scutes on other species like Lasanius suggests that mineralised tissues were present in the last common ancestor of all vertebrates, but were secondarily lost in hagfish and lampreys.^[16]

Internal differences and similarities

Both hagfishes and lampreys have a single gonad, but for different reasons. In hagfishes the left gonad degenerates during their ontogeny and only the right gonad develops, whereas in lampreys the left and right gonads fuse into one. There are no gonoducts present.^[17][18]

Hagfishes have direct development, but lamprey go through a larval stage followed by metamorphosis into a juvenile form (or adult form in the non-parasitic species). Lamprey larvae live in freshwater and are called ammocoetes, and are the only vertebrates with an endostyle, an organ used for filter feeding that is otherwise found only in tunicates and lancelets. During metamorphosis the lamprey endostyle develops into the thyroid gland.^[19]

The cyclostomi evolved oxygen transport hemoglobins independently from the jawed vertebrates.^[20]

Hagfishes and lampreys lack a

The common ancestor of both cyclostomes and gnathostomes went through a genome duplication before their split, and while a second genome duplicatio occurred in the stem-gnathostomes, the stem-cyclostomes experienced an independent genome triplication.[26]

The mouth apparatus in hagfishes and adult lampreys has some similarities, but differ from one another. Lampreys have tooth plates on the top of a tongue-like piston cartilage, and the hagfish have a fixed cartilaginous plate on the floor of its mouth with groves that allows tooth plates to slide backwards and forwards over it like a conveyor belt, and are everted as they move over the edge of the plate. Hagfishes also have a keratinous palatine tooth hanging from the roof of the mouth.^[27][28]

Unlike jawed vertebrates, which have three

The hagfish blood is isotonic with seawater, while lampreys appears to use the same gill-based mechanisms of osmoregulation as marine teleosts. Yet the same mechanisms are apparent in the mitochondria-rich cells in the gill epithelia of hagfishes, but never develops the ability to regulate the blood's salinity, even if they are capable of regulating the ionic concentration of Ca and Mg ions. It has been suggested that the hagfish ancestors evolved from an anadromous or freshwater species that has since adapted to saltwater over a very long time, resulting in higher electrolyte levels in its blood.^[29]

The lamprey intestine has a

After Miyashita et al. 2019.[32]

References