Monopterus enters the LRT with a single ventral gill opening

This newly discovered
(Britz et al. 2016) swamp eel, Monopterus, is also blind.

Figure 1. Monopterus in vivo. Color faded from deep magenta. Note the single ventral gill opening.

As in the related swamp eel,
Synbrachus (Fig. 3), Monopterus has no pectoral fins and no pelvic fins. More derived yet, Monopterus also lacks dorsal, anal and caudal fins. Essentially Monopterus has reverted back to resembling a basal chordate, like the lancelet Amphioxus (= Branchostoma, Fig. 4), including the single ventral gill opening.

Figure 2. From Britz et al. 2016. Colors added here. The upper images lack the premaxilla and maxilla, highlighted in yellow and green below.

Monopterus luticolus
(Britz et al. 2016; 20 cm in length) is an extant species of swamp eel close to Symbranchus and the first from Africa. Others are from Asia. Note the tiny blind eyes and single ventral gill opening. No fins are present on the adult. In life the color is deep magenta, not the washed out peach-color that results after immersion in preservative.

Britz et al. report,
“Several species are known for their burrowing, amphibious life-style and their ability to survive outside of water due to the possession of highly vascularized secondary air-breathing organs accompanied by substantial changes to their vasculatory systems.”

“There is no clear discontinuity in structure between abdominal and caudal vertebrae in synbranchids.”

In other words,
lowly Monopterus represents yet another attempt by fish to leave water and adopt a more terrestrial existence, this time without fins or limbs, like a worm. Pretty amazing.

Figure 3. Synbranchus, another swamp eel with a caudal fin operating eyeballs. Note the short premaxilla.

Distinct from Synbranchus
(Fig. 3), with its short, transverse premaxilla, in Monopterus (Fig. 2) the premaxilla is quite long and ventral to and separate from the maxilla. This trait is convergent with several related and unrelated fish.

Figure 4. Extant lancelet (genus: Amphioxus) in cross section and lateral view. The gill basket nearly fills an atrium, which intakes water + food, sends the food into the intestine and expels the rest of the water. This taxon also has a single ventral gill opening.

References
Britz et al. (4 co-authors) 2016. Monopterus luticolus, a new species of swamp eel from Cameroon (Teleostei: Synbranchidae )Ichthyol. Explor. Freshwaters, Vol. 27, No. 4, pp. 309-323.

wiki/Synbranchus
wiki/Monopterus