Derichthys: The Neck Eel

From Wikipedia Commons.

Derichthys serpentinus* is yet another deep-sea fish with a strikingly odd appearance - it has what appears to be a neck! On the basis of maxilla and premaxilla morphology, early workers placed the fish in its own order, Carenchelyi (Jordan and Evermann 1896), or argued that it was a synbranchiform (Gill 1905). Relying on a couple characters is of course a terrible way to go about phylogenetics, and it was eventually realized that Derichthys was a true eel despite its neurocranial oddities (Castle 1970). Derichthys is now coupled with Nessorhamphus in the clade Derichthyidae, which itself has variably been placed as a relative of Heterocongrinae (Eagderi and Adriaens 2010) or sister clade of Serrivomeridae (Mehta et al. 2010), but either way is certainly nestled deep within Anguilliformes

* Derichthys roughly means 'fish with a neck', and serpentinus refers to its pronounced resemblance to snakes, even more so than other eels.


Gill (1884) claimed that Derichthys appeared to be the only fish with a "true neck", but is this really the case? The illustration at top unfortunately has forward-bending pectorals obfuscating the distance between the head and pectoral fin base, which is about 4/5th the head length (Jordan and Evermann 1896). The dorsal fin in this species is peculiar in that it begins midway between the snout and vent (Jordan and Evermann 1896), and coupled with the constricted appearance of the fish's anterior portion, makes Derichthys look even 'neckier'. Derichthys is not the only eel with pectoral fins set far back on their body, as members of Ophichthidae do as well, albeit with much thicker 'necks'. Both Derichthys and ophichthids demonstrate 'branchial displacement', a phenomenon where connections between the cranium and gill arches are lost (as well as interconnections of the gill arches), and the gill arches are pushed back, forming an extended branchial region* (Mehta et al. 2010). Derichthys has both considerable branchial displacement and a large gape, although it is no longer clear if the two are related to processing large food items (Mehta et al. 2010). Interestingly Derichthys avoids competition with its close relative Nessorhamphus - which it overlaps in distribution and habitat - by utilizing its large gape to feed on sergestid shrimp rather than the smaller euphausiids (Hoar et al. 1997).

* I really wish I could find a picture demonstrating this.



So Derichthys isn't a complete anomaly as Gill suggested, but it would still be interesting to learn why the anterior portion of the animal would be constricted if it feeds on large-ish organisms. After looking into ophichthids, I found some of them to be so monstrously bizarre they're my new blogging priority.

References:

Castle, P. H. J. (1970). Distribution, Larval Growth, and Metamorphosis of the Eel Derichthys serpentinus Gill, 1884 (Pisces: Derichthyidae). Copeia 1970 (3), 444-452.

Eagderi, S. and Adriaens, D. (2010). Head morphology of the duck bill eel, Hoplunnis punctata (Regan, 1915; Nettastomatidae: Anguilliformes) in relation to jaw elongation. Zoology 113, 148-157. Available.

Gill, T. (1905). A New Introduction to the Study of Fishes. Science 21 (539), 653-661. Available.

Gill, T. (1884). Three New Families Of Fishes Added To The Deep-Sea Fauna In A Year. The American Naturalist 18, 433. Available.

Hoar, W. S., Randall, D. J., Conte, F. P. (1997). Fish Physiology Volume 16: Deep-Sea Fishes. Academic Press: San Diego, California. Partially Available.

Jordan, D. S., and Evermann, B. W. (1896). The Fishes of North and Middle America. Bulletin of the United States National Museum 47. Available.

Mehta, R. S., Ward, A. B., Alfaro, M. E., and Wainwright, P. C. (2010). Elongation of the Body in Eels. Integrative and Comparative Biology. doi:10.1093/icb/icq075