Moringua edwardsi (Moringuidae: Anguilliformes): Cranial specialization for head-first burrowing?

Abstract

The order Anguilliformes forms a natural group of eel-like species. Moringua edwardsi (Moringuidae) is of special interest because of its peculiar fossorial lifestyle: this species burrows head-first. Externally pronounced morphological specializations for a fossorial lifestyle include: reduced eyes, lack of color, low or absent paired vertical fins, elongated, cylindrical body, reduced head pores of the lateral line system, etc. Many fossorial amphibians, reptiles, and even mammals have evolved similar external specializations related to burrowing. The present study focuses on osteological and myological features of M. edwardsi in order to evaluate the structural modifications that may have evolved as adaptations to burrowing. Convergent evolutionary structures and possible relations with head-first burrowing, miniaturization, feeding habits, etc., were investigated. Body elongation, reduction of the eyes, modified cranial lateral line system, and modified skull shape (pointed though firm) can be considered specializations for head-first burrowing. Hyperossification can probably be regarded more as a specialization to both head-first burrowing and feeding, even though an impact of miniaturization cannot be excluded. Hypertrophied adductor mandibulae muscles and the enlarged coronoid process can be associated with both feeding requirements (it enhances bite forces necessary for their predatory behavior) and with a burrowing lifestyle, as well as miniaturization.

Burrowing and subsurface locomotion in anguilliform fish: behavioral specializations and mechanical constraints

“…Also the overall head shape, being rather pointed in P. boro, suggests it is better adapted for frontal sediment penetration. More extensive modifications in the skull that have been linked to burrowing are, however, observed in other anguilliform species, such as moringuid eels (De Schepper et al, 2005). The observed higher forward pushing forces generated by the tail in P. boro, compared with the head, supports the hypothesis that this species will perform best at tail-first burrowing, as was suggested based on morphological data (De Schepper et al, 2007a).…”

Rudimentary Eyes of Squamate Fossorial Reptiles (Amphisbaenia and Serpentes)

“…Considering that degeneration and reduction of the eyes are considered as typical examples of fossorial adaption, because these characteristics are also observed in fossorial amphibious and mammals, (Duke-Elder, 1958;Sanyal et al, 1990;Cooper et al, 1993b;CernudaCernuda et al, 2002;Nikitina et al, 2004;Schepper et al, 2005;Albert et al, 2007), Burda et al (1990) raise a question on what could explain a tendency to eye reduction in most underground animals; the authors state that the absence of selective forces that would promote functional development of the eyes propitiate a stagnation in the development of these organs. Nonetheless, the little eye can be considered as a positive feature, because the fossorial animals are constantly protected from receiving dirt on such sensitive organ; this protective role is also played by the spectacles.…”

Pisodonophis boro (ophichthidae: anguilliformes): Specialization for head‐first and tail‐first burrowing?

“…The cephalic lateral line system of P. boro is well-developed and follows the general pattern characteristic for other Anguilliformes as described by Böhlke (1989). This pattern contrasts with that of Moringua edwardsi, in which the cephalic lateral line system is aberrant (De Schepper et al, 2005); dermal cavities are present, presumably functioning as a kind of sen- sory pads, which are stimulated mechanically during burrowing or when in contact with prey. External pores are absent in M. edwardsi, impeding the connection with the environment, but avoiding the entrance of sediment.…”

“…As some parts of the jaw muscles show the aberrant fiber direction and as these muscles substantially enclose the posterior part of the skull, they may reinforce the lower jaw during head probing, thereby protecting it from being disarticulated (see below). The above-mentioned benefits of hypertrophied jaw muscles in head-first burrowing can count for M. edwardsi as well (De Schepper et al, 2005). As mentioned above, in M. edwardsi more fibers (A2d, A3) show the aberrant (ventrocaudal) fiber direction than is the case in P. boro (anterior fibers of A2 and A3 only).…”

“…Rafferty et al (2003) and Sun et al (2004) similarly found in pigs that fusions and enhanced bone growth appeared to be associated with increased suture strain. Some of these skull modifications in M. edwardsi are even more extreme compared with P. boro: paired dorsal skull bones are broader in cross section and show a larger surfaces of overlap at the midline (De Schepper et al, 2005).…”