Stress in the neurocranium of burrowing,
eel-like bony fishes.

Anguilliform or eel-like fishes are mostly found close to the bottom which is usually a complexly structured environment. Their elongated and flexible body shape seems well adapted for this way of life since it allows locomotion through narrow passages between rocks and plants. Some of these anguilliform species have become further specialized into burrowers. Just as for swimming, thrust is produced by lateral undulations of the body during burrowing while the head or tail tip is used to penetrate the substrate, depending on the species.

During burrowing, the part of the body penetrating the substrate has to endure higher loads resulting in higher stresses. It can be expected that several adaptations will have developed in the course of evolution to cope with this condition and they are most likely to be found in that part of the body used to penetrate the substrate.

Studying and comparing stress distributions in the skulls of typical head and tail burrowers will provide an understanding of the level of structural reinforcements, and thus adaptations to burrowing. In the present study, two species Pisodonophis boro (rice paddy eel) and Heteroconger hassi (spotted garden eel) were used. The rice paddy eel is less specialized and burrows both head and tail first. Garden eels are specialized tail burrowers that only burrow with their tails. To obtain the geometry for the finite element analysis, digital 3D-reconstructions based on series of histological or CT-slices of the skull of both species are made using the software package ‘AMIRA 3.1’ (Mercury Computer Systems, Inc.). Tetrahedral meshes were generated and analyzed using Strand7 (Strand7 Pty Ltd).

The results of the analyses show that certain structural features of the skull of the rice paddy eel can be regarded as adaptations for burrowing head first. A first adaptation is the increased thickness of the skull which reduces the overall range of the stress values (about ten times lower in the rice paddy eel compared to the spotted garden eel) resulting in a greater tolerance of external loads. Secondly, the stress is distributed more evenly in the dorsal part of the skull due to a more gradual transition of the interorbital septum (dorsal bony beam between the eyes) into the braincase.