Why the seahorse tail is square

Abstract

Whereas the predominant shapes of most animal tails are cylindrical, seahorse tails are square prisms. Seahorses use their tails as flexible grasping appendages, in spite of a rigid bony armor that fully encases their bodies. We explore the mechanics of two three-dimensional-printed models that mimic either the natural (square prism) or hypothetical (cylindrical) architecture of a seahorse tail to uncover whether or not the square geometry provides any functional advantages. Our results show that the square prism is more resilient when crushed and provides a mechanism for preserving articulatory organization upon extensive bending and twisting, as compared with its cylindrical counterpart. Thus, the square architecture is better than the circular one in the context of two integrated functions: grasping ability and crushing resistance.

Structure and mechanical properties of selected protective systems in marine organisms

“…The tail skeleton is composed of several articulating segments arranged into cross-sectional squares, each composed of four bony plates that surround a central vertebra (Fig. 17b) [147][148][149][150][151][152][153][154]. These plates are connected by overlapping joints that allow them sufficient flexibility for grasping as well as added strength for armored protection [154].…”

“…17b) [147][148][149][150][151][152][153][154]. These plates are connected by overlapping joints that allow them sufficient flexibility for grasping as well as added strength for armored protection [154]. In grasping, the square structure of the tail provides more surface contact and a mechanism for maintaining organization of the articulating plates [154].…”

“…These plates are connected by overlapping joints that allow them sufficient flexibility for grasping as well as added strength for armored protection [154]. In grasping, the square structure of the tail provides more surface contact and a mechanism for maintaining organization of the articulating plates [154]. When the tail (of a deceased seahorse) is crushed, it can be compressed up to nearly 50% of its original width before fracture of the vertebral column (Fig.…”

When it's hip to be square

“…Among their suite of unusual characteristics, seahorses possess a true oddity: a prehensile tail with a square, rather than round or elliptical, crosssectional shape. On page 46 of this issue, Porter et al ( 2) report that there are distinct mechanical advantages to being square. Using threedimensional (3D) printing to construct physical models, the team demonstrates that the multiplated anatomy of the square seahorse tail shows greater resistance to mechanical deformation than a similar model that has a round cross section.…”

“…Only the tail has the square cross-sectional shape, suggesting some specialized function (see the first figure). It is tempting to assume that the square shape might be related to its prehensile capabilities ( 3,4); the size of the plates decreases toward the tip of the tail, allowing for coiling into a tight logarithmic spiral when bent in a ventral direction ( 5), and the flat ventral surface might provide greater grip ( 2), although this latter notion remains to be experimentally tested. However, close relatives of seahorses (pipefishes and seadragons) lack tail prehension, but still have square tails ( 6,7).…”