Spines: what’s the point of it all?

When you hear the word “spine,” your first thought is probably a backbone: that familiar stack of vertebrae running from the base of your skull to your tailbone. At least, that’s what popped into my head when I first discussed my project with my mentor Jacob Harrison, a PhD student in the Patek Lab. However, there’s another type of spine that is often overlooked, one that is far more prevalent in nature than you might think.

Note: drawings are not realistic depictions of species

My research focuses on spines in the spiky sense. For my project, spines are defined as rigid biological structures that come to a point (J. Harrison). Barbs, quills, thorns, spines… these are all different names used across the literature for fundamentally similar structures (J. Harrison). Many of us are aware that spines exist in nature, because we’ve experienced (or tried to avoid) painful run-ins with them. However, until now I never appreciated just how diverse spines are across biology. Some organisms such as sea urchins have conical toothpick-like spines, while other species like stingrays have flattened barbs reminiscent of knife blades. Some spines are smooth, like the stingers of scorpions, while other spines display serrations of varying size, number, and orientation. For instance, while both the sea urchin and stingray have many small serrations on their spines, these serrations run in opposite directions (see Fig. 1)! Furthermore some structures, such as the raptorial appendages of spearing mantis shrimp, contain several spines at once (see Fig. 1).

Aside from being diverse in structure, spines vary widely in their function. Stingrays use their barbs defensively, embedding their spines in the bodies of predators (and sometimes, the feet of unwary beachgoers!). Meanwhile, spearing mantis shrimp use their spines for predation, skewering prey that swim above their sand burrows. This large difference in function occurs, despite the fact that both species utilize the same underlying tool of the spine.   This suggests that small changes to the structure of a spine play a role in how it is used, and ultimately begs the question: how do changes in spine morphology (or structure) influence spine function?

Fig. 1 – The structures of a sea urchin spine, stingray barb, and a spearing mantis shrimp dactyl (foreclaw)

To better understand the relationship between spine form and function, we’ll be investigating how spine structure affects puncture and draw mechanics. We decided to use 3D modeling for this project, because this will allow us to perform more controlled comparisons of changes in spine structure. First, we’ll design a basic underlying spine shape as a control, and then manipulate different aspects of that spine’s morphology (ex. serration number, size and angle) in set increments. After printing the resulting variations using the Patek lab’s 3D printer, we’ll then record the force required for each of the spines to pierce ballistics gel using a Material Testing System (MTS), which measures forces in tension and compression. This will allow us to see whether/how changes in the spine’s morphology affect its puncture/draw mechanics (i.e. how it pierces or retracts from the gel).

Rough idea of a base spine and resulting variations. We chose to model the spine after a stingray barb because 1) it’s an easily replicable shape, and 2) we know that it is definitely used to puncture things that the stingray views as a threat.

Currently I’m in the process of designing prototype spines using the 3D-modeling software 3Ds Max. Below are some printed models!

Feelin a bit like Tony Stark looking at his Hall of Armors

Because these spines aren’t precise replicas of ones found in nature, we have to be careful about what conclusions we can draw about ecological/evolutionary functions. However, the effects we observe with our basic models can still give us insight into the fundamental influences that spine structure can have on function.

The Patek Lab focuses their research on the intersection between physics and evolution, which is an inherent part of my project. I’m really excited to see what we might learn, not only because I am curious about the nature of spines and the organisms that wield them, but because I think our findings could have practical applications to people’s lives. After all, wouldn’t you want to know the structure of a stingray barb if it revealed an easier way to get it out of your foot?

Of course, there’s a lot more to explore, prepare, and test before I can say anything for sure. But still, I’m excited to take a stab at this investigation and see how it goes!

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