There’s no place like the perfectly sized home for the mighty mantis shrimp

Size matters to the small-but-mighty mantis shrimp, which show a marked preference for burrows in coral rubble with volumes that closely match their own body size or are just a bit larger—in other words, large enough to accommodate their body, but small enough that they can defend the entrance. But according to a new paper published in the journal Animal Behavior, sometimes a mantis shrimp will compromise. If a burrow is already occupied and is close to the ideal size, or a bit smaller, the mantis shrimp will fight longer and harder for that burrow—and be more likely to win the contest.

As we previously reported, mantis shrimp come in many different varieties: there are some 450 known species. But they can generally be grouped into two types: those that stab their prey with spear-like appendages (“spearers”) and those that smash their prey (“smashers”) with large, rounded, and hammer-like claws (“raptorial appendages”). Those strikes are so fast—as much as 23 meters per second, or 51mph—and powerful, they often produce cavitation bubbles in the water, creating a shock wave that can serve as a follow-up strike, stunning and sometimes killing the prey. Sometimes a strike can even produce sonoluminescence, whereby the cavitation bubbles produce a brief flash of light as they collapse.

A 2018 study found that the secret to that powerful punch seems to arise not from bulky muscles but from the spring-loaded anatomical structure of the shrimp’s arms, akin to a bow and arrow. The shrimp’s muscles pull on a saddle-shaped structure in the arm, causing it to bend and store potential energy, which is released with the swinging of the club-like claw. And earlier this year, scientists discovered that, counterintuitively, the mantis shrimp punches at half the speed in air, suggesting that the animal can precisely control its striking behavior, depending on the surrounding medium.

“Resource value assessment”

Patrick Green of the University of Exeter and J.S. Harrison of Duke University—authors of the new paper in Animal Behavior—were interested in exploring what’s known as “resource value assessment” in mantis shrimp of the smashing variety (Neogonodactylus bredini). Both male and female mantis shrimp in this species are known to compete over coral rubble burrows, which provide protection from predators and a safe space to mate and brood eggs. If a preferred burrow is already occupied, it can trigger a fight over who gets the burrow. Those competitions typically involve a ritualized exchange of high-force strikes (mantis shrimp SMASH!), with the defending mantis shrimp also using its armored tailplate to block the burrow entrance from intruders.

These sorts of animal competitions are quite common in nature, and animals seem to be able to assess the value of such “contested resources” and adjust their behavior accordingly. Such encounters are typically described in terms of a linear or categorical value assessment, in which, for example, males will fight more aggressively in the presence of females. Similarly, female parasitoid wasps will compete over the most desirable hosts in which to lay their eggs. The larger the host, the more food will be available for the offspring when they hatch, for example. Past studies have suggested that a female’s egg load seems to be a contributing factor (or selective force) in how aggressively they fight over a potential host and how likely they are to win such a competition.

Past studies have shown that mantis shrimp pick burrows whose sizes (volume) mesh well with their own body size (mass), as do hermit crabs. In the case of hermit crabs, there seems to be a tradeoff at play when it comes to resource assessment: dragging around a larger shell requires more energy but offers more protection from predators, while the reverse is true for smaller shells. Green and Harrison suggest that, when it comes to competing for a desired shell, hermit crabs may prefer shells that are the preferred size or slightly larger, while placing less value on shells that are much larger or smaller.

This would be an example of quadratic resource value assessment, in which resources are valued most highly at a certain peak level. That value decreases in either direction from that peak. In other words, there is an optimal sweet spot, or “Goldilocks zone,” where an asset is deemed to be “just right” and the animal will adapt its behavior accordingly—e.g., by fighting more aggressively when such a desirable asset is contested. Green and Harrison thought a similar quadratic resource value assessment might also apply to mantis shrimp—namely, that mantis shrimp would place a higher value on burrows with an ideal volume and would be more aggressive, and more likely to win, when fighting for control of such burrows.

To test their hypothesis, the researchers conducted two sets of experiments: “choice experiments,” where mantis shrimp could freely choose unoccupied burrows of varying sizes, and “staged contests,” where “defending” and “intruding” mantis shrimp were randomly matched in a competition over a preferred burrow. Green and Harrison predicted that their experiments would show that competitors would fight longer and harder and would be more likely to win when their body length closely matched the volume of the contested burrow—and that these factors would decrease the further that match deviated from the ideal, in either direction.

“This study is an example of maximum effort being reserved for something that’s ‘just right.’”

The researchers built mock burrows out of clear plastic tubing with a single opening, wrapped in black vinyl, with a clear area at the top to enable them to observe what was happening inside. The mantis shrimp were collected from burrows in seagrass beds along the Caribbean coast of Panama. The researchers also videotaped the staged contests (a total of 36) and intervened if it seemed like one of the fighting shrimp was in danger of significant injury or death.

They found that, overall, the occupying mantis shrimps successfully defended their burrows from intruders in 69 percent of the fights. But those odds changed dramatically in cases where the intruding mantis shrimp were competing for burrows slightly smaller than their ideal size; intruders won 67 percent of the fights in those circumstances, typically by striking first and striking more often.

“We know that animals can assess a variety of factors, including the size of the opponent and the value of the prize, when deciding whether to fight and how hard to fight,” said Green of the results. “In this case, as a smaller burrow is probably occupied by a smaller opponent, it seems mantis shrimps will compromise on the size of the home if it means an easier fight. It might be assumed that animals fight hardest for the biggest assets, but this study is an example of maximum effort being reserved for something that’s ‘just right.'”

There were some caveats, most notably sample-size constraints. Green and Harrison also acknowledged that the mock burrows were standardized, with set lengths and diameters, unlike naturally occurring burrows, which usually have more variable dimensions. And the smooth tubing is markedly different from the natural burrows formed in rock and rubble.

“Mantis shrimp are adept modifiers of natural burrows, using appendage strikes to widen too-narrow burrows and using rock and sand to fill in too-large burrows,” they wrote. “While the individuals we tested could not widen mock burrows by striking, perhaps with more time in which to establish residency, individuals would have filled in larger mock burrows.”

DOI: Animal Behavior, 2020. 10.1016/j.anbehav.2020.09.014  (About DOIs).

Listing image by Roy Caldwell

https://arstechnica.com/?p=1717749