Research reveals secret shared by comets and sand crabs

October 25, 2018

Researchers at Nagoya University find a mechanical connection between sand crab burrow widths and widths of cometary pits using a simple granular experiment.

SummaryWhen we walk on a sandy beach in summer we may find holes dug in the sand in the vicinity of the shoreline. Often these are entrances to sand crab burrows (Fig. 1). It is striking that the sizes (entrance diameters) of the burrows have a typical value (about 2 - 3 cm). Moreover, one never seems to find any very wide entrances (e.g. 10 cm in diameter). Why do crabs only dig narrow burrows? Of course, the crab's size is one factor: it doesn't need a large burrow if its carapace size is small. However, many types of crabs are much larger than sand crabs - why don't they dig burrows in the beach sand too?

Another factor must be at work. Perhaps the size of sand crab burrows is determined by the mechanical constraint of the substrate: wet sand. This simple idea is our starting point. We studied the stability and strength of burrow-like structures using a simplified model system. At the same time, we realized that the void (or hole) structure within cohesive granular matter is ubiquitous; for example, it is known that voids exist within comets.

Moreover, it is also known that voids can be prone to collapse. Therefore if we could develop a simple model system, it could be useful for explaining a variety of natural void-collapse phenomena, including those found on comets.

Results

By simply compressing a horizontal tunnel structure in a wet granular layer produced by mixing water and glass beads (Fig. 2), we observed three deformation modes: (i) shrinkage without collapse; (ii) shrinkage with collapse but no subsidence; and (iii) collapse with subsidence. Mode (i) can be observed when the initial tunnel diameter is sufficiently small. As the initial tunnel diameter is increased, the deformation mode becomes unstable. The loaded tunnel structure then experiences type (ii) or (iii) collapse depending on the experimental conditions (initial tunnel diameter and grain size). We found that the boundary between (i) and (ii, iii) is approximately 5 cm in diameter. Actually, this value is quite close to the upper limit of crab burrow sizes found in the field. This correspondence suggests that crabs make relatively narrow (small-diameter) burrows to prevent the collapse hazard. They must be smart!

In addition, through systematic experiments, we defined and measured the strength of a tunnel structure in wet granular matter. The measured result is basically consistent with similar previous studies of wet granular mechanics.

Using the obtained strength values, we also estimated the lower limit of size of pit structures found on the surface of comets. We focused on cometary surfaces covered with pit structures whose plausible origin is the collapse of voids due to sublimation of volatile materials inside the comet. The surface of a typical comet consists of a mixture of ice and solid particles. This type of mixture is also a sort of typical cohesive granular matter, like the wet granular matter we used in our experiment.

Since we know that a small void will shrink and not collapse, small pits are unlikely to be created by a void collapsing below the surface. Indeed, measured values of pits on comet surfaces appear to have a lower limit.

By combining all the experimental result and observational information (surface-material strength and gravitational acceleration that are significantly different from the Earth material), we confirmed that our shrink-collapse boundary model is roughly consistent with the observed lower limit of the size of comet pit structures. The experiment is summarized in Fig. 3.

Future perspectives

In this experimental study, the model system was extremely simplified. Although we believe that the essential behavior of tunnel structure in cohesive granular layer was properly understood in this study, much more realistic experiments should be performed to discuss the specific details. For one thing, the size thresholds might depend on grain shape. In addition, we should learn from the crab themselves, so field work is necessary to improve our understanding of crab burrows. Furthermore, this type of void collapse in cohesive granular matter could be more universal than we thought. We should consider broader applications. For instance, in November 2016 a road in Fukuoka city in southern Japan collapsed suddenly. This is also some type of collapse hazard of a void in cohesive granular layer. That is, our finding might be relevant to disaster prevention techniques as well.
-end-
The article:

"Stability of a void structure in wet granular matter and its application to crab burrows and cometary pits" is published in Scientific Reports at https://www.nature.com/articles/s41598-018-33978-8

(DOI : 10. 1038/s41598-018-33978-8)

Authors:

Ayuko Shinoda, Shin-ichi Fujiwara, Hirofumi Niiya, and Hiroaki Katsuragi

Funding:

This work was supported by Japan Society for the Promotion of Science (JSPS) KAKENHI Grant Numbers 16K13861 and 18H03679.

Nagoya University

Related Crab Articles from Brightsurf:

Notice me! Neglected for over a century, Black sea spider crab re-described
Even though recognised in the Mediterranean Sea, the Macropodia czernjawskii spider crab was ignored by scientists (even by its namesake, 19th-century biologist Vladimir Czernyavsky) in the regional faunal accounts of the Black Sea for more than a century.

Scientists detect crab nebula using innovative gamma-ray telescope
The prototype Schwarzschild-Couder Telescope (SCT)--developed by scientists at the Columbia University in collaboration with researchers from other institutions--is part of an international effort, known as the Cherenkov Telescope Array (CTA), which aims to construct the world's largest and most powerful gamma-ray observatory, with more than 100 similar telescopes in the northern and southern hemispheres.

Ship noise hampers crab camouflage
Colour-changing crabs struggle to camouflage themselves when exposed to noise from ships, new research shows.

West coast dungeness crab stable or increasing even with intensive harvest, research shows
Fishermen from California to Washington caught almost all the available legal-size male Dungeness crab each year in the last few decades.

Crab-shell and seaweed compounds spin into yarns for sustainable and functional materials
Biobased fibres are made from two renewable marine resources and with promise in advanced applications, in wovens and medical materials, among others.

From crab studies, a broader approach to identifying brain cells
In a new study, a team born in part at the Neural Systems & Behavior course in Woods Hole tests the notion that a cell's identity can be described solely by the genes it expresses.

Crab disease poses threat to shellfish stocks
Shore crabs carry parasites that pose a major threat to shellfish stocks.

Two new porcelain crab species discovered
Two new symbiotic porcelain crab species have been described. One of them, from the South China Sea of Vietnam, inhabits the compact tube-like shelters built by the polychaete worm with other organisms.

Warmer winters could lead to longer blue crab season in Chesapeake Bay
Scientists from the University of Maryland Center for Environmental Science are predicting that warmer winters in the Chesapeake Bay will likely lead to longer and more productive seasons for Maryland's favorite summer crustacean, the blue crab.

This hawk likes crab for dinner
This is the first report of a red-shouldered hawk attacking and presumably consuming any species of crab and the first report of probable ghost crab predation by a raptor in North America.

Read More: Crab News and Crab Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.