Imagine being able to breathe both on land and sea? Walking along the shore and into the depths of the ocean without any thought as to your survival. Entering the amazing underwater world without the need of supplemental oxygen. How cool would that be? A creature capable of such a feat is the crab!
How is this possible? An excerpt from The Oregon Coast Aquarium's Blog (Crabs: The Amphibious Creatures of the Sea) provided me with the answer.
Some crabs live almost exclusively on land and most can survive out of water for notable stretches of time. As long as a crab’s gills stay damp, oxygen will diffuse from the atmosphere into the water on their gills. The oxygen then moves, through the process of osmosis (the movement of particles in solution through a semi-permeable membrane to balance concentration of similar particles), into the capillary blood vessels that carry oxygen to the rest of the crab’s body. For this process to be effective, it is crucial that crabs maintain damp conditions on their gills.
Crabs have unique anatomical features that help them minimize how much water evaporates from their gills. Crabs use articulating, or movable, plates around their gills to help seal in moisture to prevent them from drying out. They also store water in their bladder, blood, and specialized pockets throughout their bodies.
Behavioral adaptations also aid crabs with their terrestrial/aquatic transitions. While on land, crabs frequently seek cool, dark, damp hiding places to decrease water evaporation from their gills. Hiding out in the rocky crevices near tide pools not only eases their breathing, but also conceals them from potential predators.
On a recent excursion along the Carmel coast, I came across several colorful striped shore crabs, (Pachygrapsus crassipes) retreating into a crevice and decided to take a closer look. Easily spooked by movement, I found a comfortable spot, set up my camera and waited for them to reemerge.
As they began to reveal themselves, I was fascinated by the beautiful coloration and patterns on their bodies in various shades of green, purple, brown and a rusty orange. Shuffling back and forth it seemed they were constantly looking for the ideal spot, with the larger crabs displaying aggression as they chased the smaller crabs away, in spite of what appears to be a perpetual smile :-)
Curious about this behavior I contacted marine invertebrate biologist Mike Gigliotti via iNaturalist and he provided the following insight.
Aggression could absolutely be a territorial behavior, maybe for feeding grounds but also "I have picked out the best crevice to hide, go away." The bigger you are, the easier it is to take or hold control of a crevice. But that doesn't mean they don't fight over females, because I'm sure the males do that if it comes to that.
I also wondered about the bubbling around the mouth and if this was due to the switch from water to air.
Yes! There are some land crabs that do it defensively where they envelop themselves in bubbles, but most of the time, it's when they've been exposed to air for an extended period of time. The bubbles are to keep the gills moist I believe.
Although also found in parts of Asia, this striking little crab is native to North America and in abundance. So the next time you venture out to the coast taking in those jaw dropping views; stop and take a peek right at your feet. You never know what hidden wonders you might find!