Published February 10, 2018 in the
“Ocean Watch” column, Honolulu Star-Advertiser ©2018 Susan Scott
A lot of us beach walkers are crabby, and I mean that as a compliment.
Last week a reader spotted two rock crabs, or aama, at Maunalua Bay that gave her pause. Barbara emailed, “Do the crabs blow bubbles? I came across a crab with fine white bubbles on its eye area. … The bubbles seemed to dissipate within a few minutes and then the crab scuttled away.”
Crabs that spend part of their lives in the water, and part out, can blow bubbles. This foaming-at-the-mouth might look like the crab is in distress, and sometimes it is, but in healthy crabs, mouth bubbling comes from the crab breathing air instead of water.
All crabs have gills, and all gills need to be wet to work properly. Crab veins bring carbon-dioxide- loaded blood to the gills. There the blood offloads this gas, a waste product of metabolism, and gets a fresh hit of oxygen. Arteries carry the recharged blood away from the gills, delivering it through the body.
Crab blood carries its oxygen with copper molecules rather than iron and, therefore, is blue instead of red.
Crab gills come in pairs, each housed in a chamber beneath the top shell near the creature’s front. To keep respiratory current flowing, the crab beats a tiny paddle, called a bailer, at the base of each of its two front claws. The water or air exits through two holes, one on each side of the mouth.
You might say, then, that a crab breathes in through its legs and out through its own version of nostrils.
When it comes to getting oxygen, the aama is flexible, extracting the essential gas from water or air, depending on where the creature is scavenging. When an aama is on a rock pushing air past its gills, because they’re wet, the exhaled breath comes out in bubbles.
Barbara saw another aama with a bulging lower flap holding a spongy egg mass, and wondered whether rock crabs give birth to live crabs.
Sort of. Females of this genus (there are eight species, one being the red Sally Lightfoot) carry fertilized eggs under the belly flap for about three weeks and then drop the hatchlings in the water.
Crab hatchlings don’t look like crabs, but rather are larvae that drift around as plankton. Survivors go through several molts before they look like crabs.
Hawaii hosts about 200 crab species in a wide variety of shapes, colors and habitats. They have one trait in common: pluck. If they can run and hide, most crabs will do so. If cornered, though, crabs defend themselves, raising their claws like little boxing gloves to fight even enormous monsters like humans.
Hawaii’s shoreside crabs can breathe air or water, run sideways, eat most anything and fight for their lives with remarkable courage. In my view, calling someone crabby is a good thing.