In today's post, we are going to be going back to the topic of native shore crabs in California. We will discuss their roles in the ecosystem and how to differentiate between two species. Although there are many species of crab found on our rocky shores, most tidepoolers come across these:
Family Grapsoidea
Hemigrapsus spp.
Purple Shore Crab (H. nudus)
Yellow Shore Crab (H. oregonensis)
Pachygrapsus spp.
Striped Shore Crab (P. crassipes)
Wow, that was a lot of Latin names! But what groups of crabs are these? Have you seen them before.
Well, lets begin with the family Grapsoidea. The name originates from Greek meaning "to grasp," which describes the pincers and grippy legs of these crabs. Crabs in Grapsoidea are fairly small (0.5- 1in) with square carapaces and eyes far apart. One may refer to this group as shore crabs, marsh crabs or (in some cases) land crabs.
Within that family, there are two common genuses of crab in California. There are Pachygrapsus and Hemigrapsus.
Within Pachygrapsus, we have the all-too-common Striped Shore Crab:
This crab is fairly common in tidepools and exposed beaches s where there are rock creases/crevices, but also prefers mud ecosystems like estuaries and mudflats.
The crab is characteristically square in shape with equal (same sized) pincers and flat legs. Their eyes are far apart, and their shell exhibits a distinct lined pattern (P. crassipes is also known as the Lined Shore Crab for that exact reason). In addition, this crab is usually mottled green/red/purple in color. Their pincers are gray early in life, and then develop into a bright rose red as they mature.
These herbivorous crabs enjoy dining on various algae, carrion (especially mussels) and occasionally other crabs, including their same species. In return, sea otters, birds, and other marine mammals eat this crab, which helps balance out the population.
Here are three distinguishing features of the crab.
It comes out at low tide to feed on algae by alternating it's pincers. It turns out that it is the only Grapsoidea crab in California that shows this behavior.
It has small, grippy hairs on it's toes that helps it grip on to rocks while running sideways or backwards. The crab is fast and nimble in and outside of water.
When it is out of water, it blows bubbles. Now, most crabs who live in saltwater do this, but the Striped Shore Crabs have a distinct way of doing it: the bubbles sometimes foam around the whole body.
You are most likely to find this crab when you go almost any rocky beach anywhere between BC, Canada, to Baja California, Mexico. No other Pachygrapsus crab exists in the west coast except for a similar species, the Pacific Mottled Shore Crab, Pachygrapsus socius, which is found south of Baja California.
P. crassipes often gets confused with the Hemigrapsus genus because they both inhabit similar ranges (and can be found living together at times). Striped Shore Crabs also tend to have the same colors as their cousins But they are very different creatures.
Hemigrapsus oregonensis or the Yellow Shore Crab is smaller crab that is more clumsier on land. Like Striped Shore Crabs, it can be found in bays, estuaries, mudflats but is rarely found in the open coast. Yellow Shore Crabs have the same diet as Striped Shore Crabs as well. This crab species contains minute leg hairs which also gives it the name Hairy Shore Crab.
Unlike Striped Shore Crabs, it rarely comes out of the water to feed, and normally prefers living under rocks at low tide.
It is usually yellowish green all over speckled with red or burgundy spots. However, juveniles can be white.
Above is a white variety of a juvenile H. oregonensis.
Here is another species of Hemigrapsus:
H. nudus, often called the "Purple Shore Crab," is both visually different and behaviorally different than P. crassipes. It is (usually) purple in color, with the exception of juveniles, which can be brown, olive green, red or white. They have a somewhat rounder carapace than P. crassipes and smaller eyes. Their legs are hairless (hence the name nudus "nude") with yellow tips. Their pincers have equal spots on them.
They normally look like this:
But juveniles of this species have more color diversity. Take a look at this green variety:
This crab doesn't even look like a purple shore crab! There are no spots on the pincers and no part of it is purple. But there are no hair legs are present, which can be very useful at times. Crabs molt (shed skin) every once in a while. Over time, juvenile purple shore crabs gain the normal purple color each time they molt. This particular variety, however, is known to persist well into adulthood.
The white variety looks exactly like the white variety of the Yellow Shore Crab! The hairless feature of the Purple Shore Crab comes in handy while identifying - while a novice identifier may not be able to tell the difference, a skilled identifyer could notice the lacking of hairs.
Features | Striped Shore Crab | Purple Shore Crab | Yellow Shore Crab |
Size | 1-1.75 inches | 1-2 inches | 1-1.5 inches |
Habitat | Rocky intertidal zones, under rocks, in crevices estuaries | Rocky intertidal zones, under rocks. (rarely) in estuaries, bay entrances, exposed area crevices | Rocky intertidal zone under rocks, mudflats, estuaries. Does not prefer exposed (direct subject to waves) areas. |
Colors | Mottled green, red purple, black stripes | Purple, (can be) green, white, yellow, brown | Green, yellow (can be) white, purple, brown |
Behaviour | Very fast, agile, aggressive in/out of water, will fight for foods/mates. Bubbles outside of water. It will come out of the rocks at low tide to feed on algae. | Agile and aggressive inside water, especially for territory. Outside of water is is more clumsy and spews out saltwater instead of bubbling. Sometimes emerges from water to feed.Species is more hyper in warmer waters. | Somewhat agile and sometimes aggressive inside water, especially for territory. Outside of water is clumsy and spews out saltwater instead of bubbling. Rarely emerges from water to feed.Species is more hyper in warmer waters. |
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