Pacific Northwest
Summer 2017
Crabs are ubiquitous and abundant in marine waters, and that’s a good thing. They are a major source of food for all kinds of creatures in the various marine ecosystems from sandy beaches to tide pools to the deep sea. And they fill many niches as predators, herbivores and scavengers. A healthy ocean needs lots of crabs.
But not THIS one — the European Green Crab is spreading now into the Salish Sea where we fear it will be as destructive as it’s shown itself on other West Coast shorelines and the East Coast. What’s so bad about one more crab among the multitudes? and how do we deal with it?
The Daily Bucket is a nature refuge. We amicably discuss animals, weather, climate, soil, plants, waters and note life’s patterns.
We invite you to note what you are seeing around you in your own part of the world, and to share your observations in the comments below.
Each note is a record that we can refer to in the future as we try to understand the phenological patterns that are quietly unwinding around us. To have the Daily Bucket in your Activity Stream, visit Backyard Science’s profile page and click on Follow.
|
Green crabs, Carcinus maenas, are native to Europe. By the early 19th c they had
invaded the East Coast of the US and are now irretrievably established there, although there are efforts to reduce their impact in certain areas. Global warming has played a role in the spread of this invader: as ocean waters have warmed in recent decades, Green crabs are now as far north as Maine. Sometime in the 1950s-60s they stowed away in bait-worm canisters shipped from Maine to California, and are considered too widely spread now to eradicate there. For decades, no Green crabs were seen in the Pacific Northwest, but then adult crabs began appearing in the quiet bays of southern Washington state open coastline in 1998. While invasive species are sometimes introduced as larvae hitchhiking in ballast water that ships pick up elsewhere, Washington state regulations prohibit dumping ballast water within 50 miles of our coast. The current hypothesis is that crab larvae drifted northward in warm ocean currents during the strong El Niño years of the late 1990s. Green crabs are so well established in our coastal bays like Gray’s Harbor state biologists consider it as impossible to eliminate them as the crabs in California and the US East coast.
What’s so bad about another crab? I mean, we already have 20+ species in the Northwest.
Well, it turns out Green crabs not only eat shellfish voraciously (East coast blue mussel and soft shell clam fisheries have been hit hard) but worse, they excavate half a foot down into seagrass beds searching for crabs and clams, uprooting plants, destroying that habitat. Eelgrass beds are among the most productive marine ecosystems.
In areas where the crabs have been able to establish reproducing populations they have had dramatic impacts on other species, particularly smaller shore crab, clams, and small oysters. While the crab cannot crack the shell of a mature oyster, it can prey upon young oysters, and will dig down six inches to find clams to eat. One green crab can consume 40 half-inch clams a day, as well as other crabs its own size.
The green crab is most often confused with native helmet crabs or hairy shore crab. The most distinctive feature is not its color – which can vary from reddish to a dark mottled green – but the five spines or teeth on each side of the shell. There are three rounded lobes between the eyes. The carapace is broader than it is long, and seldom exceeds 3.5 to 4 inches across. — wdfw.wa.gov/...
After a few Green crabs were seen in Padilla Bay and San Juan Island last summer, there was a hopeful lull — no observations over the winter. Then this spring some juveniles showed up at Dungeness Spit, on the southern shore of the Strait of Juan de Fuca, presumably having newly drifted in from the open ocean or the west coast of Vancouver Island. Baited traps were set by volunteers working with US Fish and Wildlife Service (USFWS) and Washington Department of Fish & Wildlife staff (WDFW) and over the next three months 76 were captured, before tapering off in June. That could be good news, but it might mean any crabs are hiding out as they do during mating season, uninterested in bait.
Biologists say the numbers and distribution are limited enough right now that Green crabs can be eliminated before becoming established as they are elsewhere. However given the proximity of reproductive populations and incoming currents, the Salish Sea is at risk and will continue to be indefinitely. The only way to prevent Green crabs from getting out of control is to be watching for them and catching them.
If you live in the Salish Sea area or are visiting, please keep an eye out for these invaders. Maybe we can buck the trend, keep this inland sea Green-Crab-free. There are enough threats to marine biodiversity present here already.
How to differentiate Green Crabs from our native crabs:
Crab carapace shape is the easiest way to ID them, although other features like leg length/proportions, shape of margin notches, hairs, texture and bumps are useful too. Remember, crab color is not a reliable way to identify a species.
These are the most common crabs you are likely to come across in the Salish Sea that might be mistaken for a Green Crab:
The BigGER Crabs:
Red Rock Crab
These handsome crustaceans are very abundant and an important food source for otters, gulls, fish, octopus and lots of other creatures in the Salish Sea. They, along with Dungeness crabs, are big enough to attack and eat Green crabs. Yay!
Dungeness crab
Our most famous crab, and the only commercially valuable one, is the Dungeness crab. Dungeness crabs prefer deeper water than Red Rock crabs so I usually see them as shells on the beach.
Alive, Dungeness crabs are not that bright orange we see when weathered (and cooked).
Juvenile crabs often have a different color/pattern and carapace shape than the adults of that species, as well as being smaller. Look at features like number of teeth to be sure you’re not mistaking a young, small Red Rock or Dungeness crab for an adult-size Green crab.
|
|
The smaller crabs:
First, the shore crabs, which have a similar square carapace. These can be found under rocks in the intertidal even at low tide. Crabs need to keep their gills wet constantly — that’s how they “breathe”. The shore crabs can manage that by keeping under cover, not necessarily immersed as most require.
|
|
Hairy Helmet crab:
These crabs are subtidal so I only see them washed up dead or as molts. They are also seasonal, around in spring.
Black-clawed crabs and Porcelain crabs are fairly common under small boulders in the intertidal.
(As a sidebar on the Porcelain crab’s name:
Porcelain crabs come in a variety of colors, as most crabs do, but it’s not their appearance that gives them their name. Rather, it’s their behavior of losing legs readily when attacked — an emergency tactic allowing them to escape — that makes them seem as fragile as a porcelain object. All crabs have regenerative capability, and Porcelain crabs grow back their legs more quickly than most, just a few weeks. (more about moulting and regeneration)
As a side note to crabs’ ability to deliberately shed its limbs: there is a whole industry dedicated to declawing crabs to sell those for food, which some defend as sustainable since crabs in nature can regenerate new legs. However studies have shown that declawed crabs are at a big disadvantage in nature, both in defending themselves and in finding food, having to shift from predation to scavenging. Recaptured declawed crabs indicates a 10-20% survival rate (South Florida Research Center). Declawing also removes more of the leg, leaving a bigger wound to heal. There is also evidence that crabs feel pain:
Some of the most compelling evidence for pain in invertebrates exists for crustaceans in terms of trade-offs between stimulus avoidance and other motivational requirements. Evidence of the ability for crabs to feel pain is supported by their possessing an opioid receptor system, showing learned avoidance to putatively painful stimuli, and responding appropriately to analagesics and anaesthetics. These all indicate it is likely that crabs can experience pain during declawing.
It has been argued that because crabs can autotomise (self-amputate) their claws, manual declawing along these natural fracture planes will not cause pain. However, a lack or reduction of pain during autotomy remains to be verified. Moreover, declawing results in a physiological stress response in the edible crab, as indicated by increases in haemolymph glucoseand lactate and a decrease in glycogen. This stress is evident both in the short term (< 10 min) and the long term (24 h). Further, declawing is more stressful, results in bigger wounds and causes significant mortality compared to induced autotomy.[11] -Wikipedia)
Heart crab:
Spider Crabs have long skinny legs and a carapace pointed in front. They are a favorite prey of gulls. Our most common spider crabs are these —
|
|
Did you note the differences in carapace shape, legs and other features? Would you be able to ID a Green Crab now with confidence?
Please keep a look out for these invaders, whatever coastline you happen to be on. Even where Green crabs are established, there are ongoing efforts to monitor them, such as in Northern California and Maine.
Here in the Salish Sea, Green crabs are still in the early stage of invasion and there is a chance of controlling them. It takes beach-walkers paying attention and reporting their presence. Please use the recommended protocol below.
If you believe you have found a European green crab in Washington waters please follow these steps to notify us:
- Reference our webpage on how to identify European green crabs, and distinguish them from native species. There are several native species of similar size and color.
- Take several pictures from different angles and distances to aid in confirming the identification. Be sure to place a common object or indicator of scale in the photo, so we can tell how large the crab is. Pictures are necessary and sufficient for verifying the crab’s identity.
- Leave the crab in place. It is illegal to possess a green crab in the State of Washington without a special permit. But don’t worry, we will follow up quickly if it is a European green crab.
- Email your photos and detailed location information, geographic coordinates if you have them, to crabteam@uw.edu. We will contact you to if we need additional information.
- wsg.washington.edu/...
Nature observations in your area? We’d like to hear about it in the comments!
"SPOTLIGHT ON GREEN NEWS & VIEWS" IS POSTED EVERY SATURDAY AT 5:00 PM PACIFIC TIME AND WEDNESDAY AT 3:30 ON THE DAILY KOS FRONT PAGE. IT'S A GREAT WAY TO CATCH UP ON DIARIES YOU MIGHT HAVE MISSED. BE SURE TO RECOMMEND AND COMMENT IN THE DIARY. |
(Note: I’m experimenting with using tables to show side-by-side images. Feel free to offer suggestions on technique.)