A Little Crab with a Giant Impact

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The Asian shore crab.

They were tiny and they were EVERYWHERE. When my college marine biology class had been assigned to go to this pebbly beach, look for Asian shore crabs, and record information about what we found, I had expected that we might discover a few. But by lifting some of the stones a few inches under the clear water we had sent Asian shore crabs skittering all over the place! Our professor’s reason for bringing us to this beach was that the abundant Asian shore crab didn’t even belong there. The crab was an invasive species. This memory was jogged a few weeks ago when the name Hemigrapsus sanguineus (the scientific name for the Asian shore crab) caught my eye while I was looking for an interesting creature making waves in Connecticut to write about. In 2018, Asian shore crabs remain an invasive species threatening to the environment and to certain fisheries in New England2 – and we continue the struggle to control the crabs, even if that means turning them into a seafood dish themselves4!

Before we get into what the Asian shore crab is doing as an invasive species, it is important to make clear exactly what is meant by “invasive species”. The Biology Online Dictionary defines an invasive species as a ” Non – native species disrupting and replacing native species (2018)3“. The Asian shore crab, since its 1988 arrival in the U.S., has been up to just that1.

A ship disposing of ballast water from a likely distant location. Marine Invasive species have been known to get transported in ballast water.

The first sighting of an Asian shore crab on the US coast was in New Jersey’s Cape May County1. It is believed to have arrived from its native range in ballast water – water used to lower the center of gravity of a boat that has lost a large amount of cargo so as to keep it stable. This water is taken in by the ship in one place when it is empty, and discharged into the environment wherever the ship is loaded with new cargo2.  Since 1988, however it got here, the Asian shore crab has exploded north as far as Maine and south as far as North Carolina. Populations are still growing, and it is expected that the creature will increase its range still further4. The walnut – sized crab with its distinctive banded legs7 has become a common sight indeed. Asian shore crabs have now been seen on the west coast as well, having been carried across the Pacific Ocean in tsunami debris4. Where did all of these crabs originally come from? Well, Asian shore crabs are native to the Western coast of the Pacific Ocean from Hong Kong north to Russia – an impressively vast range2.

A crab with its many eggs. Female Asian shore crabs are known to produce eggs more often per mating season than many other species of crab. This contributes to the rapid population growth of the Asian shore crab.

Some of the characteristics of this crab that have allowed it to occupy such a giant native area are also among the factors that make it so volatile as an invasive species here2. For one thing, Asian shore crabs can thrive in a broad spectrum of temperatures. The crabs also don’t mind watery homes in quite a variety of salinities1. The Asian shore crab has been described as “a highly opportunistic omnivore (Narragansett Bay Research Reserve Coastal Training Program, 2010)”, which means that it will eat almost anything1. And when an Asian shore crab eats, it is known to really pack away the food – leaving much less for other species. The burgeoning of Asian shore crab populations is also due to the fact that the crab reproduces at an epic rate. A female can mate and bear eggs up to four times per mating season, with each clutch numbering around 5,000 eggs1!

A very young larval lobster, popular prey for the Asian Shore Crab which is rapidly increasing in number.

This ravenous, reproductively busy crab that thrives in water cool or warm, salty or not1, is wreaking havoc in the lives of the species that it has joined here in New England. Some of the smaller species are harmed simply by being eaten2. For example, the crab is known to feast on lobster larvae1.  Other species are affected less directly, as the Asian shore crabs tend to set the entire food web off kilter. These species, often important economically, include fish, shellfish, and other crabs2. The Asian  shore crab is likely to replace some rock crab species completely1, and is destructive to our local biodiversity2. Biodiversity, the existence of a variety of species in an ecosystem6, is very important to the wellbeing of that ecosystem. This is because each species in the ecosystem plays a role that makes it important to the existence of the other species5.

Given the damage that the Asian shore crab is doing, the need to stave off its expansion is obvious. As with many invasive species, however2, it is nearly impossible to do so. While The Rhode Island Marine & Estuarine Invasive Species Site mentioned in 2010 that no method for controlling the Asian shore crab had been developed1, other somewhat newer sources mention ways in which people are attempting to keep the Asian shore crab at bay2, 4. It has been suggested that boats be forbidden to release ballast water in places where it is foreign. Thus, even if the Asian shore crab can’t be controlled, new individuals won’t arrive and future invasive species might be kept from joining coastal environments. Another suggestion  was that ballast water be treated before release such that invasive species can not be unleashed. Such changes, however, could be very expensive to institute1. Another idea that has surfaced has been to use the small crabs as a replacement for horseshoe crabs in fishing bait. Asian shore crabs are also being eaten in an attempt to put a dent in their populations. One restaurant serves Asian shore crabs with sushi rolls, and other recipes including the crab have been created. Asian shore crabs can be fried, accompanied by a pineapple salsa, or served in coconut milk4. Following are some Asian Shore Crab Recipes.

Some hope to use Asian shore crabs to replace horseshoe crabs like this one as fishing bait.

The Asian Shore Crab was enough of an issue in New England to be the focus of a lengthy marine biology project in 2004, and the Internet is still teeming with articles on the problem today. Clearly, though admirable efforts are being made to stave off the problems that this crab is causing, the struggle is far from over. Hopefully future endeavors will be effective, or at least we can try to keep new invasive species from arriving on our shores2.

Works Cited

1. Narragansett Bay Research Reserve Coastal Training Program. “Hemigrapsus sanguineus”.
The Rhode Island Marine and Estuarine Invasive Species Site, 2010, http://www.ri.meis.org/species/hemigrapsus.html.

2. Herbosa, Christian. “The Story of the Invasive Asian Shore Crab.”
Sea, Sand, & Sky – Save the Harbor / Save the Bay’s Boston Harbor Blog, 9 May 2014, http://blog.savetheharbor.org/2014/05/the-story-of-invasive-asian-shore-crab.html.

3. “Invasive Species.”
Biology Online Dictionary, 3 October 2005,www.biology-online.org/dictionary/Invasive_species.
Accessed 12 April 2018.

4.”ASIAN SHORE CRAB”.
Eat the Invaders, 17 May 2017, http://eattheinvaders.org/blue-plate-special-asian-shore-crab./
Accessed 13 April 2018.

5. Neira, Maria. “How does Biodiversity loss affect me and everyone else?”.
WWF, 2017, wwf.panda.org/about_our_earth/biodiversity/biodiversity_and_you/.

6. “Definition of BIODIVERSITY”.
Merriam – Webster, 2018, www.merriam-webster.com/dictionary/biodiversity.
Accessed 15 April 2018.

7. Salem Sound Coastwatch. “Hemigrapsus sanguineus“.
GUIDE TO MARINE INVADAERS IN THE GULF OF MAINE,http://www.rimeis.org/species/id_cards/invaders/h_sanguineus.pdf.

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