Naturalist Fact: Grass Shrimp

Grass Shrimp (Palaemonetes sp.)

grass shrimp

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Grass shrimp have multiple common names, including grass shrimp, popcorn shrimp, glass shrimp, glass pawns, jumpers, and hardbacks.  Compared to commercial species of shrimp, grass shrimp are quite small, rarely exceeding two inches in length.  These tiny decapods are often nearly transparent (hence the one common name of glass shrimp), and have a well developed rostrum or horn on the dorsal and forward portion of the head, much like many commercial shrimp species.  This ‘horn’ can be quite sharp and may aid in defense against predators.  Little teeth-like projections found on and around this rostrum are used to help determine the particular species of grass shrimp under a dissecting scope (different placements), although many errors in species identification are made due to the morphological similarities.

These shrimp exist in many estuarine environments, and are often associated with some sort of underwater structure such as submerged plants and very commonly, oyster reef habitats.  Structure such as submerged grasses or oyster beds provide ample protection from predators like fish, and also provide the shrimp with a food source.  These shrimp typically stay along the edges of creeks in very shallow water, but have been found up to fifty feet deep.  Grass shrimp are typically considered detritivores, feeding mainly upon dead or decaying plant matter; however their diets can consist of a wide variety of organisms.  In captivity grass shrimp have been known be cannibalistic, and will also predate on tiny polychaetes, oligochaetes and nematodes (marine worms) in the wild.  Because these tiny crustaceans often fall prey to fishes and other carnivores, they are extremely important as a link between trophic levels.  Energy and nutrients are transported between primary producers and higher trophic levels with grass shrimp being an important vector.  More research is needed to assess the populations of grass shrimp and how humans may be affecting these ecologically important organisms.

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Horseshoe crab and shorebird bonanza!

2013-04-29 horeshoe

Horseshoe crabs gather in large groups at nesting beaches in the days surrounding full and new moons.

Having inhabited the oceans for over 350 million years, horseshoe crabs (Limulus polyphemus) are a glimpse into prehistory. We spot them most often as dead carcasses or molts along the shoreline. However, right now we are seeing hundreds of live, healthy, active crabs at the waterline.

Birders at Sancho Panza marvel at the horseshoe crab "arribada."

Birders at Sancho Panza marvel at the horseshoe crab “arribada.”

Horseshoe crabs spend most of the year in deeper waters, but in the late Spring they begin to move inshore to mate in large aggregations. Hundreds are gathering at Sancho Panza Beach here on Little St. Simons Island. Mating activity peaks around the full moon and new moon cycles, and with the full moon last week, we found big groups of males fighting for the chance to fertilize some eggs, and several indentations in the soft, wet sand where undoubtedly nests had been left behind.

A female will push several clumps of eggs down into the sand with specially adapted appendages. Each clump can contain two to four thousand eggs, and over the course of her spawning season, a female will deposit around 90,000 miniature eggs! Of those 90,000 eggs, only about 10 are expected to mature into adult horseshoe crabs.

If you are familiar with our island, you know that Sancho Panza is an excellent shorebirding spot, and right now we are in the peak of Spring migration. It is quite the spectacle to see a variety of shorebirds eating to their little hearts’ content at the buffet of horseshoe crab eggs! In fact, horseshoe crabs are instrumental in the journeys of at least 20 species of migratory shorebirds along the Eastern Seaboard.

Ruddy Turnstone feeding at a horseshoe crab nest. (Photo: Pete Oxford)

Ruddy Turnstone feeding at a horseshoe crab nest. (Photo: Pete Oxford)

Sanderling feeding on horseshoe crab eggs. (Photo: Pete Oxford)

Sanderling feeding on horseshoe crab eggs. (Photo: Pete Oxford)

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Naturalist Fact: Ghost Shrimp

 

Images from georgialifetraces.com

Despite their name, ghost shrimp are actually more closely related to crabs. They are crustaceans that somewhat resemble tiny lobsters, but they have a body similar to that of a shrimp. Found year-round on Little St. Simons Island, these “shrimp” are expert burrowers. They spend their lives underground, digging and maintaining a protective tunnel system that functions as a burrow. To find a ghost shrimp burrow, simply look along the shoreline for tiny holes surrounded by fecal pellets that look a bit like sprinkles. But if you tried to catch one, you would most likely need a form of suction to remove them—they are very fast!

One of the most interesting facts about ghost shrimp is that they often share their burrow with another species. A tiny crab called Pinnixa cristata can usually be found inside a ghost shrimp burrow, using the protection and benefitting from the nutrient-rich environment. The symbiosis between the ghost shrimp and the Pinnixa cristata is referred to as “commensalism”: a relationship in which one species benefits (Pinnixa cristata) from another (ghost shrimp) without causing it any harm.

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Naturalist Fact: Blue Crab

Blue crabs (Callinectes sapidus) are a delicious resident of our island’s waters. They are members of the Order Decapoda, in which all members have ten legs. The blue crab has two legs modified as pinchers that aid in defense and feeding, six walking/climbing legs, and two legs that have adapted into a paddle shape that allow it to swim. These paddles are an excellent aid in mobility, as other species of crabs are restricted to the bottom or structures on which they can climb. Their common name comes from the bright blue color exhibited on the mature crab’s pinchers and legs, and their Latin name literally means “savory beautiful swimmer.”

Blue crabs are most abundant in our saltmarshes and tidal creeks in the late spring and summer months, and can be caught in traps, or simply by tying some bait to the end of a string, lowering it into the water, and waiting for the tug of a hungry crab. Blue crabs are omnivores, feeding on a variety of things including fish, oysters, snails, other crustaceans, and plant material. They will also scavenge the carcasses of dead marine life. Chicken is a popular bait when fishing for crabs.

As crabs grow, they periodically shed their exoskeleton, or molt. They separate from their shell and scoot out backwards, leaving the intact shell behind. Sometimes crabs will consume their molts for the calcium. The crab’s body expands (grows) while its shell is soft, and then will harden into a new exoskeleton. This process usually takes a few days. While they are in the state of “soft-shell,” the crab is very vulnerable and will take cover until their new shell has hardened.

Female blue crabs will only mate once during their lifetime and this happens after what is called the terminal molt. After the female sheds her shell for the last time and is still in the soft-shell stage, a male will mate with her and then guard her until she her new shell hardens. Later, the female will develop a spongy egg mass under the apron on her abdomen, which she will carry with her until the larvae hatch. Since females can only mate once, it is a common practice to release any females that are caught.

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