Fiddlers on the Roof
Canopy feeding crabs in a Cape Cod marsh
By Dave Grant and Nancy Church

Fiddlers at night. (Grazing and getting entangled in spider webs!)

The Swan River is one of the prettiest water bodies on Cape Cod. As the crow flies, it is about two miles from the river's source to Nantucket Sound - but twice that distance, as the swan drifts and the river meanders to its mouth on the south side of the Cape. One of several interesting features about the river is its narrow tidal inlet, which is not much wider or deeper than a motel swimming pool. It has a jetty on its west side and a small spit on the other, so it is a place that can be explored and fished in a variety of ways.

In spite of its small size, there is good tidal mixing in the lower reaches of the river and it creates a rich estuarine environment in the middle of one of the most densely populated spots along the shore. Most of the river's banks are the property lines and lawns of older homes and summer rentals, but some residents wisely allow a buffer of shrubs and trees to persist.

Motor craft are absent and on any warm day "in season" scores of canoeists, kayakers and paddle-boaters - residents and renters alike, - cruise the river between Swan Pond in Dennis, and the Sound. About 140 acres of salt marsh border the river and these wetland areas are widest downstream where the waters gently flow into the sea.

We like to explore here in September for several reasons; not the least of which is a free bungalow on the water's edge. Also, it is peak bird migration season, there is an abundance of life in the water, the crowds are gone - and so are the bugs. In the summer, gnats (no-see-ums, or sand flies - the dreaded and abundant Culcoides flies) can be maddening on calm days. (The landlord's shaggy dog story of the summer involved a couple that rented the cabin and called an exterminator to rid the area of these ubiquitous pests!)

The view from ship or shore is equally satisfying on the Swan, but close-up views of the marsh are obscured by shrubs or tall grasses along the edges. Tall, "high-vigor" cordgrass (Spartina alterniflora) at the creek's edge forms a barrier to the short, low vigor cordgrass and salt hay (S. patens) of the upper marsh. And since those areas are only flooded thoroughly on the spring tides, they are less accessible or inviting to paddlers.

One evening, while seeking the darkest possible spot for star-watching, we set up at the end of a long dock stretching across the marsh. It was a classic September night on Cape Cod - clear and calm. Too calm. After a short time orienting ourselves with the array of constellations of the Zodiac (Capricorn and Sagittarius to the south) a light fog moved in off the sound and began to hide all but the brightest stars. So we had to be content with what was directly overhead, the dazzling points of the Summer Triangle, one of which is Deneb, the tail of Cygnus - the Swan. The star watch soon became a marsh study and instead we explored with flashlights.

Wetlands are wonderful at night, and peaceful conditions made this one especially magical. But the darkness offers little rest for the weary and the hunted. We could easily make out the wispy calls of migrating songbirds overhead, and there were rude squawks from herons and the slurps of bass gulping down their prey. The crescent moon hanging on the horizon meant spring tides too and the water was quite high. On the flood it not only inundated the whole marsh, but reached halfway up the stems of the stunted, foot-high grasses of the upper marsh.

We made the obligatory attempt to catch blue crabs off the deepest pilings (Our net was too short and the tide too high), and then settled down to harassing bait fishes with our lights.

Killies (or "chubs" as they are called locally) tend to freeze in place at the bottom.
(Is it a deer-in-the-headlights response, or do they not notice the light?)

Menhaden ("bunker" locally) twitch and drift aimlessly as the school disintegrates, and often swim right into the dip-net.
(Are they too blinded by the light or does this inhibit their normal defensive response which is to "ball up" into a tight crowd?)

Shiners ("silversides" locally) go berserk and start leaping out of the water in such numbers that it sounds like big raindrops if you target a large school.
(Does the reflection of the light off their schoolmates startle them, or is this a "stadium wave" response that is initiated by one startled fish?) Hold the light close to the surface and fish will leap up at it. Mullet are noted jumpers too, and when I visited Tarpon Springs, Florida I learned that the old Portuguese "spongers" believed the mullet always jumps three times - trying to leap into Heaven. I mused that perhaps these short-lived bait-fishes were, like humans, moving towards "the light" to escape the peril and their fate on earth.

Shiners under the light

In the shallow, upper marsh, several creatures immediately attracted our attention. At the tops of the half-submerged grasses we found fiddler crabs (Uca) perching out of the water by gathering several stems and pulling the tips together like teepee poles for support. My first thought was that they were simply caught away from their burrows by the rapidly rising tides "springing" up. Since the only ones we could examine closely were females, I also speculated that after the breeding season, they were no longer welcome in the burrows of the males, and so were wanderers trying to stay out of the reach of fishes, but subsequently we found males too.

Then we noticed tiny snails on the same stems. Was it possible the fiddlers were preying on them? Again, closer inspection revealed that some crabs were in a resting position (their eyes were folded into the sockets) but we did note that the snails were also concentrated on the dry tops of grass. Eventually we saw (and heard) fiddlers zero in on snails.

The saltmarsh snail, Melampus bidentatus, leads a particularly regimented life in the marsh and illustrates the complicated links between the tides and tidal creatures in this dynamic environment. No bigger than a pea, the snail is easily overlooked unless you know where to look for it; moving up and down the grass stems ahead of the tide. Since it is an air breather like us and its lung cannot function when submerged, the snail must constantly be on the move.

I've been told that Melampus "senses" the tide by the vibrations that resonate ahead of the rising and falling waters. In mythology, Melampus (the man) was a soothsayer who could understand the language of animals, and on one occasion, by eavesdropping on feeding termites, this gift allowed him to escape a building before the roof collapsed.

Melampus (the snail) escapes the marsh for just a short period in its early life. Their reproduction is also tied to the tides, and gelatinous egg masses are laid before the spring tides so that they will be covered by marsh detritus and kept moist until the next spring tide; which occurs in two weeks - the time it takes for the eggs to develop. That next moon tide releases the larvae into the plankton community, and the veligers take another two weeks to develop. The lucky ones which are not eaten and manage to drift back up to the highest parts of the marsh on the next spring tide, develop into adult snails. Although vast numbers of larvae must be lost each season, the process couldn't be better synchronized, and the snails are present in most marshes I've visited. Obviously the system works!

Melampus

Movement in the grass attracted our attention. Something was slithering along, parting the stems in an orderly manner. To our surprise it was a blue-claw crab; not living up to its name Callinectes (beautiful swimmer) but crawling along, half out of the water. Initially we assumed it was stranded and heading back to deep water, but it soon became obvious that the crab was feeling its way around the marsh - searching. The night was getting even more interesting.

By reaching out with its claws, the crab would gather an "armful" of stems, and by bending them down and methodically propping its way upward, was combing the grass for prey. This was confirmed visually in front of us, and audibly by soft crunches that we began to notice coming from another grass "cowlick" nearby. Apparently, other blue crabs were searching the meadow, gleaning snails (and presumably, fiddler crabs) from the tops of the marsh grass.

Our curiosity piqued, we returned the next night, and right on schedule with tide, we observed several blue crabs harvesting in the same manner. Watching them, I thought that if crabs had voices, they'd be working their way through the meadow humming something like:

"Sowing in the noontide and the dewy eve…;
We shall come rejoicing, bringing in the sheaves.'
(Knowles Shaw - The Singing Evangelist 1874)

Predator, scavenger or filter feeder? - crabs occupies a flexible position in the food web, depending on the situation. Apparently fiddlers are not confined to scavenging minute particles on the surface of the marsh during the day; and blue crabs may be more cosmopolitan in their habits than we thought. Vince Guillory, who hails from Louisiana (where half the Gulf Coast crabs are harvested), describes the blue crab's feeding strategies as:
"Raptorial" - hunting for large prey by swimming with outstretched claws;
"Interface feeding" - from the surface of objects and on sediment surfaces; and
"Plankton feeding" - consuming small suspended material.

Watching them in action, it's obvious crabs present a challenge to their neighbors throughout the marsh; sometimes in ways that surprise us. By exploiting the upper marsh at night and on moon tides, crabs manage to short-circuit the food chain and utilize energy sources that otherwise might not be available to them. We propose a fourth feeding strategy as described above; which, until we can come up with something more descriptive (or a better alliteration), we'll call "canopy feeding."

On the third night, a cold front, wind, bitter cold and heavy rains prevented us from making more observations. After that, the waxing moon and falling tide levels kept water and crabs out of the upper marsh, but the cold, crisp nights were great for star watching. Now we searched the sky with binoculars for faint star clusters - Messier objects like the Beehive in the constellation Cancer. But to our dismay the star chart indicates that at this time of year, Cancer - the Crab is blocked by the Sun during the day.

NOTE: Dave Grant is the Director of Brookdale College's Ocean Institute at Sandy Hook and previously wrote about hermit crabs and ocean crabbing (spending one entire year measuring crabs without suffering a single pinch.)

Nancy Church is the Interpretive Services Coordinator at Waquoit Bay National Estuarine Research Reserve; and was Director of Education at the Cape Cod Museum of Natural History.

Crabs and other Crustaceans links. Fish and snail links.

http://www.blue-crab.net/crab2.htm

Guillory, V., P. Prejean, M. Bourgeois, J. Burdon, and J. Merrell. 1996. A biological and fisheries profile of the blue crab, Callinectes sapidus. La. Dept. Wild. Fish., Fish. Mgt. Plan Ser., No. 8, Pt. 1.
Millikin, M. R. and A. B. Williams. 1984. Synopsis of biological data on the blue crab, Callinectes sapidus Rathbun. U.S. Dept. Comm., NOAA Tech. Rept. NMFS 1.

Warner, W. W. 1976. Beautiful Swimmers. Atlantic Monthly Press Book, Little Brown and Co., Boston.

Williams, A. B. 1984. Shrimp, lobsters, and crabs of the Atlantic coast of the eastern United States, Maine to Florida. Smithsonian Institution Press, Washington, D.C.