UNDERWATER DRAMA

One night late in December, a small group of us decided to check out Puget Sound at Seahurst during a minus 1.5 foot low tide. At extreme low tides, you can find all sorts of marine invertebrates close up and personal. Unfortunately, low tides occur at night during the winter, so you have to go out with a strong flashlight and, most of the time, bundled up in parka and rain gear to have this experience. We were lucky: moderate temperature, no wind, no rain at 10 pm. The water was calm and clear as a windowpane, and the exposed beach was covered with critters.

It takes a while to get the hang of it. You have to distinguish stationary animals, many of them sessile filter-feeders, from multi-hued rocks and gravel and sand. Many of them are exposed by the receding tide, while others shelter in crevices, among algal fronds, or under rocks. On this visit, we looked mostly on the surface, as there were very few rocks of the right size to turn. But by doing this, you can find many more critters, including several species of fishes. But always replace the rocks carefully!

Mollusks of numerous kinds, including limpets, lined chitons (Tonicella lineata) and rock oysters (Pododesmus cepio), were scattered over the large boulders that were exposed by the low tide. There were scattered plumose anemones (Metridium senile) hanging down obscenely from rocks and Christmas anemones (Urticina crassicornis) looking like prettily patterned blobs. We found one attached to a movable rock, so we took it out in a foot of water to let it expand. We came back to it 10 minutes later, and it was nicely expanded for photos.

As we were photographing it, a kelp crab (Pugettia producta) approached from stage right. It came up to the anemone and started feeling in its tentacles. We wondered if it might be snipping them off to eat, but we couldn't be sure. More likely it was feeling for organic matter left among the tentacles, perhaps part of the anemone's last meal. Several times after inserting a cheliped among the tentacles, it brought it back to its mouth. After a short while of doing this, it started running one of its longer legs through the tentacles, almost in a grooming manner. Perhaps the leg was able to detect food particles?

 

Presumably the thick chitin of the crab's legs protected it from the anemone's stinging cnidocytes, and a coating of anemone mucus might have conferred additional protection.

As we were watching this, a red rock crab (Cancer productus) approached the kelp crab, and it scuttled away about a meter and sat among the eelgrass. The second crab didn't interact with the anemone but then found a dead Dungeness crab (Cancer magister), perhaps to check it out as dinner, but our lights may have scared it away. We had earlier found a pair of Dungeness crabs mating in an eelgrass bed.

Sea stars were the most conspicuous megafauna appearing in our lights, including sunflower stars (Pycnopodia helianthoides), many of them young, and ochre sea stars (Pisaster ochraceus), with fewer mottled sea stars (Evasterias troschelii). We turned one Pycnopodia over that had a hump in the middle and found what I thought was a fish skull clutched in its hot little tube feet. We grabbed the skull and started pulling on it and out from what must be an amazingly large stomach came a spotted ratfish (Hydrolagus colliei) carcass, largely digested!

On a previous occasion at the same site, we had found a Pycnopodia that had ingested a whole rat. We assumed the rat had been dead in the water rather than that the starfish had gone on walkabout up in the rocks. This huge sea star really seems at the top of the intertidal Puget Sound food chain!

Dennis Paulson

IT’S SHOREBIRD TIME AGAIN

Every year in April and May the Pacific Northwest experiences a mad rush of thousands and thousands of shorebirds—sandpipers and plovers—on their way north to their Arctic and Subarctic nesting grounds.

This is nowhere more obvious than around two big coastal estuaries, Grays Harbor and Willapa Bay. These estuaries are extremely productive of the invertebrates that the shorebirds eat. Prey animals are present at very high densities in the mud and sand of these estuaries, for example about 10,000/square meter at Bottle Beach, on the south side of Grays Harbor, one of the most productive areas.

This seems astronomical but pales in comparison with 100,000 individuals/square meter of one species of amphipod in the Bay of Fundy. Nevertheless, it is sufficient density to support the tens of thousands of shorebirds that visit the area for up to a month each spring. Individual birds stay only a few days, fattening up for a flight that may be only 200 kilometers to Roberts Bank in British Columbia or as much as 900 kilometers to the Copper River delta in Alaska, then on to arctic breeding grounds.

The invertebrates that fuel these birds are primarily amphipods and polychaete worms. The amphipods are about 5-6 mm long, and some of the polychaetes are in the same size range. Other polychaetes are called “thin worms,” up to 30 cm long but only 1 mm in diameter! They can be in such high densities as to almost bind the mud together.

The three most abundant species in spring migration are Western Sandpiper, Dunlin, and Short-billed Dowitcher, in descending order. Most of the big flocks you see will include these species. There are many other species present as well, and searching them out gives the observer variety as well as spectacle. Other common species include Black-bellied and Semipalmated Plovers, Greater Yellowlegs, Whimbrel, Marbled Godwit, Ruddy Turnstone, Red Knot, Sanderling, and Least Sandpiper, with numerous others even less common but out there somewhere.

No migration is evident at the beginning of April, but by the middle of the month, Greater Yellowlegs and Short-billed Dowitchers have arrived in numbers. During the last two weeks of the month, all the species move in, most of them in full breeding plumage, and peak right around the first of May. Numbers fall off after that but are still impressive through mid May, and some of the later migrants are present until the end of that month. Species that peak late include Red Knots and Long-billed Dowitchers.

Herb Wilson of Colby College, Maine, found that these hordes of shorebirds had little effect on the invertebrates on which they fed. He erected “exclusion cages” at Bottle Beach, meter-square cages that kept shorebirds out, and after the migration compared the numbers of invertebrates under each cage with the original numbers there and the numbers just outside. He found no fewer invertebrates outside than inside; thus the numbers had not been reduced by the birds.

The spectacle can be seen in feeding areas at lower tides and at roosts at higher tides. The ocean beach itself has throngs of shorebirds on peak migration days, but it is discouraging to see the high levels of human activities (joggers, horseback riders, dog runners, mopeds, pickup trucks) that disturb each flock again and again on a busy weekend day. The birds get back to feeding immediately after each disturbance, and we can only hope that they are able to take in enough nourishment to make it to their next destination on time.

Dennis Paulson