Beach-walking in late March

crow behaviors, elegant swans, boring (literally) clams, and a robbery on ice

What’s a restless naturalist to do, when most of the ground in covered with soggy snow and strong signs of spring seem reluctant to appear? Although the varied thrushes and juncos are singing, and the skunk cabbage has poked up in some places, some of us get a little impatient for more. One option for our edification is walking the beaches in hopes that something of interest will show up (it almost always does!).

On the way to Crow Point via the Boy Scout trail, small gaggles of garrulous geese flew from the river over to the far end of the wide, flat meadow. (Do they ever stop talking, except maybe when sleeping??)

Crows were beach-combing, dozens of them, all scattered along the line of the advancing tide, perhaps nabbing small prey that were activated by the onward rush of water.

Overhead, winging northward, were three elegant trumpeter swans. Although most trumpeters nest in the Interior, a few nest near the north end of Lynn Canal. It’s always a treat to see them, if only in passing, and that was the main ‘payoff’ for post-holing my way out to the beach. There had been strangely little foot traffic out this way, so post-holing was the only way to go.

On another day, for no accountable reason, I suddenly was possessed of the notion to look for mermaid’s purses. These are the egg and embryo cases of skates (relatives of rays and, more distantly, sharks). So I went to a beach where I’ve occasionally found them before, washed up on a big tide. Bingo! I found four of them: one black, dry, and dead, two much fresher, yellowish green, but ripped open, perhaps by an enterprising raven, and the embryos gone. The other one was intact, and I tossed back into the water, in case the embryos had survived their stay on the beach.

A short chunk of timber had washed up on the shore of Eagle Beach. It was completely riddled by smooth tunnels, about the diameter of a finger. Aha! Teredos had been at work. A.k.a. shipworms, for their unpopular habit of mining into wooden ships, teredos are really molluscs, albeit with a worm-like body. They can get to be over two feet in length, long, soft, and quite slender, with two very small, ridged shells at the front end. Those shells grind their way into wood as a teredo rasps its way along. The wood particles are digested, with the help of symbiotic bacteria that live in special cells in the animal’s gill. Teredos also eat plankton, which are inhaled by a siphon at the rear end of the body, and pass over the gill on the way to the mouth.

A teredo gets its start when a free-swimming larva finds a suitable piece of wood and settles down, attaching itself by means of byssal threads, such as one sees on mussels and other sedentary molluscs. The larva softens the wood at the attachment point, and then transforms into the adult shape and starts boring. Naturally, the tunnel gets bigger as the teredo grows and moves ahead.

Teredos are somewhat related to piddocks, another kind of clam that tunnels into hard substrates (and which have appeared in these essays before). Piddocks are clearly recognizable as a type of clam, but the stout shell is more curved and has ‘teeth’ along the edge. Piddocks use their burrows just for shelter and filter-feed on plankton, like ‘normal’ clams. A likely evolutionary link between teredos and ordinary piddocks is a group of distinctive wood-borers that use symbiotic bacteria to aid digestion, as teredos do, but the shells are more complex and substantial than those of teredos, a little more like the huskier ones of piddocks. Tunneling into wood (and rock, in the case of piddocks) is the way of life for a considerable array of clams, apparently, a surprising twist on our conventional view of more familiar, sediment-dwelling clams.

At Twin Lakes, the ice was dotted with open holes. A friend watched a river otter that was actively fishing—it caught and ate almost a dozen fairly small fish in a couple of hours. Then it caught a starry flounder; an eagle was watching and came down to snatch it away, but the otter saw the eagle coming and dove quickly, holding onto its prey. Later, the otter caught a big staghorn sculpin, hauled it out onto the ice, and began eating it, looking around for that thieving eagle. Down came the eagle, very fast, and ripped the sculpin right out from the otter’s feet. The otter dove beneath the ice, and the eagle had a good breakfast.

otter-just-before-eagle-stole-fish-Jos
The moment before the theft. Photo by Jos Bakker

Thanks to Aaron Baldwin, ADF&G, for expanding my knowledge of teredos and their relatives.

Mermaids’ purses

…and their cartilaginous currency

alaska-skate-egg-case-by-gerald-hoff-afsc
Photo by Gerald Hoff, Alaska Fisheries Science Center

This is a fanciful name for the egg cases of skates, which are cartilaginous fishes related to rays and, more distantly, to sharks. When some friends found a few of these egg cases on North Douglas beaches, I got interested in learning more about them. Not being a marine biologist, I had to do a bit of digging, but I got a start with the help of a genial skate biologist at NOAA. Here are some of the things I think I have learned.

Skates put their eggs into tough, leathery cases. The cases are deposited in traditional nursery sites that are used year after year. According to the skate biologist, there are two skate species in our area that use relatively shallow-water nurseries and are the most likely ones whose egg cases occasionally appear on our beaches. These two are the big skate and the longnose skate. Beachcombers should be able to distinguish the egg cases of these two species quite easily: Egg cases of big skates are typically more than eight inches long with short stubby horns at the four corners; those of the longnose skate are about four inches long, with slender horns. When the cases are laid, they often bear tufts of sticky threads that help stick the case to the seafloor; these threads may get worn off on cases that wash up on beaches. Most egg cases that show up on beaches are empty; the young skates are gone.

Most skates, including the longnose, put only one egg in each case, but the big skate may have as many as seven or so in each case. There is little available information on how many egg cases a female skate produces each year, but it is only a few hundred at most or, in some instances, much less. The growing embryos are well endowed with abundant yolk and the cases require considerable material and energy to make; this large parental investment per embryo means that the number of embryos must be fairly small. The case stays closed for several weeks, and then slits open in the horns, letting in sea water and oxygen; the embryo develops a temporary filament on the tail, and this undulates in one of the horns to facilitate the movement of water in and out. The eggs incubate in their cases for a long time, averaging about nine months for the big skate (but some other skates living in very cold water, such as the Bering Sea, may have incubation periods of several years!).

Predatory snails can bore into the cases and eat the contents; in some situations, over 40% of egg cases have been depredated by snails. In the Bering Sea, the intensity of predation was lower in nurseries with high densities of egg cases, suggesting that there is some safety in numbers—(this is referred to as predator satiation, or predator swamping, or the selfish herd effect). After the lengthy incubation, when the eggs finally hatch, the ‘pups’ are miniature versions of their parents. They have many predators, and juvenile survival is undoubtedly very low.

Both big skates and longnose skates can, but apparently seldom do, live for more than about twenty years. Big skates in the Gulf of Alaska were estimated to mature at about five to nine years of age, longnose skate at age nine to twelve years. Females mature somewhat later in life than males, particularly in big skates.

Skates eat a variety of fishes and invertebrates, including crabs, octopus and squid, and worms.

alaska-skate-by-alaska-fisheries-science-center
Photo by Alaska Fisheries Science Center

Around the world, some skate populations have crashed dramatically, due to overharvesting plus numerous by-catch captures in fisheries directed at other species; by-catch captures are just discarded. Both big skates and longnose skates in Alaska are increasingly subject to both directed harvest and by-catch mortality, and fisheries biologists report declining numbers of these species. Therefore there is both management and conservation concern for these species.

The North Pacific Fishery Management Council recently identified six skate nursery areas in the Bering Sea as Habitat Areas of Particular Concern. These areas are not closed to fishing but will be monitored for changes in skate egg density and other factors. Consultation will be needed for proposed activities that might modify habitat or otherwise impair skate reproduction, such as drilling or laying of cable in these areas. There was no report of any quasi-protective measures for the Gulf of Alaska.

All you beach-walkers, take note: if you find a skate egg case in your peregrinations, please send a good digital photograph of it, with precise location information, to jerry.hoff@noaa.gov. He is keeping records in a database and will identify the egg cases you find.