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On the Trails: Mermaids' purses

Posted: March 1, 2013 - 1:00am
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An Alaska skate, which has been studied intensively in the Bering Sea.  Courtesy of Alaska Fisheries Science Center
Courtesy of Alaska Fisheries Science Center
An Alaska skate, which has been studied intensively in the Bering Sea.

The name “mermaid purse” is a fanciful moniker 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 the National Oceanic and Atmospheric Administration.

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 percent 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.

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.

• Mary F. Willson is a retired professor of ecology.

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