WHY DO STARFISH BEHAVE LIKE THIS?

by Whit Gibbons

March 8, 2015

My grandson Parker sent me an email with a beach photo and one sentence that said, "Do you know why they are behaving like this?" Before I saw the picture I assumed he was referring to the antics of some high school- or college-age boys and girls on the beach. Turns out the photo was not of people but of starfish. It looks like an old screensaver I once had depicting countless starfish on the ocean's bottom disappearing in the distance. But the screensaver starfish were on the sandy bottom of the sea. The photo from Parker was taken at low tide, and the ocean was no longer above the starfish. For the most part, except for some that washed up in small tidal pools, they were stranded starfish destined to perish on a sandy beach near Charleston, S.C.

As is often the case when I encounter rarely seen natural phenomena, I turned to an expert for an answer. I contacted Meg Hoyle who runs Botany Bay Ecotours in Edisto, S.C. "Amazing sight to see!" she responded. "Like so many things in the ocean, there is so much we DON'T know, and we never know what we will find. If you love nature, that's what makes a trip to the beach so exciting. Starfish strandings are typically a winter or early spring phenomenon, but the full explanation of why they are stranded is not known."

Meg indicated that some ecologists have observed that mass strandings can occur after several days of strong east or northeast winds. The winds send waves that sweep the ocean floor of creatures that cannot swim away from shore and carry them up and in toward the coast. "Starfish and jellyfish can move, but they are not great swimmers and are at the mercy of strong waves or currents," Meg explained.

She noted that the starfish phenomenon has been reported from other islands along the South Carolina coast as well as other parts of the world. Mass gatherings of animals are often related to reproduction in a species. But the complete reproductive cycles and associated behavior are known for only a tiny fraction of the world's starfish species. Nonetheless, although scientists may not know all the details of the process, a breeding congregation is certainly vulnerable to natural events such as high winds and wave action.

Some conservationists have suggested coastal dredging or commercial trawling as possible causes, but many beach-stranding phenomena may be completely natural. "What I like to remember when I see such events," said Meg, "is that these are the ones that did not survive naturally and then to imagine how many are still out there. Another coastal creature I see millions of in the winter are tiny sea cucumbers that wash ashore. It is truly mind boggling."

Meg mentioned another seasonal event of coastal waters that happens with high predictability - horseshoe crab nesting. "In St. Helena sound south of Charleston on the April full moon, thousands of horseshoe crabs end up stranded. One night I was walking along the beach and had to stop because there were so many horseshoe crabs, there was no place to walk without stepping on them!" Imagine that with every step you took you crushed a big china plate that cracked underfoot.

Such an experience would certainly be memorable and induce wonder at Mother Nature's ways. Horseshoe crabs live exclusively in ocean waters, and a mating strategy that strands them on a beach seems a bit counterproductive. But like many other marine species, a single horseshoe crab can release thousands of eggs. "High mortality rates are not unusual and apparently can be tolerated as part of the survival strategy by these and other species."

So the answer to the question "do you know why they are behaving like this?" turns out to be "not really." Scientists have theories but no one has a full understanding of this phenomenon - but that need not diminish one's awe upon encountering such an event.

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