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Crab, Grunion and Moth Migrations

Mini “migrations” of animals can inspire and some (bugs) may revile, but these natural events are never boring.

Posted: December 18, 2009

By Clay Jackson

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horseshoe crab
If this ventral view of a small horseshoe crab reminds you of a spider, you aren’t alone. These “crabs” are in an entirely different class of animals from true crabs.
horseshoe crab
The molted exoskeleton pictured is only a bit larger than a JFK half dollar; larger adult females can attain total lengths (including the tail) of 2 feet.
jellyfish migration
While they may not migrate in the classic sense, large aggregations of jellyfishes do show up from time to time in beach areas, much to the chagrin of humans swimming nearby.

I have three mental reference points when it comes to animal migrations.

The first involves a photo in a circa 1970s National Geographic. That particular shot was taken from a tundra hillside with the camera lens pointed downslope toward a large amoebic mass on the valley floor. The caption informed that this living, congealed blob consisted of thousands of members of the Porcupine Caribou Herd (the whole herd is 123,000 individuals). How I dreamed of being behind that camera lens.

Growing up near the Central Flyway (a narrow bottleneck on the Great Plains along which millions of birds fly south each autumn to escape the harsh northern latitudinal winters and return by – though heading north – in springtime) one of the harbingers of autumn (my favorite season) was the southbound Vs of Canadian geese up in the sky. You would often be tipped off that a V of “honkers” was several hundred feet directly overhead by their incessant, yes, honking, as they communicated with one another. I always wondered what those in the rear might be “saying” while staring directly into the tailpipes of those in front. The way these flying Vs work is that the lead goose breaks “trail” while those that follow draft off of those in the lead. When the lead goose tires it drops back to the rear and lets another assume the lead. Wolves do the same thing when traveling single file in heavy snow.

My last migration memory involves what are commonly called miller moths (a common moniker for multiple species). Each year, Colorado (as well as other locales in the West) experiences a mothy invasion – in 1991 it was a biblical plague of sorts – of zillions of these dusty, dull gray-brown flying insects as their eggs hatch on the Great Plains and they travel en masse to higher elevations to escape the summer heat. They eventually end up in the mountains, but not before brief layovers in the cities of the Front Range Urban Corridor (FRUC). The FRUC extends 200 miles from Pueblo, Colorado, to Cheyanne, Wyoming, and parallels the mighty Front Range.

This populous metropolitan statistical area has experienced its own migration in recent years of hundreds of thousands of bipeds, many economic transplants using real estate windfalls to find better deals in the Intermountain West. From 2000 to 2008, FRUC experienced growth of 15 percent, or an increase from about 3.6 million to 4.2 million latte-sipping, SUV-driving, house-buying humans.

Anyway, back to the millers. During the 1991 miller invasion, the harmless insects were seemingly everywhere. Open the door and in they came. Many people even used Shop-Vacs to suck them up. I remember exiting the front door of the house I was living in and promptly getting hit in the face by a squadron of them, as they detached themselves like so many ceiling-clinging bats from the top inside frame of the screen door.

What of Sea Creatures?
You knew if you were patient enough that I’d eventually get around to talking about fishes and such. Some of the more celebrated migrations in the animal kingdom include the annual travels of thousands of Pacific gray whales along the west coast, on their way to warmer Mexican calving waters and eventually returning north with their well-grown calves in tow. Their annual roundtrip migration of 19,000 to more than 27,000 miles is believed to be the longest migration among mammals.

There are also the famous, often romanticized, accounts of Pacific salmon and steelhead runs, extending along both sides of the Pacific Rim – but for our purposes, from Alaska to Central California – where the mature fishes fight their way upstream to spawn and then die. See my previously posted blog about the Idaho steelhead fishery (“Greetings From the Trout Capital of the World, posted Oct. 2, 2009).

There are opportunities on both the Left and Atlantic coasts to view some of these migratory breeding events firsthand.

West Coast Grunions
In the spring and summer, on California beaches, during full- and new-moon-lit nights and thus during the highest tides, 4- to 5-inch-long silver, minnow-looking Leuresthes tenuis come ashore. The female grunion back their tails into the wet beach sand to deposit eggs. While they are doing this, males come along and wrap themselves around any available, half-protruding female and deposit sperm. The eggs remain buried for 10 days and at the next series of high tides, the next generation of grunion emerge from the sand and head to the ocean.

The California Department of Fish and Game website publishes schedules each year of grunion runs. These are fairly accurate given the clocklike regularity of the spawning cycles of these fish. (Go to www.dfg.ca.gov/marine/gruschd.asp to see the dates and times the grunion are expected to run.) At the entrance of the Los Angeles Harbor, Cabrillo Beach is one good location to witness this amazing natural phenomenon. For 15 years I’ve lived just a few clicks down the road from Cabrillo, but I have yet to take my kids out to see the grunion. I think I’ll make a special effort in 2010 to take my two daughters on our own run to see the grunion run in either March or April. I’ll try and see if I can put together a video and blog about it when the time comes.

East Coast Horseshoe Crabs
Referred to as “crabs,” these alien-looking creatures are actually more closely related to spiders than they are to crabs. They are in a different class of animals than crabs, Merostomata and Malacostraca, respectively. Considered “living fossils,” these unusual animals have nearly identical-looking fossil relatives going back to the Triassic and Devonian periods, or 230 and 400 million years ago, respectively. These interesting animals are harmless to humans. Rather than having iron-based blood like ours, which flows red when exposed to the air, horseshoe crabs have blue copper-based blood.

The Atlantic horseshoe crab (Limulus polyphemus) is found throughout much of the Gulf of Mexico as well as along the northern Atlantic Seaboard. One of the largest populations is found in the Delaware Bay area between Delaware and New Jersey. It is here that thousands of adult horseshoe crabs head upbay each spring to deposit eggs and sperm and establish the next generation. With males coming ashore first (once again during full and new moons), the larger females dig holes, deposit eggs while dragging a male hitched to their carapace. The male “hangers on” do so with specialized front claws. During spawning, female horseshoe crabs may drag males about for months at a time – talk about your free loaders. As the female finishes laying its eggs and begins to move away, the male is dragged over the nest hole where it deposits its sperm.

Every year about 10 percent of the breeding population of horseshoe crabs succumbs to being tossed on their backs (much like turtles) by incoming waves. In response, the Just Flip ‘Em campaign was started by a regional nonprofit as a way to encourage beachgoers to aid floundering crabs when they happen across them.

Like grunion runs, Delaware Bay’s annual horseshoe crab migration presents an excellent opportunity to experience an unusual species with unusual behavior all without having to don diving gear. Also in an increasingly less diverse world, it is an opportunity for parents and children to learn together about a facet of the complexity that is our oceans.

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Crab, Grunion and Moth Migrations

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Reader Comments

Dot    Hawk Junction, ON

5/1/2012 4:46:03

Great article

Bryan    Stilwell, OK

2/9/2010 9:35:04

interesting

Tommy    pocatelloq, ID

1/21/2010 7:57:29

hores shoe crabs are like one of my most favorite sea critter oh man.

sk    nh, CT

12/23/2009 3:52:56 PM

interesting

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