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Place 07 · The Northeast Coast · Chapter 2 of 7
The Crab That Feeds a Flight
Every May, a bird the size of a robin lands on a Delaware Bay beach after flying up from South America, and nearly doubles its weight in about two weeks by eating the eggs of an animal that has been around for 445 million years.
About 15 minutes · 1 video · 1 3D model · 2 quick activities
Start here
A date set by the Moon
In May and June, on the high tides around the new and full moon, horseshoe crabs crawl out of Delaware Bay by the million to spawn. Those are the spring tides from Place 02’s tide clock: the highest water of the month, which lets the crabs bury their eggs high on the beach, out of reach of the waves. Delaware Bay hosts the largest horseshoe crab spawning population in the world, and it did not pick the date by accident.
Watch this
See it move
Deep Look (KQED and PBS) — Once a Spawn a Time: Horseshoe Crabs Mob the Beach. Five minutes of Delaware Bay’s spring spawning in close-up 4K. It is, technically, a nature documentary about a very crowded date night.
While you watch, look for
- When and where the crabs come ashore, and what they leave behind
- Who shows up to eat the eggs
- Something you did not expect. Hold onto it.
The short version
Three things going on here
The crab
Not a crab at all
Horseshoe crabs are more closely related to spiders and scorpions than to true crabs, and fossils of their relatives go back 445 million years, long before the first dinosaurs. They have ten eyes, take at least nine years to grow up, and a single female can lay up to 100,000 eggs or more in a season, in clusters of about 4,000. The tail spike is not a weapon: it is for flipping back over when a wave rolls them.
The bird
A robin that refuels
The rufa red knot, a robin-sized shorebird, can fly more than 9,300 miles each spring from as far south as Tierra del Fuego to the Canadian Arctic. Delaware Bay is its main refueling stop, and there it gorges on crab eggs until it nearly doubles its weight in about two weeks. Red knot numbers fell about 75% in some key areas since the 1980s, mostly because there were fewer eggs, and the bird was listed as threatened in December 2014.
The blood
Blue blood, busy labs
Horseshoe crab blood is blue because it carries oxygen with copper, not iron, and it contains LAL, a substance that clots when it meets bacterial toxins. Drug makers use it to make sure injectable medicines and medical devices are safe, so if you have ever had a shot, a horseshoe crab probably helped. Crabs are caught, part of their blood is drawn, and they are released, but an estimated 15% of them die afterward.
Photos: Tyler Mascola, teyi 徐, cottonbro studio / Pexels
Play with it
Turn a living fossil over
This is a 3D scan of a real Atlantic horseshoe crab from the teaching collection of the Paleontological Research Institution in Ithaca, New York, right here in the Finger Lakes. Spin it, flip it over, and find the eyes on top, the legs and gill plates underneath, and the tail spike.
3D model: “Horseshoe crab: Limulus polyphemus (PRI)” by the Digital Atlas of Ancient Life on Sketchfab (CC0), specimen from the Paleontological Research Institution, Ithaca, NY; model by Emily Hauf.
Two questions to answer while you are in there
- The shell looks like armor, but horseshoe crabs still get flipped by waves. Find the tail spike: how would a crab use it to turn itself back over?
- This body plan has barely changed in hundreds of millions of years. What does that tell you about how well it works for life on a sandy, wave-washed shore?
Try it yourself
Check yourself
Try it
True or False on the Beach
Eight statements about crabs, birds and the tide. Which ones hold up?
Try it · interactive
Red Knot Refuel
Plan a red knot’s two-week stop in Delaware Bay: set how many crabs spawn and how many are caught for bait, watch the eggs on the beach and the bird’s weight climb, and see whether it reaches departure weight in time to make the Arctic.
Open Red Knot Refuel in its own window (opens in a new tab).
Words you will hear
Four terms, that is all
Spawning
Releasing or laying eggs to reproduce
Spring tide
Extra-high tide around new and full moon
Stopover
Refueling place on a long migration
LAL
Crab-blood extract that detects bacterial toxins
Check your understanding
Practice the whole chapter
Everything in this chapter, in eight quick questions. Retry as often as you like. Read the feedback on every answer, right or wrong, because that is where the learning is.
Practice
Practice: The Crab That Feeds a Flight
Eight questions covering the video, the three cards, the four terms and the interactive above.
If you want more
Go deeper
- Our World Ocean 14.6 — Spring and Neap Tides — LibreTexts · 4 minutes
- U.S. Fish & Wildlife Service — Rufa Red Knot — USFWS · 5 minutes
- Atlantic States Marine Fisheries Commission — Horseshoe Crab — ASMFC · 5 minutes
Photo credits (Pexels): Dusty Ruth, Александр Максин, Tyler Mascola, teyi 徐, cottonbro studio.