Every May and June, on the nights around the new and full moon high tides, Delaware Bay does something it has done for longer than the continents have been arranged the way they are now. Horseshoe crabs come out of the water. Not a few — by some counts, tens of thousands crawl onto Delaware and New Jersey beaches on a single peak night, moving up the sand in the dark to lay eggs at the tide line. Volunteers have counted them on the same three Delaware beaches every spring since 1990 — Kitts Hummock, Ted Harvey Wildlife Area, and North Bowers Beach — a survey now coordinated by the Delaware National Estuarine Research Reserve. It may be the longest-running wildlife count most people in Delaware have never heard of.
An animal older than the question of what it is
Horseshoe crabs are not actually crabs. They are chelicerates, a lineage more closely related to spiders and scorpions than to anything that ends up in a crab pot, and they have been coming ashore to spawn in roughly this same way for an estimated 445 million years — long enough to have survived the extinction event that ended the dinosaurs, and several mass extinctions before that. Their blood is blue, colored by copper rather than iron, and a compound drawn from it — Limulus Amebocyte Lysate, or LAL — is still the standard test used to screen vaccines, IV fluids, and implanted medical devices for bacterial contamination. If you have had a vaccine or a surgical implant, a horseshoe crab's blood was almost certainly involved in confirming it was safe.
What is waiting for them
None of that is why most people who know about the spawning actually go looking for it. They go for the birds.
Red knots are unglamorous-looking shorebirds — about the size of a robin, gray in nonbreeding plumage, easy to walk past — that make one of the longest migrations of any animal on Earth. According to the U.S. Fish and Wildlife Service, some rufa red knots fly more than 9,300 miles one way, from the far southern tip of South America near Tierra del Fuego to breeding grounds in the central Canadian Arctic, then make the trip again in reverse that autumn — up to 19,000 miles a year.
Delaware Bay is where a large share of that population stops to refuel before the final leg north. Federal wildlife biologists estimate that 50 to 80 percent of the entire rufa red knot population passes through the bay some springs. The birds often arrive lean, having burned through much of their body mass on the flight up from the south Atlantic coast, and they need to nearly double their weight in about two weeks to have a realistic chance of reaching the Arctic and breeding successfully. The only food dense and abundant enough to make that possible, in that narrow window, is horseshoe crab eggs — laid in such numbers along the bay shore that a red knot can eat almost nothing else and still gain weight faster here than at any other stopover recorded anywhere in the world, according to FWS.
One red knot, banded with the orange flag code B95, was resighted in spring 2014 at an estimated 21 years old or older — meaning it had very likely made this exact flight, to this exact bay, more than twenty times. Most red knots do not live nearly that long. B95 is the exception, not the rule, but it is a real, individually tracked bird, and its record is the detail that turns an abstract migration statistic into an actual animal's actual life.
A bird that nearly lost its bay
That dependency is also the reason the whole system nearly broke. Through the 1990s and 2000s, unregulated harvest of horseshoe crabs for bait drove a steep decline in the number of crabs spawning on the bay, and with fewer eggs on the beach, red knot numbers fell with them. The U.S. Fish and Wildlife Service estimates the rufa red knot population dropped roughly 75 percent between the 1980s and the 2000s — a decline serious enough that the subspecies was listed as threatened under the Endangered Species Act in December 2014.
The response, on the Delaware side, was land as much as regulation. Mispillion Harbor, on the bay's western shore near Slaughter Beach, is uniquely sheltered by a jetty, with shallow water that warms quickly in spring and gently sloping sand that crabs favor for laying eggs. Starting in the mid-2000s it became a focus of conservation work: the Conservation Fund purchased most of the harbor's shoreline in 2006 specifically to keep it from being developed, and ownership was transferred to the State of Delaware in stages, with the final parcel handed over in 2021. It is now one of the most reliable places on the bay to watch both animals doing what they came for, on the same tide.
Delaware also tightened its own harvest limits and joined a coordinated, multi-state management plan for the crab fishery. It has been slow, unglamorous work — closed seasons, bait restrictions, decades of the same volunteers counting the same beaches on the same moon-timed nights — but state and regional surveys reported in 2026 show the horseshoe crab population nearing the highs last recorded before the decline began. Red knot recovery lags behind crab recovery, as it should, since a bird population rebuilds more slowly than an invertebrate one, but for the first time in a generation the trend line points the right direction.
The claim we are not making
You will see Delaware Bay described, in state and federal wildlife material, as the largest horseshoe crab spawning aggregation and the most significant red knot stopover in the world. That description is widely used — by federal wildlife agencies, regional conservation groups, and independent nature writers alike — and it is repeated here on that basis, not on any count of our own. Nobody has surveyed every beach on Earth where a horseshoe crab might spawn. Treat "largest in the world" as a widely sourced, attributed claim rather than a fact this publication independently verified.
Seeing it without becoming part of the problem
Spawning peaks over two or three weeks in May, timed to the new and full moon evening high tides, and state agencies generally point visitors toward the same handful of public bay beaches. The rule repeated by every agency involved is the same: do not pick crabs up or flip them over out of curiosity. A crab that looks stranded upside down is often mid-spawn, and handling disturbs the process and can injure the animal. Watch from a distance, keep dogs leashed, and give the shoreline room — the eggs those birds are flying thousands of miles for are buried just under the sand you would be standing on.
Sources: U.S. Fish and Wildlife Service, "Rufa Red Knot" species profile and Delaware Bay Rufa Red Knot and Horseshoe Crab FAQ; DNREC, Delaware Bay Horseshoe Crab Spawning Survey; The Conservation Fund, Mispillion Harbor protection history; Smithsonian National Zoo and Conservation Biology Institute, horseshoe crab natural history; Spotlight Delaware, 2026 reporting on horseshoe crab population recovery. Hero image is an original Delaware Beach Finds illustration, not a photograph. Public-domain U.S. Fish and Wildlife Service photographs of red knots and horseshoe crabs at Mispillion Harbor and Prime Hook (credit: Gregory Breese/USFWS; Michael Carlo/USFWS) have been identified and are approved for swap-in under the same filename per this site's photography workflow.