No. 042 November 1, 2026 9 min read

Fifteen Percent of the Ones Released Alive

A million horseshoe crabs were bled for endotoxin reagent last year, and four-fifths of the widely quoted death toll is a multiplication rather than a body count. The synthetic replacement has been compendial since May 2025 and the bleeding went up anyway.

In 2024, biomedical facilities on the U.S. Atlantic coast bled 1,008,943 horseshoe crabs. That figure is public, reported by the Atlantic States Marine Fisheries Commission. The figure that gets quoted alongside it — about 185,000 of those crabs died — is roughly four parts arithmetic to one part body count, and working out which part is which took me most of the day.

The reason anyone bleeds a horseshoe crab is a quirk of its immune system. Limulus polyphemus has no antibodies, so it defends itself by gelling: its blood cells carry granules of clotting protein, and a protease cascade dumps them on contact with bacterial debris. The first enzyme in that cascade, Factor C, binds lipopolysaccharide — the outer membrane of Gram-negative bacteria — and triggers the rest. Fred Bang noticed in 1956 that dead bacteria clotted crab blood as well as live ones, which is the whole trick: the reaction detects the molecule, not the organism, so it finds endotoxin in a sterile vial where a culture finds nothing. In 1977 the FDA accepted lysate from those cells as a release test for drugs and devices that touch blood, replacing a test that involved injecting rabbits and taking their temperature.

1,008,943

crabs bled in 2024

ASMFC coastwide, biomedical only

15%

assumed post-bleeding mortality

95% CI 4%–30%

82%

of the 2024 death toll

is that rate, not counted bodies

May 2025

USP <86> became official

rFC first accepted by FDA in 2012

Where the 15% comes from

The method is simple enough to reconstruct. Facilities report crabs collected, crabs bled, and crabs observed dead before release; the commission applies a flat 15% post-bleeding mortality rate to every crab released alive, adds the observed deaths, and reports the sum.

The arithmetic checks against the published series. In 2023, facilities collected 1,113,644 crabs and bled 1,038,673; fifteen percent of the bled figure is 155,801, exactly the post-bleeding mortality reported, and adding the 22,431 observed deaths gives the published total of 178,232. Run the same calculation on 2024 and the modelled component is 151,341, leaving about 33,400 counted carcasses inside that 185,000. Four-fifths of the headline death toll is a multiplication, and the multiplier is a point estimate with a factor of seven and a half inside it.

That is not a criticism of the multiplication. There is no way to follow a million released crabs around Delaware Bay, and a literature-derived rate is the honest alternative to pretending they all live. The problem is what happens to the rate once it leaves the assessment document.

The 15% is a point estimate from a meta-analysis of 47 bleeding-mortality studies. Smith and colleagues, publishing in Frontiers in Marine Science in 2020, give its 95% confidence interval as 4% to 30%. That is a factor of seven and a half between the ends. And the ends get used selectively: the 2024 petition to list the species under the Endangered Species Act says in its body that “up to 30%” of bled crabs die, and in its executive summary firms this up to “at least 30%” — the top of the interval, restated as a floor. I could not find anyone quoting the 4%.

Estimated 2024 biomedical crab deaths

Counted dead before release

~33,400

At 4% post-bleeding (interval low)

~73,700

At 15% post-bleeding (ASMFC)

184,693

At 30% post-bleeding (interval high)

~336,000

Bled, then sold for bait

counted against bait quota

The spread in the underlying studies tracks handling more than bleeding. Hurton and Berkson’s controlled work found 2.6% mortality in crabs put through capture, transport and holding without being bled at all, against 8.3% in crabs bled and stressed; at 40% of blood volume removed, mortality was 6% in unstressed animals and 15.4% in stressed ones. The much-cited 30% comes from a Massachusetts study of females — Leschen and Correia, 2010 — which I am taking on a review’s authority, the original sitting behind a paywall I could not get past. A flat coastwide rate averages over the difference between a careful facility and a careless one, which is precisely the variable management could act on.

The study that points the other way

The strongest piece of evidence I found is also the most awkward for the standard story. Smith’s group used nineteen years of the Fish and Wildlife Service tagging database — 174,936 tagged animals with known sex and bleeding status — to compare bled and unbled crabs tagged in the same places in the same years. Annual survival was not lower for bled crabs. If anything it ran slightly higher, which the authors attribute to selection: biomedical facilities cull visibly injured or heavily worn animals, so the bled group starts out healthier.

What they did find was a lower probability of recapture for bled crabs, strongest in males — median relative risk around 2.0 in Delaware Bay. Recapture here mostly means being found spawning on a beach, so a bled crab that shows up less often is plausibly a crab that spawns less, which matters more to the population than whether it is technically alive. Telemetry work by Owings and colleagues found bled crabs spawning less frequently and sitting in deeper water, the effect fading inside two weeks.

So the long-term data do not show the acute kill the 15% implies, and the sublethal data show something the mortality accounting does not capture at all. Both cut against the tidy version of the story, in opposite directions.

Legally invisible numbers

Underneath all of this is a data structure I had not appreciated. Per-facility biomedical numbers are confidential under federal and state law, and ASMFC applies a “rule of three” — at least three contributors aggregated before a figure can be published — so only coastwide totals exist in public. Before 2019 the biomedical data were not in the population modelling at all; the 2019 and 2024 subcommittees got access to the confidential series but could not print it.

So the only publicly checkable quantity in the whole controversy is a coastwide sum, and nobody outside the commission can tell whether a million crabs were bled gently by one operator or roughly by another. The 1998 management plan set a biomedical mortality threshold of 57,500 crabs, which has been exceeded every year since 2008 except 2016 — 2023’s total was more than three times it. The response was not to act but to conclude that a biologically meaningful threshold could not be set without coastwide population estimates, so the trigger stayed, unarmed, and fires annually. The 2024 assessment rates the coastwide stock “good,” Delaware Bay “good,” and New York “poor,” which is its own kind of answer.

The replacement is not the same test

The usual framing is that a synthetic replacement has existed for years and industry dragged its feet. The feet-dragging is real: recombinant Factor C has been commercially available since 2003, the FDA accepted it as an alternative method in 2012, the European Pharmacopoeia took it in 2016, and USP’s own chapter <86> only became official in May 2025. Lilly’s Emgality was approved in November 2018 on an rFC assay, so a recombinant reagent has carried an FDA-approved product for eight years.

But the two assays are not measuring the same thing, and that is the part the framing loses. The crab cascade has a second input branch: Factor G, which fires on (1,3)-β-D-glucan — fungal cell wall material, cellulose residue — rather than endotoxin. The recombinant reagents rebuild the endotoxin branch and leave Factor G out. So lysate responds to things rFC ignores, and when the two disagree on a dirty sample, “rFC read lower” and “lysate false-positived” are the same observation with different owners.

The comparison literature reads accordingly. A 2021 paper in the European Journal of Pharmaceutical Sciences reported that recombinant reagents substantially under-recovered endotoxin across 128 water samples and could not be shown non-inferior to lysate; the journal issued an expression of concern that May, after reviewers argued the low-grade waters chosen could not support a general claim. I am working from trade-press accounts of it rather than the paper. The rebuttal that convinced me most — that those waters were full of glucans and cellulose, and that a lysate lacking Factor G read the same samples much lower — came from Kevin Williams, whose career includes Lonza and bioMérieux, both of whom sell recombinant reagents. The cleanest recent comparability study, on naturally occurring endotoxin from seven facility isolates, was published this July with all five authors employed by bioMérieux, measures recovery against the mean of the lysate results rather than a known concentration, and uses a 50–200% acceptance window wide enough to pass most things.

USP’s resolution was not to settle any of this. Chapter <86> admits both single-enzyme rFC and the three-enzyme recombinant cascade, and requires each laboratory to validate the method for its own products. The question “is recombinant as good as lysate” was given no global answer, by design.

What I take from it

The ethical arithmetic everyone is doing — a million crabs, fifteen percent, so many thousand dead — is the shakiest layer of the structure and the one carrying all the public weight. Underneath it sit a confidence interval nobody quotes the bottom of, a long-term dataset finding no survival penalty, a sublethal effect no number tracks, and legally sealed per-operator data.

What is not in doubt is the direction. Bleeding roughly doubled between 2016 and 2023 and crossed a million for the first time, which is the opposite of what you would expect from the replacement narrative. The 2024 count came in slightly below 2023’s, the first dip in years; one year is not a trend, and the collected figure for 2024 is not yet in the public series. And biomedical mortality is still the smaller share of the problem — 19.4% of directed-use mortality in 2023, with the bait fishery taking the rest.

The honest summary is that a reagent sourced from a living animal was replaced by one that works differently, in a market where every published comparison has a vendor behind it, under an accounting regime where most of the deaths are inferred and none of the inputs can be audited. That is a more interesting situation than a villain.