# How does a Horseshoe Crab find the Water after Breeding? **URL:** \ **Category:** Factual Questions\ **Created:** [July 13, 2022, 3:07pm UTC](https://boards.straightdope.com/t/how-does-a-horseshoe-crab-find-the-water-after-breeding/967877 "2022-07-13T15:07:19Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![tofor](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tofor/32/13727_2.png) [@tofor](https://boards.straightdope.com/u/tofor)\ **Post date:** [July 13, 2022, 3:07pm UTC](https://boards.straightdope.com/t/how-does-a-horseshoe-crab-find-the-water-after-breeding/967877/1 "2022-07-13T15:07:19Z")
This year my family and I were able to visit New Jersey during Horseshoe Crab breeding season. To breed, the males attach themselves to the back of a female, and the female crawls out of the ocean at high tide onto the beach where she digs a hole and lays her eggs. The males then fertilize the eggs in the nest. After this, the crabs return to the water. Sometimes they get turned over by the surf and stranded on the beach. Visitors are encouraged to gently turn the crabs over so they can return to the water. Once a crab is turned right side up, it often walks in a circle before making a beeline for the surf. It looks for all the world like it’s trying to get its bearings. Occasionally, they would head off in completely the wrong direction instead. This led me to wonder, how does a Horseshoe crab on the sand detect where the water is? I have been utterly unable to find an answer to this question. Is it sight (But they often breed at night. Maybe that’s why they prefer full moons?)? Is it the sound of the surf? Something else? ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [July 13, 2022, 3:14pm UTC](https://boards.straightdope.com/t/how-does-a-horseshoe-crab-find-the-water-after-breeding/967877/2 "2022-07-13T15:14:44Z")
> [@tofor](#): > > (But they often breed at night. Maybe that’s why they prefer full moons?) Maybe, or maybe they prefer the higher high tides that you get when the Moon is full (or new). Or maybe it’s a combination of both. To add to the list, it could also be smell, or it could be that they’re perceiving the slope of the beach and heading downhill. ---
**Author:** ![Darren\_Garrison](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/darren_garrison/32/92_2.png) [@Darren\_Garrison](https://boards.straightdope.com/u/Darren_Garrison)\ **Post date:** [July 13, 2022, 7:59pm UTC](https://boards.straightdope.com/t/how-does-a-horseshoe-crab-find-the-water-after-breeding/967877/3 "2022-07-13T19:59:01Z")
They do have _a lot_ of eyes. > **[Anatomy](https://www.horseshoecrab.org/anat/vision.html)** > > A comprehensive look at horseshoe crab anatomy. Includes detailed drawings by Shuster, Milne and Barlow, and data for both the novice and researcher. ---
**Author:** ![jnglmassiv](https://avatars.discourse-cdn.com/v4/letter/j/f07891/32.png) [@jnglmassiv](https://boards.straightdope.com/u/jnglmassiv)\ **Post date:** [July 13, 2022, 8:17pm UTC](https://boards.straightdope.com/t/how-does-a-horseshoe-crab-find-the-water-after-breeding/967877/4 "2022-07-13T20:17:55Z")
I was just coming back to this thread from earlier today to post a link to that website. I’ll bet they write back if @tofor asks them. They really, REALLY like horseshoe crabs. Neat website. ---
**Author:** ![purplehorseshoe](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/purplehorseshoe/32/2996_2.png) [@purplehorseshoe](https://boards.straightdope.com/u/purplehorseshoe)\ **Post date:** [July 13, 2022, 8:27pm UTC](https://boards.straightdope.com/t/how-does-a-horseshoe-crab-find-the-water-after-breeding/967877/5 "2022-07-13T20:27:48Z")
> [@Darren\_Garrison](#): > > They do have _a lot_ of eyes. Wow, light-sensing organs all along their tail?!? Neat! ---
**Author:** ![tofor](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tofor/32/13727_2.png) [@tofor](https://boards.straightdope.com/u/tofor)\ **Post date:** [July 13, 2022, 9:36pm UTC](https://boards.straightdope.com/t/how-does-a-horseshoe-crab-find-the-water-after-breeding/967877/6 "2022-07-13T21:36:17Z")
> [@jnglmassiv](#): > > I’ll bet they write back if @tofor asks them. They really, REALLY like horseshoe crabs. Neat website. I’ve written as you suggest, expressing my vast curiosity about their favorite sea creature. ---
**Author:** ![nearwildheaven](https://avatars.discourse-cdn.com/v4/letter/n/90db22/32.png) [@nearwildheaven](https://boards.straightdope.com/u/nearwildheaven)\ **Post date:** [July 13, 2022, 10:12pm UTC](https://boards.straightdope.com/t/how-does-a-horseshoe-crab-find-the-water-after-breeding/967877/7 "2022-07-13T22:12:18Z")
They also have pale-blue blood that looks like Downy fabric softener. There’s a company that “bleeds” them to produce a product for the medical laboratory and vaccine industry. [https://www.nhm.ac.uk/discover/horseshoe-crab-blood-miracle-vaccine-ingredient.html](https://www.nhm.ac.uk/discover/horseshoe-crab-blood-miracle-vaccine-ingredient.html) I first learned this on an old “Dirty Jobs” episode where Mike Rowe was on a fishing boat, and before throwing a horseshoe crab back, the head fisherman got a syringe and withdrew a small amount of blood. He knew how much he could draw without harming the animal, and then tossed it back. ---
**Author:** ![Jackmannii](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jackmannii/32/311_2.png) [@Jackmannii](https://boards.straightdope.com/u/Jackmannii)\ **Post date:** [July 13, 2022, 10:17pm UTC](https://boards.straightdope.com/t/how-does-a-horseshoe-crab-find-the-water-after-breeding/967877/8 "2022-07-13T22:17:50Z")
> [@tofor](#): > > Sometimes they get turned over by the surf and stranded on the beach. Visitors are encouraged to gently turn the crabs over so they can return to the water. Hey, that’s a [Call To Action](https://www.horseshoecrab.org/act/flipem.html)! But it’s apparently OK now. 😇 ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [July 13, 2022, 11:21pm UTC](https://boards.straightdope.com/t/how-does-a-horseshoe-crab-find-the-water-after-breeding/967877/9 "2022-07-13T23:21:57Z")
The horseshoe crab lays on its back, its belly baking in the hot sun, beating its legs trying to turn itself over. But it can’t. Not with out your help. But you’re not helping. Why is that, @Jackmannii ? ---
**Author:** ![Tim\_T-Bonham.net](https://avatars.discourse-cdn.com/v4/letter/t/46a35a/32.png) [@Tim\_T-Bonham.net](https://boards.straightdope.com/u/Tim_T-Bonham.net)\ **Post date:** [July 14, 2022, 12:14am UTC](https://boards.straightdope.com/t/how-does-a-horseshoe-crab-find-the-water-after-breeding/967877/10 "2022-07-14T00:14:31Z")
> [@Chronos](#): > > … heading downhill. This seems like the most obvious answer. Beaches slope down toward the sea, (water runs downhill, that’s why the sea is where it is) so following the slope down will inevitably lead to the sea. Walk in a circle to figure out which way is uphill and which downhill, then head down. Evolution would favor the crabs smart enough to follow this simple rule. ---
**Author:** ![tofor](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tofor/32/13727_2.png) [@tofor](https://boards.straightdope.com/u/tofor)\ **Post date:** [July 14, 2022, 12:58am UTC](https://boards.straightdope.com/t/how-does-a-horseshoe-crab-find-the-water-after-breeding/967877/11 "2022-07-14T00:58:03Z")
> [@Jackmannii](#): > > Hey, that’s a [Call To Action](https://www.horseshoecrab.org/act/flipem.html)! But it’s apparently OK now. Hey now, I have no opinion on whether any of you should travel to Delaware (I had the wrong state when I said New Jersey earlier) and flip stranded Horseshoe Crabs over. All I can say is that my young children were absolutely delighted to be saving creatures one at a time, all the way down the beach. Which reminds me, we saw something potentially horrifying that same day. We were on a small beach separated from another small beach by a large river we were unable to cross. On that other beach were two adults with an unmarked pickup truck. They appeared to be picking up the stranded crabs and _throwing them in the bed of the truck_. Now it’s _possible_ they were picking up only dead crabs, we were too far away to see for sure, but it sure looked like they were grabbing them all. Cleaning up the beach, probably? But why collect them, when all you have to do is “Just Flip 'Em”? (I don’t mean you _personally_ of course. Do what you want, I’m not calling you to any particular action.) ---
**Author:** ![Jackmannii](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jackmannii/32/311_2.png) [@Jackmannii](https://boards.straightdope.com/u/Jackmannii)\ **Post date:** [July 14, 2022, 1:35am UTC](https://boards.straightdope.com/t/how-does-a-horseshoe-crab-find-the-water-after-breeding/967877/12 "2022-07-14T01:35:57Z")
I flipped two beetles out of a saucer of water today, saving them from certain drowning. I couldn’t resist their piteous cries. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [July 14, 2022, 8:01am UTC](https://boards.straightdope.com/t/how-does-a-horseshoe-crab-find-the-water-after-breeding/967877/13 "2022-07-14T08:01:49Z")
I suppose It’s also possible that their eyes are able to sense the polarisation of light - reflected light off water has particular polarised properties, so turning in a circle could also be sensing that ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [July 14, 2022, 8:28am UTC](https://boards.straightdope.com/t/how-does-a-horseshoe-crab-find-the-water-after-breeding/967877/14 "2022-07-14T08:28:45Z")
> [@tofor](#): > > On that other beach were two adults with an unmarked pickup truck. They appeared to be picking up the stranded crabs and _throwing them in the bed of the truck_. Now it’s _possible_ they were picking up only dead crabs, we were too far away to see for sure, but it sure looked like they were grabbing them all. Cleaning up the beach, probably? But why collect them, when all you have to do is “Just Flip 'Em”? People sell horseshoe crab shells for quite a high price (higher prices still if it’s decorated or painted and for sale on Etsy). I would suspect those people were collecting for that purpose - and probably didn’t care a whole lot about whether the animals were dead or alive. ---
**Author:** ![DinoR](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dinor/32/2821_2.png) [@DinoR](https://boards.straightdope.com/u/DinoR)\ **Post date:** [July 14, 2022, 11:14am UTC](https://boards.straightdope.com/t/how-does-a-horseshoe-crab-find-the-water-after-breeding/967877/15 "2022-07-14T11:14:25Z")
> [@tofor](#): > > But why collect them, when all you have to do is “Just Flip 'Em”? You can’t chop them up for bait or harvest blood for the biomedical industry if you “Just Flip 'Em.” Generally, one or both of those uses are legal, but limited, in the US native species’ range. It could also have been poachers without permits. That brings up harvesting their roe to eat as an option too. ISTR having had horseshoe crab roe from an Asian native species before. ---
**Author:** ![tofor](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tofor/32/13727_2.png) [@tofor](https://boards.straightdope.com/u/tofor)\ **Post date:** [July 14, 2022, 11:52am UTC](https://boards.straightdope.com/t/how-does-a-horseshoe-crab-find-the-water-after-breeding/967877/16 "2022-07-14T11:52:21Z")
> [@Mangetout](#): > > People sell horseshoe crab shells for quite a high price (higher prices still if it’s decorated or painted and for sale on Etsy). They were not treating them as though they cared about the shells much, so > [@DinoR](#): > > You can’t chop them up for bait bait seems more likely? ---
**Author:** ![Ruken](https://avatars.discourse-cdn.com/v4/letter/r/f475e1/32.png) [@Ruken](https://boards.straightdope.com/u/Ruken)\ **Post date:** [July 14, 2022, 2:25pm UTC](https://boards.straightdope.com/t/how-does-a-horseshoe-crab-find-the-water-after-breeding/967877/17 "2022-07-14T14:25:01Z")
Some animals can see polarized light. Reflections off water are polarized. IIRC this is how some parasites drive insects to water. It doesn’t seem like a horseshoe crab has much vantage to see the water though if there are any obstructions. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [July 14, 2022, 2:40pm UTC](https://boards.straightdope.com/t/how-does-a-horseshoe-crab-find-the-water-after-breeding/967877/18 "2022-07-14T14:40:48Z")
Apparently the horseshoe crab’s eyes can sense polarised light. It’s not just the direct reflections from the water that will provide directional cues though ---
**Author:** ![TriPolar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripolar/32/3008_2.png) [@TriPolar](https://boards.straightdope.com/u/TriPolar)\ **Post date:** [July 14, 2022, 2:56pm UTC](https://boards.straightdope.com/t/how-does-a-horseshoe-crab-find-the-water-after-breeding/967877/19 "2022-07-14T14:56:04Z")
They can turn themselves over using their tail for leverage. I’ve seen them do this. It takes them a long time but eventually they can do it. It’s more difficult for them in loose beach sand where their tails just dig into the sand. ---
**Author:** ![blondebear](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/blondebear/32/1022_2.png) [@blondebear](https://boards.straightdope.com/u/blondebear)\ **Post date:** [July 14, 2022, 3:16pm UTC](https://boards.straightdope.com/t/how-does-a-horseshoe-crab-find-the-water-after-breeding/967877/20 "2022-07-14T15:16:13Z")
Radiolab’s current episode is appropriate to this thread: [Baby Blue Blood Drive](https://radiolab.org/episodes/baby-blue-blood-drive-2207) I was in Cedar Key FL recently and was a bit surprised to see that Horseshoe Crabs were a thing on the Gulf coast. The Florida Conservation Commission has has a public program where you can volunteer as a Florida Horseshoe Crab Watcher. [Next page](https://boards.straightdope.com/t/how-does-a-horseshoe-crab-find-the-water-after-breeding/967877.md?page=2)