# Sharks sensing a drop of blood in water

**URL:** <https://boards.straightdope.com/t/sharks-sensing-a-drop-of-blood-in-water/187709>\
**Category:** Factual Questions\
**Created:** [July 11, 2003, 2:01pm UTC](https://boards.straightdope.com/t/sharks-sensing-a-drop-of-blood-in-water/187709 "2003-07-11T14:01:42Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![Qazzz](https://avatars.discourse-cdn.com/v4/letter/q/aeb1de/32.png) [@Qazzz](https://boards.straightdope.com/u/Qazzz)\
**Post date:** [July 11, 2003, 2:01pm UTC](https://boards.straightdope.com/t/sharks-sensing-a-drop-of-blood-in-water/187709/1 "2003-07-11T14:01:42Z")

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It seems a commonly touted fact about sharks is that their acute sense of smell allows them to sense a single drop of blood from miles away in a body of water. [Here](http://science.howstuffworks.com/shark3.htm) is just one such site that makes this claim.

Anyway, what I’m wondering is – must the blood be diluted throughout the water in order for some of the “blood molecules” to actually reach the shark, which is miles away? Or does the shark sense the blood from the moment the drop falls in the water? If this is the case, how would that work? What is the communication between the blood and the shark?

Thanks.

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**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 11, 2003, 2:19pm UTC](https://boards.straightdope.com/t/sharks-sensing-a-drop-of-blood-in-water/187709/2 "2003-07-11T14:19:39Z")

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The blood has to be dispersed in such a way that there is some of it near the shark.

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**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\
**Post date:** [July 11, 2003, 2:25pm UTC](https://boards.straightdope.com/t/sharks-sensing-a-drop-of-blood-in-water/187709/3 "2003-07-11T14:25:41Z")

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An actual blood molecule (many of them) needs to reach the shark for it to sense it.

Sharks do, however, have electrical sensors built in that allow them to detect erratic movements and this will be much faster than the time it takes to detect blood (essentially instantly). An animal in distress convulsing in the water will attract the shark. Once it is closer it has a better shot at sensing blood in the water. Erratic movement + blood = damaged animal that probably can’t defend itself = easy meal.

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**Author:** ![Qazzz](https://avatars.discourse-cdn.com/v4/letter/q/aeb1de/32.png) [@Qazzz](https://boards.straightdope.com/u/Qazzz)\
**Post date:** [July 11, 2003, 2:32pm UTC](https://boards.straightdope.com/t/sharks-sensing-a-drop-of-blood-in-water/187709/4 "2003-07-11T14:32:32Z")

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So they don’t actually sense it from miles away, right? They sense blood which has diluted throughout a few miles of water. Hmmmmmmm.