A wizard can choose a crab as a familiar with the find familiar, and crabs have a blindsight of 30ft and are amphibian.

I can understand why crabs would have blindsight in water, as they can notice movement in the water and (apparently) changing salt levels to detect enemies. But it makes no sense for a normal crab to have blindsight out of water.

But from a purely mechanical standpoint, nothing is stopping me from having a crab familiar, and getting 30 feet of blindsight by sharing senses with my familiar. Essentially having a constant "detect invisibility" seems a bit strong, though, for a 1st-level spell.

Is this correct? Do crabs have blindsight out of water?

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    – V2Blast
    Commented Jun 19, 2020 at 7:14
  • \$\begingroup\$ Coconut crabs are known for their sense of smell, as as adults are wholly terrestrial, I wouldn't be surprised if that is what gives D&D crabs 'blind sight'. \$\endgroup\$ Commented Jun 19, 2020 at 9:50

2 Answers 2


Yes, the crab has blindsight out of water

The statblock for the Crab adds no additional qualifiers to its senses, so the Crab has blindsight both in and out of water.

But it makes no sense for a normal crab to have blindsight out of water.

There are lots of things in DnD that make no sense. A mule can ride a phantom steed, for example. But it doesn't ruin the experience, as the great thing about DnD is that you change the game to whatever makes for the most fun experience. If you want a crab familiar that's blind out of water because it improves your immersion, then it's absolutely your right, with DM approval, to modify the statblock accordingly.

In terms of balance

It shouldn't be a concern. If you want to see this as an alternative way of casting See Invisibility, consider that a Crab can only see out to 30ft, while a PC casting See Invisibility can see out to their full range for the duration of the spell (1 hour, no concentration required). Additionally, it requires a full Action to switch to the familiar's perspective, meaning that the combat utility of the Crab familiar's blindsight is essentially null.

Additionally, a crab has a passive perception of 9, meaning that any reasonably stealthy invisible opponent will have no trouble defeating it with an average Dexterity (Stealth) roll, so its utility as a sentry is similarly poor.

  • \$\begingroup\$ So you would allow a wizard in your games to use a crab familiar to have constant 30ft blindsight on demand? \$\endgroup\$
    – Greebo
    Commented Jun 18, 2020 at 19:26
  • \$\begingroup\$ Possibly the question should be reframed as "Do you think crabs should have...". As I mentioned in the question, I know they do from a purely mechanical standpoint, but I, at least, feel like it doesn't makes sense! \$\endgroup\$
    – Greebo
    Commented Jun 18, 2020 at 19:31
  • 3
    \$\begingroup\$ @Greebo added my comments on balance \$\endgroup\$
    – Andrendire
    Commented Jun 18, 2020 at 19:36
  • 4
    \$\begingroup\$ @Greebo: Note that "Do you think crabs should have blindsight out of water" is a very different question from whether they actually do have it; whether they "should" have it is a primarily opinion-based question. It's fine to ask "Am I correctly understanding the rules", but we can't really answer questions about "do you think the rules make sense", because there's no way to choose a single best answer; every answer would be equally valid. \$\endgroup\$
    – V2Blast
    Commented Jun 19, 2020 at 7:16
  • \$\begingroup\$ A creature that has blindsight can see invisible creatures, meaning that no matter how well an invisible creature hides in plain sight, the blindsight creature can see it. rpg.stackexchange.com/a/99975/45088 \$\endgroup\$
    – L0neGamer
    Commented Jun 19, 2020 at 7:53

Yes, the crab has blindsight out of water

There is nothing saying that the blindsight turns off out of water, so it stays active.

It's not unbalanced

The crab has 30ft blindsight, and 9 passive perception. Quite useful, but look at the other familiar options:

  1. Bat: Flying, 60ft blindsight, +1 perception, advantage on perception to hear
  2. Cat: +3 perception, 40ft movement, 30ft climb, advantage on perception to smell
  3. Hawk: +4 perception, 60ft fly, advantage on perception to see
  4. Octopus: +4 Stealth, advantage on stealth underwater
  5. Owl: 120ft darkvision, 60ft fly, doesn't provoke opportunity attacks, advantage on hearing and sight!
  6. Rat: advantage on perception to smell
  7. Raven: +3 perception, can mimic sound!
  8. Spider: Can use its web to sense location of creatures
  9. Weasel: +3 perception, advantage on hearing and smell!

Clearly crab isn't the strongest choice. The bat is basically a flying crab, that can blindsight twice as far and has advantage on hearing, and 2 better perception.

If you want to detect invisibility, remember that you can still detect an invisible creature with hearing, smell, or even sight by noticing signs of their presence. Your best bet would probably be the weasel, followed by the owl, bat, at, hawk, or rat.

A common tactic is to have an owl fly around above the party at all times, keeping an eye out and listening carefully for any threats. When there's a threat the owl can fly down to 100ft from you and telepathically tell you what it detected. It can do this indefinitely, even if the party is sleeping. This is a lot more strong than having 30ft around you to detect invisible creatures.


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