Tiny Caribbean Shrimp

I went out diving today with Patrick from Red Baron Divers on Grand Cayman. He is a keen photographer and an excellent critter spotter. He was keen to show me these tiny shrimps (only about 1-2mm long - v v small) that live in the centre of solitary gorgonian hydroids.

I haven't got a Scooby what they are. Any ideas?

These images are cropped!

Alex

Hi Alex,

Are you sure they are a shrimp? I can't identify any legs in the photos, perhaps some type of lice?

Doug

Looks like an amphipod. Possibly a gammarid. Really need to look at them isolated.

Tom

I know nothing.

All suggests seem reasonable. Thanks guys.

I will stress that they are very very small. About the size of a pygmy seahorse eye!

I will go back out and shoot them with some more magnification - when I get the chance.

Alex

Here is a view of the whole scene. Alex

holy cow ... that is frickin tiny ...

Hi Alex -- Definitely a gammaridean amphipod. I'll forward it to a friend who's an expert in the group but there's probably not enough body visible for him to know what family. We have images of more Caribbean gammarideans on our Guana website at http://www.nhm.org/guana/bvi-invt/bvi-surv/pera-g03.htm.

Cheers, Leslie

Thanks Leslie,

I realise that the pix are not perfect. But it is probably identifiable from its association with the hydroid?

Alex

It might be identifiable from the hydroid association but there are several ifs:

- if we knew what species the hydroid is

- if this association has ever been seen or reported

- if the amphipod & hydroid have ever been collected & identified together

- if this is a case of obligate commensalism* rather than the amphipod just sitting on the hydroid

On the other hand, maybe the color pattern is distinctive enough that someone working in live amphipods will recognize it. That's possible for some species. I hope my friend - Don Cadien, a research associate here - will know more.

Leslie

* Alex will know this but for others that might not an obligate commensal is an animal that can only live together with a specific host. Like whale lice (cyamid amphipods) - each species of cyamid can only live on a certain species of whale.

Hard to say that it is exclusively commensal with the hydroid. But according to Patrick they are always there. You can't see them at the time - they are so small - but you can see them later in your pix on the computer!

Thanks as always for your help, Leslie.

Alex

Alex -

Having looked at thousands of gammarid amphipods in several states of decay under the microscope I concur with Leslie. I have done research on dietary analysis of fish and these guys (or parts) are usually are found in the stomachs of smaller fish. Identifying down to species is very difficult, especially when they are half digested. We usually accept "gammarid amphipod" as the lowest taxonomic level of identification that we need to break down to. Impressive shots considering that they really are tiny - the only reason i could see them easily in a petri dish was because we had dyed them bright pink with a Rose-Bengal solution. Keep sending us good stuff from GC...

regards...

- MP

Hi Alex --

I've sent your images to experts on both amphipods & hydroids and none of them knew anything about this particular association. That's pretty much what I suspected, that only sharp-eyed divers/ UW photographers would know about this. The usual benthic sampling methods would leave the animals jumbled up with a variety of other organisms so the association would be lost. You'll need to hand collect some infested hydroids & send them off to a taxonomist if you want identifications. My local 'droid expert would be happy to oblige.

Sorry, Leslie

PS - What was the depth?

Hi Alex

if you wish I can forward our pictures to my colleague Wim Vader in Norway (Univ. of Tromso), he did several reviews of amphipod associations, I have a selection of his papers on associates of urchins, sponges, crabs, sea anemones and jellyfish .. nothing on hydroids though

all I can say it's Gammaridea (superfamily), not necessaily Gammaridae

nice pictures, as usual !

cheerio

I can see how you would find these tiny hydroids so appealing, Alex. In their miniature size, they are very intricate in design and the way they sway back and forth in the current, turning a complete 180 degees as if on a ball-pivot, they are just another wonderful creation of nature to admire. Kudos for the great photo! They are quite difficult to photograph due to their constant sway and the shallow depth of field you had to work with in order to get in close.

These hydroids are quite abundant in south Florida and the Keys as well. I usually find them bundled together and attached to soft corals such as the different variety of sea rods and also on fire coral. Interesting enough, the few photos I have in focus also shows your little critter hiding in its center. It's very interesting to imagine how these two creatures work together (if that). I will definitely have to sit and observe them further to try to figure something out.

Now if we can only find an anemone fish that lives within its center too!?!?

Please post any other information you happen to find out.

Alex

just an update on your amphipods

Wim Vader thinks it might be a pleustid or a pontogeneiid, or something else, but he doesn't think it is a stenothoid, the most common amphipod family on hydroids.

I will contact a pleustiid specialist in Ottawa, CA

cheers

Art

Alex

sorry, your hydroid amphipod cannot be identified further

Ed Hendricks in Ottawa wasn't even sure about the family

need microscope shots or dead bodies

cheers

Art

Hi Alex -- I've been carrying your images on my laptop in case I ran into more 'pod people. right now I'm at a taxonomic workshop in Panama with Jim Thomas, the top amphipod expert for the Caribbean. Like everyone else so far he said there's not enough showing to really id the critter but he also said that it's likely to belong to 1 of 3 families: Amphilocidae (which are commonly found on hydroids), Stenothoidae (also hydroid associates), or Cyproidae (former members of the Amphilocidae). This specific association is unknown to him.