Snail ID: Hydatina physis - 3/20/09
Hi again!
<Hi, or should I say bonjour!>
I need your help for a snail identification.
<It’s actually a neat little sea slug!>
I sent you a picture...
<Yes indeed, and it’s a beaut.>
I'm pretty sure it's not reef safe
<Well, it’s a predator alright, but one that feeds exclusively on Cirratulid Polychaetes (aka ”hair-worms”) so everything else is safe. As with many sea slugs and Nudibranchs, they’re beautiful but difficult to keep long term because of their specialized diets.>
...but can you tell me its name?
<Yep, it’s Hydatina physis, a cephalaspidean sea slug that carries a thin shell around on its back. It’s also commonly called variations of the Brown-Lined Paper Bubble or Bubble shell. Please see the following link for more information: http://www.seaslugforum.net/showall.cfm?base=hydaphys >
Thank you very much!
<Thank you for sharing such a terrific photo of a beautiful little creature! Take care, Lynn>

Critter ID? Operculums and Possible Sand Anemone - 3/19/09
<Hello Christopher, Lynn here this morning.>
A shell of a what?
<More like what's in the shell!>
One might think that after 40+ years of fishkeeping, 8 years of outright reef obsession, and being supported by the bountiful resources of WW, I could tell, or at least easily discover, if something is a mollusk or a worm, or neither.
<Heheee! I’ve been in the hobby since the early 70’s and believe me, it happens.>
I recently had the opportunity to go bigger (50g to 125g).
<Nice>
After the 6 months of preparation, moving, acclimation, quarantine, and additions to the new system, I finally have my dream tank, short of glass walls everywhere. Thanks to advice found on your site, I have had great success, the move went swimmingly and the only surprises have been how much both my reef inhabitants and I enjoy the larger space and there are these shells. I have identified dozens of the hundreds of “hitch-hiking” organisms that have populated my system over the years and continue to pop up. While moving the old sand bed as a “start” for the new larger one I decided to sift out some of the rubble that had accumulated from generations of free limpets, tubeworms, Chitons, Nerites, Vermetid snails and from the not-so-free snails, hermit crabs, coral base rocks and the like. (A pair of maroon clowns can tend to break a lot of things around their house over the years) Among the remains, I discovered those of an animal that I had never seen before but it had obviously thrived and multiplied for some time. They appear to live or have lived on the glass bottom under the sand. What is left are disk-shaped hard shells with a completely flat, almost polished bottom and an intricate spiral growth pattern on the top. I have attached a photo of the “shell remains” that I found in great numbers (dozens, perhaps hundreds) in the sand. The three at the top are turned upside down to show the bottom surface.
<They’re snail operculums/opercula - the trap door at the opening of a snail that protects the soft animal inside from predation, desiccation, etc. Some are thin, and flexible, while others are thick and calcareous. Yours are obviously of the latter variety and most appear to be from bygone Turbo snails. See these links for more information/comparison: http://aphriza.wordpress.com/2006/12/21/trapdoor-on-the-seafloor/
http://seashellplace.com/catalog/images/others_eye_green.jpg
http://farm4.static.flickr.com/3261/2319171639_4443687a10.jpg
http://en.wikipedia.org/wiki/Operculum_(gastropod) >
I later remembered a photo I took a few years ago of some other unidentified animals that I occasionally saw poking out from the sand. After scouring your site at the time I assumed them to be some sort of small anemone. I never could get a real up-close observation of it and the only photo I got was this poor one taken before I read all of your cautions about using a zoom lens to take a close up in my aquarium. Perhaps it will be clear enough for you to tell what kind of creature it is and whether it is perhaps the maker of the shells?
<I sure wish I could help with this one, but I can’t see it well enough to be able to determine what it is. If I had to guess, I’d say that it might be some kind of anemone, possibly a sand anemone (Phyllactis spp). Please see the photo at the following link for comparison:
http://www.flickr.com/photos/lemurdillo/3044460357/in/set-72157605663802360/ >
As you can tell it is about twice the size of a typical xenia polyp. It had what appeared to be a plume of translucent fleshy brownish speckled tentacles protruding from the 4-5” sand bed next to the base rock. I am stumped by these guys and can’t seem to find images or descriptions of similar organisms.
FYI: 125 Gallon, 20 gal sump/refugium, deep sandbed, all parameters are good.
Lots of healthy live rock with colorful algae, sponges, tunicates, worms, tiny seastars, pods, macro algae, etc, etc. and mature colonies of soft corals introduced years ago and re-established over the rockwork.
Corals: Mushrooms, leathers, xenia and Zoanthids
A theme of relationships:
2 – Maroon Clownfish (Premnas biaculeatus) mated pair (7 yr)
1 – BTA (Entacmaea quadricolor) hosts above in separate “bommie” (6 mo)
1 – Pink Spotted Watchman Goby (Cryptocentrus leptocephalus) (4 yr)
1 – Tiger Pistol Shrimp (Alpheus bellulus) (3 yrs)
5 – Lyretail Anthias (Pseudanthias squamipinnis) (5 mo)
1 – Blue Devil Damsel (Chrysiptera cyanea)
1 – Midas Blenny (Ecsenius midas) (1 yr)
4 – Canary Wrasse (Halichoeres chrysus) (1-5 yr, 3-4 mo)
2 – Cleaner Shrimp (Lysmata amboinensis) (4 yr)
Grateful to have and to share it all. Any comments are welcome.
Christopher Williams
Santa Barbara, CA
<Hope that helps! Take care, Lynn Zurik – Everett, Wa>