Dear Lindsay,

I am afraid we got no follow-up on whether Hydatina would eat other polychaete worms. In the discussion you refer to I described how another worm-eater, Melanochlamys cylindrica, would eat worms in captivity it would never encounter in the field. Because of its 'suction pump' foregut it was able to ingest most worms presented to it, as long as they were stationary. Hydatina has evolved a different type of foregut with a very long 'proboscis-like oral tube' [see message #21498] which I guess is designed to be inserted down worm tubes. Whether it would work with free-living worms without tubes I am not sure. All I can say is I have not yet found the remains of 'tubeless' worms in the gut of Hydatina.

Hydatina physis sometimes occurs in quite large numbers so it would be an interesting little 'experiment' for some one to attempt - just tempt some Hydatina with a variety of worms and see how hungry they are.

Best wishes,
Bill Rudman

Dear Gary,

Thanks for these nice photos. In your photo of the shell we can see not only the gill [which I have marked g] but also the penis [p]. Hydatina is one of the few opisthobranchs which retains an external penis like this, almost all the others have an eversible penis which is usually hidden in an internal sac when not in use. In cephalaspideans this sac is usually on the right side of the head.  Interestingly, many marine snails have an external penis situated in much the same position as Hydatina.

Thanks also for the photo of its food worm. Have you actually seen it feeding? Why I ask is that when I first discovered the food of this species I was relying on stomach contents. Because of the extremely elongated oral tube I suggested that it must capture its prey by everting this long 'proboscis' down the burrow of the worm, or the crevices in which it lives. Valda Fraser's photos [#21498] showing an everted black tube certainly supported this hypothesis, but I have never witnessed Hydatina actually feeding, so I would be interested in an 'eyewitness' account if you have seen it in action.

• Rudman, W.B., 1972. The anatomy of the opisthobranch genus Hydatina and the functioning of the mantle cavity and alimentary canal. Zoological Journal of the Linnean Society, 51: 121-139.

Best wishes,
Bill Rudman

Dear Linda,

Thanks for an Atlantic example - of which we have very few. In fact I think yours is the first on the Forum from the west Atlantic. This Caribbean animal used to be considered a separate species, Hydatina vesicaria, but there appears to be no recognisable anatomical differences between it and the Pacific H. physis.

I have included a close-up alongside of the head as it shows part of the Hancocks Organ which is the chemosensory organ in primitive cephalaspidean sea slugs. Usually it is a pad or groove on each side of the head, but in species of Hydatina it consists of a row of folds or 'flaps'.

Best wishes,
Bill Rudman

Dear Valda,
Thanks for these interesting photos. I have never seen Hydatina with its proboscis [= oral tube] extendedlike this but when I studied these animals 30 years ago I did find on dissection that they had a very long muscular oral tube. Some specimens also had remains of cirratulinid worms in their gut. Cirratulinid worms live in crevices or in sand burrows and feed by extending long sticky tentacles out over the nearby sand and rock to pick up detrital food particles. I suggested that during feeding, the long oral tube was extended out of the mouth so that the jaws and teeth on the end of it could be inserted into crevices and burrows to grasp these worms. There are large muscles attached to the oral tube which would then drag it back into the body. Your photos show that the oral tube does indeed extend some distance outside the mouth. I suspect it can extend at least twice as long again as in your upper photo.

Species of Micromelo, and some species of Bullina [B. lineata does, B. roseana doesn't] have a long oral tube like this. In 1972, I had no information on the feeding choice of Micromelo and Bullina but since then I have discovered cirratulinid worms, or parts of them, in the alimetary canal of both Micromelo undata and Bullina lineata.

• Rudman, W.B., 1972. The anatomy of the opisthobranch genus Hydatina and the functioning of the mantle cavity and alimentary canal. Zoological Journal of the Linnean Society, 51: 121-139.
• Rudman, W.B., 1972. Studies on the primitive opisthobranch genera Bullina Ferussac and Micromelo Pilsbry. Zoological Journal of the Linnean Society, 51: 105-119.

Best wishes,
Bill Rudman

Dear Ian,
It's nice to get photos from the same locality showing the range in colour.
Best wishes,
Bill Rudman

Thanks Marina,
Hydatina physis seems to be another of those species which is often found in big aggregations. It is certainly nice to get some more photos of this Atlantic form of the species which Odhner gave the name H. stromfelti [see message #1835]. In your photo of the three animals together, you can see in the top right animal the cream coloured gill extending out from the mantle cavity within the shell. It penis, just below the gill, is probably impregnating the upper left animal. It seems the threesome is someone successful as it seems the top left animal is mating with the bottom animal.  In the photo showing the two animals there is also an egg mass at the bottom right.
Best wishes,
Bill Rudman

Thanks Marina,
Best wishes,
Bill Rudman

Dear Liza,
I can only guess without a photo, but your animal was probably Hydatina physis, which although a sea slug (opisthobranch) is not a nudibranch. Nudibranchs are a group of sea slugs in which the shell is completely lost except in the larval stages. Have a look at the Fact Sheet for Hydatina physis, and other attached messages, for further information.
Best wishes,
Bill Rudman

Thanks Ian,
It's nice to get another observation of egg-laying behaviour in Hydatina. As I discussed in an earlier message from Japan, it seems that this species produces its whole egg mass before anchoring it in the substrate, which is rather different from most opistobranchs which cement the egg ribbon down as it is being layed. I found indications of the same procedure with Pupa kirki in New Zealand so perhaps it is a common behaviour within the Acteonoidea.
Best wishes
Bill Rudman

Thanks Yukari,
Hydatina and the related animals in the superfamily Acteonoidea are considered very primitive opisthobranchs. Apart from obvious primitive signs such as having an external shell, these animals all have a large external penis. In all other opisthobranchs the penis is internal and only everts during mating.

From your photos, and others I have seen, the egg ribbon of Hydatina also interesting in the way it is formed. It seems as though the whole ribbon is completely produced before it is attached to the substrate. The egg ribbon of Hydatina is attached by a mucus thread or string which is anchored in the sediment. It seems that anchoring the ribbon is the last stage in the process and only occurs after all the ribbon is exuded from the genital opening. A quite different process than that of nudibranchs and other opisthobranchs which gradually attach the ribbon as it is being exuded from the genital pore.
Best wishges
Bill Rudman

Thanks Paul,
This species is quite variable in its colour shading. It can range from this very dark form to a pale pinkish white in which the bluish border is almost invisible.
Best wishes
Bill Rudman

Dear Kayoko,
I don't think the feathery object is part of the Hydatina. Could it possibly be a fish scale or something similar that has got stuck on the edge of the foot? I have dissected a few of these animals over the years and I can't recall any organ that looks like this, so I can only guess that it's something from the sand that has got stuck on the foot.
Best wishes,
Bill Rudman

Dear Leanne & David,
Its possible they are mating but it could be that they have caught the scent of the cirratulid worms they eat.
Best wishes,
Bill Rudman

Dear Marina & Patrice,
It's nice to see living animals of this East Atlantic form of Hydatina physis. This is the form that Odhner (1931) desribed from the Canary Ids as Hydatina stromfelti. In an earlier message I discuss my anatomical studies of this form, but I only had preserved animals to look at.
Best wishes,
Bill Rudman

Thanks George,
I have only ever found them once in large numbers but I regularly am told of people seeing large aggregations of them together. I guess like Aplysia aggregations its just a case of a simultaneous settlement of lots of larvae together all reaching maturity about the same time. The problem with trying to study something like this is that there is no way to be sure just where and where it will happen again.

One intersting feature you can see in your photos is how the colour of the animal can vary considerably depending on the lighting. It is not just an artefact photography and can appear this way when viewed with the naked eye.
Best wishes,
Bill Rudman

Thanks George,
Yes these are quite spectacular animals. If you have some photos of them clustering together, mating etc I would be interested in seeing them.
Best wishes,
Bill Rudman

Dear Mary Jane,
The common name, at least in Australia, of Hydatina physis - The Rose-petal Bubble Shell - is one of the few common names for opisthobranchs which does justice to the animal. It has an amazing variability in colour from almost translucent white to this deeper reddish ink. Both it and
Bullina lineata can have the brilliant flourescent blue border which is so difficult to capture on film.

Best wishes,
Bill Rudman.

Dear Peter,
Your quotations remind me of those medieval bestiarys where weird animals were described and even weirder tales told about them.

I certainly know of no record of any species of Hydatina swimming. The H. hydatis she mentions is probably the European Haminoea hydatis, but I am sure it does not swim either.

The most likely candidate would be the aplysiid Akera bullata Muller, 1776. which has a Haminoea-like shell and is a well-known, if clumsy swimmer.

Best wishes,
Bill Rudman.

Dear Marco,
Many years ago I looked at the shells and animals of animals of H. stromfelti from the Canary Islands, H. vesicaria from Atlantic and H. physis from parts of the Pacific and concluded that they were all one species. I could find no anatomical differences and no consistent shell differences. I have recently looked at a range of shells in the Australian Museum collections and can find more variation within shells from one part of the Indo-West Pacific (eastern Australia) than I can between oceans. My feeling therefore remains that there is one circumtropical species, much like we now consider there is one circumtropical species of the related Micromelo.

Reference:
• Rudman, W.B., 1972. The anatomy of the opisthobranch genus Hydatina and the functioning of the mantle cavity and alimentary canal. Zoological Journal of the Linnean Society, 51: 121-139.

Best wishes,
Bill Rudman.

Dear Ian,
Years ago I looked at the stomach contents of H. physis from various localities around the world, as well as other related species, and in all cases the worm remains were easily identified as belonging to the Cirratulinae, a subfamily of the Cirratulinidae. Over the years I have checked the odd stomach and again have only found similar worms, suggesting they are fairly specific feeders.

However as Marphysa would be a lot easier to collect, I guess anything is worth a go. I remember keeping a new Zealand species of worm-eating aglajid, Melanochlamys cylindrica, alive in aquaria for many months on worms that it could not possibly feed on in nature. Species of Melanochlamys feed by rapidly sucking in polychaete worms much like sucking in a long piece of spaghetti. I often found them stuck head down in coralline algae turf, one end of a eunicid or nereid worm in their mouth and the other end desperately trying to escape into a crack or burrow. Collecting nereid worms for them to eat was a time consuming task so on one occasion I decided to test them on Sabellaria kaiparaensis, a worm which builds a tube from sandgrains and clusters in large colonies forming sandy reefs on the shore. Large numbers of worms could be quickly extracted by breaking off sections of the reefs. I would then freeze them in conveniently sized 'ice-blocks' and throw a block into the aquarium every few days where it would melt. The worms would thaw out and the aglajids would eat them with great enthusiasm. It seems in that case the aglajids were eating certain worms on the shore, not so much from choice, but because they were able to catch them.

It is possible with Hydatina that it eats cirratulid worms because they are relatively easy to catch in the areas in which it lives. If so then perhaps it would eat other types of worms if they were offered. To have any chance of success though, I suspect they would have to be freshly dead, as alive they would probably move to fast for Hydatina.

For those with no idea about polychaete worms, cirratulids have no head appendages but have long thread-like tentacles along most of the body but especially around the head. They live in crevices, empty burrows or under rocks partially embedded in the sand or mud. They live surrounded by a 'coccoon' of debris, and they extend their tentacles out like the head tentacles of terebellid worms, to collect food particles. On the other hand, eunicid worms like Marphysa, although living in burrows, are quite active worms emerging either partially or fully from their burrow to feed on carrion, or in some cases, algae.

Ian, if your enquirer has any success with Marphysa, I would be interested to hear about it. Collecting cirratulinid worms would probably be quite difficult and require quite extensive environmental disturbance, turning embedded rocks and digging and scraping around to find them.

Best wishes,
Bill Rudman.

Dear Laureen,
Common names are a real problem. Usually only the author who invented them uses them and so they are hardly 'common'. It would be better if publishers treated their readers with some intelligence and stopped insisting on authors making up common names.

The animal with this name is Hydatina physis, which you'll see from the photo has a proportionally large frilly body which could be likened to rose petals. The cephalaspid Bubble Shells are an interesting group of Sea Slugs with modern representatives showing all the evolutionary stages the ancestors of today's Sea Slugs must have gone through to become shell-less slugs. Have a look at Pupa and Bullina to see two more snail-like representatives. Also have a look at the pages on Detorsion - how snails became slugs, What is a slug?, and torsion and nudibranch gills for general information on the topic.
Best wishes,
Bill Rudman.