Exploring the Underwater World 2023

Umbrella Sea Slug - Umbraculum umbraculum.

During November and December, the water has been warming up in the lagoon at Kingston and in November, when snorkelling in Slaughter Bay searching for nudibranch, I found an interesting lagoon slug which I had not seen before. This was about the size of the palm of my hand and was an orange colour and appeared to have coral or weed growth on top. My husband and I managed to get a few photos of the sea slug which was on the inner ledge of the coral when I swam towards the outer edge of the inner lagoon, and it was near the upper edge of the coral reef, about less than a metre deep at low tide.

Checking the images at home, I have explored on my computer, researching for other images of nudibranchs and sharing photos with friends and Susan Prior, I eventually learnt this was a sea Slug named the Umbrella Slug, it is called Umbraculum umbraculum.

I posted images on Facebook ID Please (Marine Creature Identification)

ID Please (Marine Creature Identification) | Facebook, and members on this group identified my sea slug as Umbraculum umbraculum or Umbrella Slug.

I read that the species of Umbraculum umbraculum, Umbrella Slugs, rhinophores and eyes, and when I photographed the Umbrella Slug these can be seen in my images.

Rhinophore, are the head anatomy, a rhinophore is one of a pair of chemosensory club-shaped, rod-shaped or ear-like structures which are the most prominent part of the external head anatomy in sea slugs, marine gastropod opisthobranch. Rhinophores allow sea slugs to find their food and other slugs to mate with, avoid predators, and even sense changes in water pressure and vibrations. Different nerves within the rhinophores send information to the brain.

I learnt more about the Umbrella Slugs at this web site.

Umbrella Slug (Umbraculum umbraculum) · iNaturalist Australia (ala.org.au)

Umbraculum umbraculum are a species of large sea snail or limpet, a marine gastropod mollusc in the family Umbraculidae. It is found in tropical to warm temperate parts of the Indo-Pacific and Atlantic Oceans, where it feeds on sponges. Umbraculum umbraculum is a large mollusc growing to a length of up to 20 cm (8 in), with a relatively small external shell on the dorsal surface. This shell is a flattish cone and is usually encrusted with epiphytic organisms, both plant and animal. The mantle is roughly circular in outline, and is covered with pustules; the ground colour of the mantle varies, usually being orange or brownish, and the pustules are usually white. The mollusc does not elongate when crawling and the location of the head is revealed when it thrusts out its rolled rhinophores from beneath the shell. At the base of the rhinophores are a pair of eyes, and beneath is a cleft which contains the mouth. The radula is broad and is armed with many small teeth.^[3][4] The sole of the foot is smooth and flat; the gills are bipinnate and are located in the right side of the body.^[5]

Umbraculum umbraculum is found in the Pacific Ocean in both the tropics and in warm temperate regions. Its range includes Indonesia, Australia, New Zealand, Hawaii, Mexico and Panama.^[4] It also occurs in the Red Sea, the Mediterranean Sea and the warmer parts of the Atlantic Ocean. In 2016, researchers reported that it had been observed for the first time in the Bay of Biscay in surveys undertaken between 2011 and 2014, in what seemed to be a northward shift in its range; this "tropicalisation" was attributed to a rise in sea temperature.^[5] It occurs in both shallow water and at depths down to about 275 m (900 ft).^[4]

Umbraculum umbraculum feeds on sponges and its colouring often makes it well camouflaged. It rasps the surface of the sponge with its radula, opening the cleft on its base wide so that it can press its mouth against the surface of the sponge. This slug has been found on Tethya spp., Aaptos aaptos, Ancorina alata and Plakina trilopha.^[3] The eggs are laid in a long, coiled ribbon and the larvae are planktonic.^[5]

References

1. Rosenberg, G.; Gofas, S. (2010). Umbraculum umbraculum (Lightfoot, 1786). In: Bouchet, P.; Gofas, S.; Rosenberg, G. (2010) World Marine Mollusca database. Accessed through: World Register of Marine Species at http://www.marinespecies.org/aphia.php?p=taxdetails&id=141879 on 23 July 2020
2. ^ Goemans, Bob. "Umbraculum umbraculum: Umbrella slug". SaltCorner. Retrieved 24 July 2020.
3. ^ ^{a b} Rudman, W.B. (7 March 1999). "Umbraculum umbraculum (Lightfoot, 1786)". Sea Slug Forum. Retrieved 23 July 2020.
4. ^ ^{a b c} Miller, Michael D. (2004). "Umbraculum umbraculum". Opisthobranch of the week. The Slug Site. Retrieved 23 July 2020.
5. ^ ^{a b c} Arias, A. & Crocetta, F. (2016). "Umbraculum umbraculum (Gastropoda: Heterobranchia) spreading northwards: additional evidence to the "tropicalization" of the Bay of Biscay". Cahiers de Biologie Marine. 57: 285–286.

Umbraculum umbraculum - Wikipedia

Umbraculum umbraculum, common name the umbrella slug, is a species of large sea snail or limpet, a marine gastropod mollusc in the family Umbraculidae.^[2] It is found in tropical to warm temperate parts of the Indo-Pacific and Atlantic Oceans, where it feeds on sponges.

I wanted to learn the difference between Sea Slugs and Nudibranch and found this link interesting reading. One site suggested that nudibranchs have gills outside and easy to see. The nudibranchs also have a very potent chemicals they store in their skin make them seriously distasteful to potential predators. They therefore make little attempt to hide and so are often the only nudibranchs on open display.

Nudibranch or sea slug? Whats the difference? - More Fun Diving

Often the term “nudibranch” is used for all sea slugs. However, while all nudibranchs are sea slugs, not all sea slugs are nudibranchs.

“Sea slug” does not stand for a specific taxonomic group but they are part of different clads in the class of Gastropoda (snails and slugs including those in freshwater and on land). So, in other words “sea slug” does not say anything about the relations between the different species but it is a common name for snails or slugs that live in the ocean.

The taxonomy has been changed around a lot during the last 10 or so years and I got quite confused reading scientific papers and reading nudibranch identification books. It looks like there is still a lot of work to do and a lot will change further.

If you go snorkelling, good luck finding these amazing underwater creatures. I think I have found a few Norfolk Island nudibranchs and sea slugs so far when snorkelling in the Slaughter Bay and Emily Bay.

Check out Susan Prior Facebook page, Norfolk Island Reef, to view more images of these amazing creatures, many of which can be very small. Norfolk Island's Reef (norfolkislandreef.com.au)

Here are images of this amazing Sea Slug, I think it suits the name Umbrella Slug.

Betty Matthews

December 2023