Some notes about Glaucus atlanticus

May 9, 2006
From: Irina Roginskaya

Dear Bill,
I am so glad the Forum is back! We really felt orphaned without daily opisthobranch news from all the world!

I am sending you two photographs of Glaucus atlanticus I took (as you can see from their quality) in predigital epoch) (Zenith-E camera). Three specimens of G. atlanticus were collected by pleuston net during the cruise no.59 of R/V "Vytiaz" in the Pacific Ocean.(Sta.7505, 34 17'9 N, 143 52'0 E, June 25 1976) together with their food objects hydrozoan cnidarians, the Chondrophorina: Velella with a keel-like "sail" elevated above the surface of water and disk-shaped Porpita ( the latter on photo 1) . The body length of the Glaucus in spirit 12.0 mm, 13.5 mm (dissected) and 16.5 mm. Diameter in spirit ot the disk of Porpita = 10 mm. The radula of dissected specimen contained 14 rows of teeth. This specimen was in the process of egg-laying , as the bead -like egg-strings were appearing from the oviduct. It seems to me that there were no photos in the Forum of G. atlanticus from this area.

Locality: 34 17' 9 N, 143 52' 0 E, surface of water, Open Ocean, Pacific Ocean, 25 June 1976, collected by pleuston net. Length: 12.0 mm, 13.5 mm, 16.5 mm in spirit. Photographer: IrinaS.Roginskaya

As recently a lot of messages, touching upon the feeding, egg-laying, cannibalistic habits etc. of  Glaucus appeared in the Forum I couldn't resist the temptation to add some information concerning Glaucus, mainly from the observations of the late Dr. A.I.Savilov from the P.P.Shirshov Institute of Oceanology , Moscow,  Russia (Savilov, 1956,1968). Perhaps these works are unknown to you and colleagues as being published in Russian only.

Dr.A.I. Savilov (1913-1969) - marine biologist, the well known specialist of the surface fauna of the ocean, keen observer, amateur graphic artist, participant of many scientific ocean cruises. You can find in his papers a lot of life history characteristics of bouyant organisms: Glaucus, Glaucilla, Fiona, Janthina, Physalia, Velella, Porpita, Actiniaria etc. The fragments of  text I cite here are from two papers of A.I. Savilov [my translation - so please be understanding] .

From Savilov,1956, page 65:
"Glaucus is a carnivore, feeding on Velella. It prefers to keep freely on the surface of water and approaches the prey only being hungry. For long hours during the drift of "Vytiaz' we looked closely from the board at the surface of the ocean in hope to reveal Glaucus in the middle of floating past Velellas. At last we were lucky to discover it. In the slightly splashing water we noticed an exemplar of swimming Velella and behind it broadly spreading their papillae the pair of Glaucus keeping on the surface. The velvety-blue coloured molluscs were nearly invisible on the background of the blue-green ocean depth. Sometimes when the Glaucus were thrown away by wave or a splash of water, they rather quickly, paddling with the bundles of papillae as with oars, rowed back to Velella."

This contrary to the opinion of T.E.Thompson and I.D.MacFarlane (1967) that "small contractions and movements of the cerata and metapodium are not directional in function. Glaucus is thus planktonic rather than pelagic" and later of T.E.Thompson (1976), considering glaucid nudibranchs to be passively planktonic as the movements of their cerata "do not appear to result in translocation". P. Bieri was aware of these field observations of Savilov (1956), concerning the intentional swimming of free-living Glaucus atlanticus to its prey. (Mr. Saburo Nishimura kindly lent him his translation of Savilov's report). But R. Bieri stated that the specimens of Glaucus in his experiments - maintained in jars with running water - could not swim. I think that as these laboratory observations may not reflect the conditions in the open sea, the behaviour of his Glaucus could be greatly affected by maintenance in captivity. By the way, A. Fredol characterizing Glaucus wrote: "Quoique paresseux, il nage avec vitesse". If the observations of Savilov (1956) are not accidental we can consider Glaucus to be a pelagic organism rather than a planktonic one.

P. Bieri (1966) already described the cannibalistic habits of his specimens of Glaucus:
" Glaucus frequently attacked one another and bit off pieces of tail and 'arms". One 20 mm long individual ate all but the head and trunk of a 15 mm long individual in 30 minutes." And further in the Discussion: "Glaucus prefers Physalia. On the other hand, when food is in short supply or after periods of prolonged starvation, the niche differentiation probably breaks down and then even cannibalism is practiced by Janthina and Glaucus. (The same situation as described by Peter Whiter (Whiter, P. 2006 .March 28) and earlier by R. Armstrong (Armstrong, R.1999. Feb.26).

A.I. Savilov (1968) describes the egg-laying of Glaucus.:
" For egg-laying, Glaucus needs floating objects. They attach their egg-masses to the skeleton plates of dead Chondrophora and small driftwood". He also emphasizes the importance of Chondrophora as distributional and topical factor (page 292). " The skeletal plates of Chondrophora after being picked clean by predators, retain their bouyancy for a long time and are frequently used for attachment of egg masses. Some species (Glaucus, Halobates) utilize these plates for laying their cocoons and eggs, the others - for attachment of their juveniles during the first days of their life (Lepas ), until their own floats are produced; some species inhabit skeleton plates either permanently (Fiona, Turbellaria) or temporary (Planes, Isothea, Metapennaeus)".

There is similar data about the egg-laying of Glaucus  in the monograph of W.J. Dakin (1953). He indicated (pages 270-271) that " Requiring something a little more substantial than water on which to deposit its egg masses , Glaucus feeds on Velella, lays its eggs on the skeleton framework of the creature it has been eating".  Maybe only in the absence of suitable substrata are specimens of Glaucus  forced to deposit their egg-strings directly on the surface where they float. (Rudman,1999. Feb 25). Thompson and MacFarlane (1967) admit both modes of egg-laying: " Strings of eggs each consisting of 12 to 15 eggs are shed into the sea, or attached to the prey".

• Armstrong, R.1999 (Feb.26) Re: Glaucus in New Zealand. Message in Sea Slug Forum. Australian Museum,Sydney. Available from http://www.seaslugforum.net/find/616
• Bieri R. 1966. Feeding preferences and rates of the snail, Janthina prolongata, the barnacle Lepas anserifera, the nudibranchs Glaucus atlanticus and Fiona pinnata and the food web in the marine pleiston. Publications Seto Marine Biological Laboratory, vol.XIV, no.2, pp.161-170, PlatesIII-IV.
• Dakin, W. J. 1953. Australian Seashores. Angus andRobertson.
• Fredol, A. 1866. Le Monde de la Mer. Deuxieme edition. vii+693 pp. Paris. Librairie de L.Hachette etC.
• Rudman , W.B. 1999. (Feb 25). Comment on Glaucus in New Zealand. Message in Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/616
• Savilov, A.I. 1956. Floating biocoenosis in the Pacific Ocean based on the material of the Expeditions of the Institute of Oceanology, Academy of Sciences USSR. Priroda, Vol.45, no.3,pp.62-68.
• Savilov A.I. 1968. Pleuston of the Pacific Ocean.Pages 264-353 in the Monography "Pacific Ocean. Biology of Pacific Ocean ". Part 2. Moscow. Academy of Sciences of the USSR. Institute of Oceanology."Nauka".
• Thompson T.E. !976. Biology of Opisthobranch Molluscs.Vol.1, 207 pp, London The Ray Society. No151.
• Thompson T.E. and Mac Farlane I.D. 1967  Observations on a collection of Glaucus from the Gulf of Aden with a critical review of published records of Glaucidae (Gastropoda, Opisthobranchia). Proceedings Linnean Society London. Vol. 178, no.2 pp 107-123.
• Whiter, P.A 2006 (Mar 28). Cannibalism in Glaucus? Message in Sea Slug Forum. Australian Museum, Sydney Available from http://www.seaslugforum.net/find/16189

Irina Roginskaya

irina7@hotmail.com

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