By Dr Deborah Robertson-Andersson
Bubble raft snails or Violet Snails or Purple sea snails are a very high priority on the beach shell collectors list. The first time I saw a whole one I was in KwaZulu Natal. When I saw it I pranced up and down the beach, so much so that my research companion thought I was having some sort of fit.
The shells are very fragile so you only find whole ones if a light onshore wind has been blowing and there are gentle waves breaking, which in Cape Town is a very rare thing. The family Janthinidae is very small and only consists of about 8 species, 6 of which are found in South Africa. What makes the wash ups this month quite unique, are that there have been 4 different species found at the same beach.
The snails hang upside down from the sea surface and are kept suspended by a raft of clear mucus-chitin coated bubbles which are produced by the foot of the snail. They use their feet to agitate the water, creating bubbles, which they bind together with mucus. They cannot swim and are blown around by the wind. They are blind and find their prey by touch. The snails predate on “By-The-Wind-Sailors” and “blue bottles”. The purple colour comes from a gland that is secreted as an anaesthetic to prevent the dying prey from shedding their tentacles.
Fertilization occurs inside the female, but as males lack a penis, there is no direct mating. Instead, the males release their sperm into a case that drifts to a female, where the sperm fertilizes the eggs. The eggs develop internally and are born live, with the tiny purple snails immediately able to build their own rafts. If the bubble raft ever breaks apart, the snail will sink into the ocean depths and die.
Violet snails are preyed upon by fish, birds, sea turtles, other molluscs and nudibranchs. The snails sometimes have goose barnacles attached to their shell as hitchhikers.
Species found on our beaches:
Janthina janthina (Linnaeus, 1758)
Janthina janthina is a carnivore, feeding on relatively large floating prey animals such as By-The-Wind-Sailors (Velella velella), other raft snails (Janthinidae), Halobatidae (marine water striders the only true marine insects) and Siphonophora (Cnidaria – jellyfish). They are found world wide in waters over 10 °C. This is the most common species and washes up from Cape Town to Mozambique. The snail’s shell is reverse counter shaded and are paler on the top of the shell compared to the bottom and this is because they live upside down and helps in hiding them from below and above. This is the only violet snail whose young are brooded within the ovaries of the female and the young are expelled from the shell as late larvae.
Janthina prolongata De Blainville, 1822
Janthina prolongata gives birth to mobile young which are all males, it then changes sex from male to female as it gets older. The young are brooded in capsules on the underside of the float (see the picture above). A single float can carry 250 to 400 egg capsules of varying ages. Each capsule can contain 75 to 7500 embryos per capsule. The young will be born as fully formed snails. This species is a carnivore and feeds on: Blue Buttons (Porpita), By-The-Wind-Sailors (Velella velella), other raft snails (Janthinidae), Haliobatidae (water striders) and Siphonophora (Cnidaria – jellyfish). They are canabalistic and will eat members of their own species. They occur in the same places as Janthina janthina and they often wash up together.
Janthina pallida W. Thompson, 1840
Janthina pallida are often found in wash ups with Janthina prolongata. If you look really hard you may see one of these but they are quite rare. It is often called the pale Janthina as it’s much lighter in colour than other Janthina. It occurs from 32 °N to 23 °S.
Janthina exigua Lamarck, 1816
Janthina exigua is also known as the dwarf Janthina. They are normally found in the open ocean in a belt on either side of the equator in waters of 15° – 25 °C. It is fairly common and is usually easily recognised by its smaller shape and darker colour.