Solar powered sea-slug
September 10, 2002
From: Michel Barrabés
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Dear Bill,
On July 25th 2002, I found a slug looking like Spurilla neapolitana at Bassin d'Arcachon, France [Atlantic coast]. It was in a meadow of Zostera nana in the intertidal zone. So it was out of water 6 hours a day ... It had the brown network of symbiotic algae you describe, but it's the first time I seen a Spurilla in such a place. I have found it previously at about 5 to 10 meters, on rocks or sand, but at depth where light is very poor in our green waters. And the previous specimen I found where pink or orange or white, but without the brown network. I have sent photos of this pinkish form in a separate message.
PHOTOS: 7 PM on 25/08/02 about 30 mm long (there were 3 specimen from 10 mm to 30mm with their spawn, similar to that of Spurilla) 0.50 m deep at full tide near "Ile aux oiseaux" in the middle of Bassin d'Arcachon, among a meadow of Zostera nana.
Best regards,
Michel Barrabés
France.
m.barrabes@libertysurf.fr
Barrabés, M., 2002 (Sep 10) Solar powered sea-slug. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/7886![](/images/m7886d.jpg)
Dear Michel,
Thanks for the interesting photos. Firstly animals in intertidal sea grass beds are largely protected from drying out during low tide by being 'wrapped in' the sea grass leaves which retain considerable amounts of water between the leaves.
Secondly, many of the sea slugs which keep zooxanthellae in their tissues, either have 'forms' which lack zooxanthellae, or go through stages in their life when they lack zooxanthellae. I suspect that those without zooxanthellae are either unable to find zooxanthellae or else are living in an environment in which zooxanthellae cannot survive. As you suggest, poor light at depth may be the reason in this case. Many slugs, including Spurilla obtain their zooxanthellae from the sea anemones they feed on. If the sea anemones that Spurilla is eating lack zooxanthellae then that would explain the pink forms.
What we don't know is how vital the zooxanthellae are for the slug's survival. It is possible that in some species the zooxanthellae are essential, but in most it seems they can survive without them. Whether they can grow to maturity and lay eggs without the nutrition provided by the zooxanthellae is a mystery still to be answered.
Best wishes,
Bill Rudman
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