Mention slugs to people and you are likely to get "Ugh!"
as a response - and from gardeners a more hostile reaction. This is
because slugs are slimy and they also eat plants, often in competition with humans
when we are growing plants for food or decoration.
To a Natural Historian, slugs are fascinating, as they are
gastropod molluscs that have lost their shell during evolution. Adult gastropods,
with few exceptions, move using muscular contractions and expansions of the
foot, sliding over a film of secreted slimy mucus. Production of mucus is also important
in providing a body covering to prevent desiccation and as a deterrent to predators.
Nevertheless, many slugs are predated and gardeners are encouraged to provide
areas for hedgehogs or predatory birds. One solution to the pest problem that is rarely
mentioned is the collection of slugs for food, just as we gather some types of
snails (Roman snails, winkles, whelks, etc.). Here is a recipe adapted from one
posted on the "eattheweeds" website [1]:
Allow slugs to feed on salad
leaves and then kill them in a vinegar-water mixture before boiling them in
water, repeating this three times. The prepared slugs can now be used to make
patties.
Beat 3 eggs with 3 tablespoons of
double cream and add the mixture to 3 tablespoons of flour, the same quantity
of cornmeal, and 10 slugs (chopped into bite-sized pieces). Whisk, form into
patties, and then fry in butter. Serve with bread and salad.
Some of you may find this low on your list of recipes to try,
but slugs are attracting the interest of at least one celebrity chef [2] and
they are a source of protein that is readily available. Hugh
Fearnley-Whittingstall recommends slugs cooked in a tomato sauce [2].
Less familiar to us than terrestrial slugs are those that live in
the sea and these are not easily gathered in quantity, so are unlikely to
be utilised widely as food. There are reports of sea slugs being eaten by
humans, but these accounts arise from confusion with sea cucumbers (holothurian
echinoderms) that are eaten in many parts of the World. Holothurians formed
part of the famous dinner of the Acclimatization Society in 1862 where views on flavour and consistency were mixed [3]. However, true sea slugs are now
finding their way on to speciality restaurant menus [4] – and note that the
article refers to them as "trash from the sea". It is an unfortunate
term for remarkable and beautiful animals.
It was a sea slug, Marionia
sp, that I "adopted" at the UCL Grant Museum of Zoology [5]. It is
mounted with the foot facing the viewer and this was done to allow students,
and other observers, to see the dimensions of the foot and thus the area in
contact with the substratum (see above left).
Unfortunately, preservation in spirit removes pigment and the living
specimen would have been light orange in colour, providing camouflage (see above right). Reference
to the Sea Slug Forum [6] shows the range of sea slugs and some of their extraordinary
adaptations and beautiful colours. These colours are not for our appreciation,
of course, although a Creationist might disagree, but to provide warning or camouflage.
Warning of what? I'll come to that question, but first we need to know a bit
more about the body plan of sea slugs and how they are different to snails.
During evolution, some of the internal organs of gastropods
underwent torsion, with the gut opening forward into the mantle cavity at the
shell entrance. Slugs, including sea slugs, have undergone de-torsion, with the
anus being towards the rear, nearer to their ancestral positions. The anterior
end of the gut of snails and slugs features a rasping tongue, or radula, and
this is used to remove food from the surface of stones or vegetation, or for
scraping away the tissue of living plants and animals. Marionia feeds on corals [7] and the Grant Museum specimen, labelled
Marionaria quadrilatera (but actually
Marionaria blainvillea) would have
fed on these colonial animals while living in the Mediterranean Sea.
A characteristic of corals is the ability of individual polyps
to retract and to use stinging cells as a means of catching food, just like
their relatives the jellyfish (many corals also generate their own food by
harbouring algal cells within their tissues). The stinging cells, or
nematocysts (see below), have a trigger that releases a thread through which
toxins are injected to immobilise prey, a series of small barbs holding the
thread into the tissue. They also serve in defence and a grazing sea slug will be
at risk of being attacked by these stinging threads as they eat the coral
tissue, even when polyps are retracted. The evolution of stinging cells in coelenterates
is an amazing adaptation, but there are some sea slugs that eat coral tissue
without the nematocysts discharging. Not only that, intact cells can be moved
to extensions of the body, called cerata, on the back of the sea slug through
branches of the gut that extend into the projections. Any animals attacking these
sea slugs now receive stings from coral nematocysts and the bright colour of
the molluscs probably acts as a warning.
Slugs may not appeal to everyone, but we should all be amazed
at some of their adaptations. How these evolved can only be speculated upon, but there are few examples in nature as
extraordinary as the evolution of nematocysts by coelenterates, followed by
their hijacking by some sea slugs. It's worth thinking about if you feel tempted
to eat some.
[3] G.H.O.Burgess (1967) The
Eccentric Ark: The Curious World Of Frank Buckland. New York, The Horizon
Press.
The UCL Grant Museum of Zoology gave permission for me to use the photograph of the preserved Marionaria specimen.
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