Category: defence (Page 4 of 4)

Snails use their shells as a weapon against parasitic worms

Parasitic roundworms that invade a snail’s shell may be trapped, encased and fixed permanently to the inner layer of that shell, as Robbie Rae shows.

Thanks to its shell, a snail is protected against damage, predators, heat and cold, drought and rain. But there is more, as Robbie Rae discovered. The snail also uses its shell as a defence system to eliminate parasitic roundworms (nematodes). These parasites attack snails since snails appeared on earth, about 400 million years ago. It is obvious that snails had to evolve a defence mechanism against these enemies, but until now, no defence mechanism was known.

Encapsulation

In his lab, Rae exposed grove snails (Cepaea nemoralis) to the nematode Phasmarhabditis hermaphrodita for several weeks. This bottom dwelling animal, less than 2 millimetres long, is able to penetrate and kill many snail and slug species, but some snails are resistant, as for instance the grove snail. Rae studied the interaction between the grove snail and the worms to find out how the snail eliminates the parasites.

It turned out that the cells on the inner layer of the shell do the job. They adhere to an invading worm, multiply, and swarm over the parasite’s body until it is entirely covered. Engulfed by the cells, it is fused to the inside of the shell and dies. By this procedure, grove snails not only encapsulate this lethal roundworm, but they use the immune reaction also to kill other, less dangerous nematodes, as experiments showed.

In nature, this is common practice. Rae collected grove snails and white-lipped snails (Cepaea hortenis) from the wild and observed that many snails had different species of roundworms attached to their inner shell surface, up to 100 worms in one shell. Also the garden snail (Cornu aspersum) – like the other two snail species an inhabitant of Western Europe – uses its shell to eliminate invading worms by encapsulation.

Old defence

Finally, he examined a large number of snails from museum collections, to conclude that many snails of many different species had nematodes attached to their shells. Trapped worms proved to be fixed permanently; they even can be found in snails that died a few hundred years before. As this defence mechanism is found to be widespread among the large and old clade of terrestrial snails and slugs, it must have evolved about 100 million years ago. Even some slug species eliminate parasitic roundworms by this mechanism. During evolutionary history, their shells have become reduced and internalised, but in many species they retained the ability to trap, encase and kill roundworms.

The vineyard snail (Cernuella virgata) is one of the species that is unable to get rid of the roundworm Phasmarhabditis hermaphrodita. Apparently, the parasite evades its immune reaction in one way or the other. As many slug species are also susceptible to this parasite, it is formulated into a biological control agent to be used against herbivirous slugs.

Willy van Strien

Photos:
Large: grove snail, Cepaea nemoralis. Kristian Peters (Wikimedia Commons, Creative Commons CC BY-SA 3.0)
Small: nematodes fixed to the inner layer of a grove snail’s  shell. © Robbie Rae

Source:
Rae, R., 2017. The gastropod shell has been co-opted to kill parasitic nematodes. Scientific Reports 7: 4745. Doi: 10.1038/s41598-017-04695-5

Cloud of semiautonomous pincers protects sea urchin

When a hungry fish approaches, the collector sea urchin releases a cloud of small biting, venomous pincers that deter its enemy before it has attacked. This peculiar defensive strategy is revealed by Hannah Sheppard-Brennand and colleagues.

In addition to their prominent spines, sea urchins and sea stars possess numerous smaller appendages as well: little pincer-like heads on a movable stalk, called pedicellariae. The pincers have different functions, such as catching food or removing debris – and tormenting predators. For in spite of the unattractive appearance that sea urchins and sea stars have, predators such as fish will pick at their tube feet and other soft tissue. Some species have pedicellariae with teethed jaws and a venom sac to deal with such predators, and anyone who once stepped on such a sea urchin will remember the painful experience.

Cloud

One species, the collector sea urchin Tripneustes gratilla, deploys these venomous pedicellariae in an unique way, as Hannah Sheppard-Brennand and colleagues show: when harassed, this sea urchin will release a cloud of them in the surrounding water. Upon release, the pincer-like heads behave semi autonomously; they are mobile, have sensory structures and will bite and deliver their venom when they touch a supposed enemy.

Safe

Fish are deterred by such a swarm, as lab experiments revealed, and they will leave before they have bitten the sea urchin. Protected by his unique defensive army, the collector sea urchin is able to forage safely on grass and eelgrass during the day, when other sea urchin species have to take shelter, as well as during the night.

Willy van Strien

Photo:
Collector sea urchin Tripneustes gratilla with a cloud of released pedicellaria heads. © Hannah Sheppard Brennand

Source:
Sheppard-Brennand, H., A.G.B. Poore & S.A. Dworjanyn, 2017. A waterborne pursuit-deterrent signal deployed by a sea urchin. The American Naturalist 189, online March 27. Doi: 10.1086/691437