03 February 2009

How to perform CPR on a slug

A reader who had recently lost a "beloved" pet slug e-mailed this inquiry: Can slugs have heart attacks?

Interesting question. But first, what do we mean when we say a human has had a heart attack? MedicineNet describes a heart attack as follows:

The death of heart muscle due to the loss of blood supply. The loss of blood supply is usually caused by a complete blockage of a coronary artery, one of the arteries that supplies blood to the heart muscle. Death of the heart muscle, in turn, causes chest pain and electrical instability of the heart muscle tissue.
If this leads to irregular heart beats or to the complete stopping of the heart, then the blood flow, and consequently the oxygen supply to the brain will stop. If the heart beat is not restored within about 5 minutes permanent brain damage or death will follow.

The circulatory anatomy of gastropods (slugs and snails) is rather different than that of humans. The best description of the gastropod vascular system has been given by Fretter and Graham1 for the semi-terrestrial snail Littorina littorea.
Arteries distribute the blood to the main parts of the body, branching over and over again until they are minute and end by pouring their contents into a series of blood spaces around the main organs and in between the muscles and connective tissue layers of the body. From this series of spaces other vessels take the blood to the excretory and respiratory organs, whence it is returned to the heart. The vascular system of [Littorina littorea] differs from the more familiar vertebrate pattern in a number of important respects: the heart receives primarily oxygenated blood to circulate to the body...; there is no capillary system between the arteries and veins but a series of indefinite haemal spaces, though these are not necessarily of dimensions very different from capillaries. All the organs are directly bathed in blood, and the circulation must be slower than in the vertebrate and less definite in its course.
Gastropods don't have coronary arteries either. But I suppose a blockage could develop in one of the main blood vessels. Such a blockage would certainly interfere with the functioning of a slug's heart and the circulation of blood throughout its body. But I suspect that if a slug's (or a snail's) heart stopped beating for whatever reason, the slug wouldn't just drop to the ground and pass out—it's already flat on the ground, to begin with—but would continue to carry on its sluggish activities until the oxygen concentration of the blood in the blood spaces became about equal to that of the tissues. At that point the diffusion of oxygen from the blood to the organs would stop and the slug would probably become quiescent for a while, lowering its metabolic rate and thus reducing its oxygen requirement. But, if its blood circulation didn't recover, it would sooner or later die.

In humans, the word "attack" in the phrase "heart attack" implies a quickly worsening condition with a sudden onset and a potentially detrimental outcome. If my reasoning is correct, in slugs and snails, on the other hand, even if the onset of a pathological condition in the heart were sudden, its effects on the overall well-being of the animal would be gradual. Therefore, there would be no "attack", so to speak. Forget the CPR, then.

A slug in intensive care. It didn't recover and may have died of stress-induced multiple organ failure.

If you think your slug has eaten too much and is suffering from heartburn instead, give it one tablet of Hajmola and e-mail me in the morning.

Finally, I send my deepest condolences to my reader. I hope you didn't forget to pickle your loved one in alcohol. Send it over and I'll perform an autopsy. That may help us determine what killed it. May your next slug live longer.

1. Fretter & Graham. 1994. British Prosobranch Molluscs.

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