|MadSci Network: Zoology|
Charlotte asked [872032656.Zo]:
"How does the edible periwinkle survive out of the water if it has gills?"
As usual, I'm going to approach my answer to this question by a few easy stages:
My McGraw-Hill Dictionary of the Life Sciences gives the following definitions:
- [VERT ZOO] The respiratory organ of water-breathing animals. Also known as branchia.
- [ANAT] Either of the paired air-filled sacs, usually in the anterior or anteroventral part of the trunk of most tetrapods, which funtion as organs of respiration.
A tetropod is an air breathing vertebrate; in other words, an amphibian (e.g. frog), a reptile (e.g. snake or lizard), bird or mammal. Vertebrates are not the only animals to "lungs". Many arachnids (e.g. scorpions and spiders) have structures called "book lungs" and terrestrial gastropods (slugs and snails) have a single "lung" which they use to breathe air.
The point of this diversion is simply that animals use a variety of different types of organs to respire (get oxygen in and carbon dioxide out). In some cases, rather similar organs are used in air and water. For instance, King Crabs have "book gills" which are quite similar in basic structure to the book lungs of arachnids.
So there is no very clear difference between the structure of gills and lungs except for a few consequences of the environments in which these organs are used:
Periwinkles, and the other animals that people refer to as "snails", are classified by zoologists as members of the Class Gastropoda in the Phylum Mollusca (they are often referred to as "gastropod molluscs").
Gastropods, like the other molluscs, evolved in the ocean so most use gills to extract oxygen from the water. (The actual structure of the gills can vary a lot among the different gastropods.) For protection, the gills are often (but not always) contained inside a chamber (or space) inside the animal's shell and over its head called the mantle cavity. The location of the mantle cavity and the gills is marked on the diagram.
Some gastropods—the land snails and slugs are well known examples—moved onto land and, in the process, lost their gills. Instead of gills, these animals extract oxygen from the air using the lining of their mantle cavity. This tissue became heavily supplied with blood vessels to aid this process. (This is also shown on the diagram.)
Just to confuse the issue, some of these "lung using" snails—called pulmonates—moved back to the sea and are now common, particularly on rocky shores but also in other environments (e.g. mangrove forests). These animals seem to have no trouble surviving in the water using a lung, so perhaps it is no surprise that snails with gills can survive out of water. And now to answer the question...
Snails don't really move very fast, so don't really need much oxygen. Rocky shores are environments where there is usually plenty of oxygen. When the tide is in, the waves are crashing about, mixing lots of air up with the water. When the tide is out, air is about 21% oxygen, so there is no shortage then either.
Periwinkles keep the mantle cavity filled with water. This ensures that the gills are both supported and moist, and can function effectively. As the oxygen in the water is used up, it will be replaced by oxygen diffusing in from the air. The animal may even be able to take up some oxygen across its skin (many aquatic invertebrates can). Studies have been done of some snails showing that there is very little difference in their respiration between being in and out of the water.
That is my long answer to Charlotte's short question. The reason I wandered around a bit was to illustrate the variety of ways in which animals, particularly invertebrates (animals without backbones), breathe. And, I guess, to show why a simple question—to a scientist—can sometimes result in an involved answer!
School of Biological & Environmental Sciences
Northern Territory University
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