Slugs, as terrestrial members of the phylum Mollusca, circulate a fluid that distributes necessary substances throughout their bodies. While similar to vertebrate blood, its composition and function are fundamentally distinct. This fluid serves as a circulatory system, moving molecules and removing waste, which is the basis for the term “bleeding” when a slug is injured. Understanding this fluid requires exploring the unique biology of invertebrates.
The Fluid: Hemolymph
The circulatory fluid in a slug is called hemolymph, which combines the functions of blood and lymph found in other animals. Hemolymph is primarily a water-based plasma, containing dissolved salts, carbohydrates, lipids, and various proteins. Unlike vertebrate blood, hemolymph does not contain traditional red blood cells, which alters its method of oxygen transport. This fluid distributes nutrients and hormones while collecting metabolic waste products.
The primary difference lies in the respiratory protein used to carry oxygen throughout the tissues. Vertebrate blood uses hemoglobin contained within specialized cells, but hemolymph utilizes a different protein dissolved directly into the fluid. This composition is adapted to the slug’s relatively low metabolic rate and the structure of its internal anatomy.
Circulation: An Open System
The movement of hemolymph occurs via an open circulatory system, meaning the fluid is not continuously confined within vessels. The slug possesses a heart, which pumps the hemolymph through a short aorta and into a network of open spaces called sinuses. These sinuses form a large body cavity known as the hemocoel, where the fluid directly bathes the internal organs and tissues.
This direct contact allows for the exchange of oxygen, nutrients, and waste without the need for an extensive capillary network. The hemolymph then collects around the organs before returning to the heart to be re-oxygenated and recirculated. This low-pressure circulation is sufficient for a slow-moving organism with a low oxygen demand.
Why Slug Blood Isn’t Red
The color difference between vertebrate blood and slug hemolymph is due to the oxygen-carrying molecule. The respiratory protein in slug hemolymph is hemocyanin, which differs from hemoglobin in its metal composition. Hemocyanin uses copper atoms to bind oxygen, whereas hemoglobin uses iron atoms.
When hemocyanin binds to oxygen, the copper imparts a pale blue or blue-green tint to the hemolymph. When oxygen is released to the tissues, the protein becomes colorless or grey, making the fluid appear translucent. This contrasts with the bright red color produced by oxygen-bound iron in hemoglobin.
What Happens When a Slug Is Injured
When a slug sustains a physical injury, the open nature of its circulatory system results in the spillage of hemolymph directly from the wound site. This loss of fluid is the closest equivalent to “bleeding.” The slug defends against fluid loss using specialized circulatory cells called hemocytes.
These hemocytes aggregate at the injury site, similar to how platelets function in vertebrate clotting, attempting to seal the breach. The process of hemostasis is often slower than in animals with closed systems. Furthermore, the slug’s mucus contains adhesive components that aid in wound closure.