KINGDOM ANIMALIA: INVERTEBRATES

MOLLUSCS

BLUE MUSSEL (Mytilus edulis)

Mussels, clams, oysters, snails, octopuses, and squid belong to a group of animals called molluscs. These animals have a soft body, which may be encased in a shell. A well-known mollusc of the Northern Hemisphere found around Nova Scotia is the blue mussel. This animal has a smooth shell that is pointed at one end, a glossy bluish black on the outside, and violet on the inside. It takes five to seven years to reach 7 cm, and they can grow up to 10 cm.

Blue mussels are well adapted to their life in tidal zones. They attach themselves with tough threads to rocks, pilings, and buoys. The narrow end of the mussel is pointed toward the breaking waves to minimize damage. If young mussels settle in a poor feeding site, these threads can be absorbed, allowing the mussels to move to new area. Mussels have two openings between their shells, or valves. One opening takes in water containing food and oxygen, the second opening is for the release of water and waste.

Mussels usually spawn in spring, releasing eggs and sperm into the water. Some females may spawn as many as 12 million eggs. When the drifting larvae settle to attach and become adults, they begin to compete for space with barnacles and brown algae, like rockweed. Whether rockweed, barnacles, or mussels dominate a shore reflects progressively greater wave actions. Blue mussels can be so abundant that they form extensive beds along a shore. When underwater, they can be eaten by crab, dog whelks, and starfish. At low tide, their predators include minks, raccoons, and various seabirds.

ANNELIDS

LEECH (Class Hirudinea)

There are about 9,000 species of annelids, or segmented worms, found worldwide. Two-thirds of the annelids are marine worms. The most familiar annelids are earthworms and freshwater worms (oligochaetes) and leeches (hirudineans).

Leeches are found primarily in fresh water, with some species occurring in marine waters and moist areas on land. They have 34 body segments, are flattened from front to back, and occur in a variety of patterns and colours. Most leeches are between 2 cm and 6 cm in length. Leeches are hermaphroditic, which means each leech has both male and female organs for reproduction. They have suckers at both ends of their bodies that allow for attachment to surfaces while resting, feeding, or mating.

Leeches are active predators, sucking in small animals or taking tissue and blood from animals such as snails, fishes, birds, and mammals. True blood-suckers have cutting plates, or "jaws," for cutting through skin and tissues. Their guts are specialized for holding large amounts of fluid, such as blood. Bloodsuckers can consume 2 to 10 times their weight in blood, require 200 days to digest a meal, and may need to feed only twice a year.

The medicinal leech (Hirudo medicinalis) was used for centuries to suck blood from human patients. Fevers and other illnesses were once believed to be caused by too much blood. In recent years, there has been research into using leeches in surgery. Bloodsuckers, like the medicinal leech, release an anticoagulant called hirudin when they bite. Anticoagulants prevent the blood from clotting and the leech can suck the flowing blood.

INSECTS

TWO-SPOTTED LADYBIRD BEETLE (Adalia bipunctata)

The largest group of insects, the Coleoptera, has 370,000 species worldwide and 24,000 in North America. These are the beetles. They can be found from the equator to the polar regions, and in every terrestrial and freshwater habitat. All have toughened front wings that act as cases to cover a second pair of wings. The majority of species feed on plant matter, but there are scavengers, parasites, and predators in the group.

One abundant and well-known group are the ladybird beetles, commonly called ladybugs. There are 400 different species in North America. These beetles are shiny, brightly marked, and almost round. Ladybirds range in length from 1 mm to 10 mm. The wing case colour may be red, black, orange, or yellow and will have contrasting spots in one of the other colours. The bright markings of the adults warn predators of their distasteful or toxic body fluids.

Adults often overwinter in large numbers. The common, two-spotted ladybird beetle often hibernates in attics and cool areas of buildings. They may be seen at windows in fall or spring. Both adults and larvae of this and other ladybird beetles are predators. They consume vast numbers of aphids, scale insects, mites, and other insects. Some species are used by humans as biological controls for garden and orchard pests.