Isopoda
Introduction
(Source:
Biodiversity Institute of Ontario)
(Source:
Biodiversity Institute of Ontario)
With over 10,000 species, the order Isopoda is the second most diverse group of crustaceans. Isopods can be found in all types of habitats, but most species live in the sea or on land. Looking under garden rocks or rotting logs, you will often discover them. They are the small, hard-shelled creatures which resemble miniature armadillos, that roll into a tiny ball when disturbed. You might know them as rolly-pollys, or sow bugs. The most striking characteristic of isopods is their dorsoventral compression.
Morphology
(Source:
Biodiversity Institute of Ontario)
(Source:
Biodiversity Institute of Ontario)
The body of isopods is divided into three sections; the cephalothorax, the thorax and the abdomen. The compound eyes of isopods are dorsal and unstalked. Isopods are negatively phototactic (reathey ct to light by moving away from it), and usually remain hidden under rocks and debris. Their first antennae are short, while the second set is much longer and reach ahead of the isopod to "feel' for signs of food sources as well as potential danger. The first pair of legs are called gnathopods ("jaw-feet") because they are modified for grasping and tearing food. The other thoracic segments each bear a pair of walking legs. The last abdominal segment is a result of the fusion of the last four abdominal segments to form a posterior shield in all North American species. Uropods are the last pair of abdominal appendages that are biramous and extend beyond the end of the abdomen. Mature females have large, inwardly directed, platelike oostegites at the inner base of their anterior (front) legs. They form a shallow chamber on the ventral surface of the thorax called a marsupium. It serves to hold and protect developing eggs or young.
Reproduction
(Source:
Biodiversity Institute of Ontario)
Isopods reproduce sexually and breeding occurs throughout the year. Thus, egg-bearing females may be found year-round, but most are found during the spring and summer months. During mating, males grasp females and carry them under their bodies using their periopods. They may remain attached for months. The male presses his ventral surface against the side of the female, placing his genital pores close to one of the female's genital pores. Sperm are released while the male pleopods vibrate rapidly. The male then moves to the other side of the female and repeats the process with the other genital pore. After 1 to 10 months, the eggs are
fertilized and pass into the marsupium. The incubating eggs and newly hatched young remain in the marsupium for up to one month. The newly hatched first instar young look much like miniature versions of the adult. The number of instars in unknown, but there are thought to be at least 15 before maturity is reached. Most freshwater isopods have a lifespan of approximately one year.
Ecology
(Source:
Biodiversity Institute of Ontario)
Freshwater isopods are primarily benthic and occur in the littoral zone of ponds, lakes, streams and springs, but they have been recorded at depths of as much as 55 m. They are the poorest swimmers of all freshwater crustaceans. In fact, they can barely swim at all and locomotion is usually restricted to a slow crawl. For this reason, they are exclusively associated with the substrate, or aquatic macrophytes where they live a somewhat secretive existence. Most species are scavengers, feeding primarily on dead and injured organisms, as well as green, decaying vegetation. As a result they form an important link in food chains by recycling dead and decaying material back into living tissue. Isopods have also been shown to be an important food item in the diet of many stream fishes.
Idiosyncratic Inverts
The tongue-eating isopod,
Cymothoa exigua, lives in the mouth of the rose snapper (
Lutjanus guttatus) and causes tongue stub and the floor of the fish's mouth and resembles the missing tongue!