THE XERCES SOCIETY FOR INVERTEBRATE CONSERVATION Aquatic Invertebrates in Pacific Northwest Freshwater Wetlands |
Home Intro to ID taxa invertebrate ecology |
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Introduction to Wetland invertebrate Natural History |
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In every wetland, hundreds of macroinvertebrates that represent scores of species are likely to be in every square foot. Each of these organisms has particular needs for survival. Some have adapted over as long as hundreds of millions of years to survive in their environment or colonize new habitats. The topics below give you some idea of the adaptations and life histories that have kept them going for so long. Click on one of the categories below to get a brief introduction to a variety of topics on invertebrate ecology.
Respiration A few macros, including some beetles (Coleoptera), flies (Diptera), and snails (Gastropoda), actually do breath air either by capturing air at the surface then taking it with them as a bubble, or breathing it through an body opening or tube. Some even breathe the gases held within plant tissues. Most other macroinvertebrates must utilize the limited oxygen that is dissolved in the water. Many have gills that assist in extracting the oxygen, other pull it into their body cavity where specialized tissues collect the oxygen, still others must just transport the oxygen across thinner membranes of the body. Gills come in a variety of shapes and sizes and are regularly used in identification of macroinvertebrates. Mayflies (Ephemeroptera) have flat, plate-like gills that often have visible veins or trachea that help transport the oxygen. Many caddisflies (Trichoptera) have finger-like gills on their thorax and/or abdomen. Others may have long filaments on the sides of the abdomen that function as gills. Feeding Macroinvertebrates are often categorized by what they eat and how they eat it in a classification system called "function feeding groups". There are 6 major groups, but some macroinvertebrate groups may be omnivorous (characterized by more than one feeding habit).
In this guide, macroinvertebrates may fall into one of the following "habit categories" that describe how they move in water and in association with their substrate. These categories are based on those used in Merritt and Cummins' Introduction to the Aquatic Insects of North America.
Macroinvertebrates are the preferred food of many fish, birds, and other macroinvertebrates. Some have very little protection against such predators. Midges, for example, have soft bodies, aren't very fast, and are tasty to eat. They are extremely successful though, because there are just so many of them that there are still plenty left after the predators are full. There are a couple species of midges, however, that will tuck themselves under the wing pads of other macroinvertebrates to pupate, trusting in the ability of their host to move away from danger while they are in the particularly defenseless pupal stage. Many of the more mobile macroinvertebrates rely on escape (by swimming, scurrying under rocks, or burrowing) to retreat to safety. Some spend their days on the undersides of rocks, then come out to feed at night. Others will use the current to take them to a safer location (a trick that can easily backfire since fish often watch for such prey). There are a few macroinvertebrates that offer their own protection. The soft bodies of snails, mussels, clams, and limpets are protected by bone-like shells. Sometimes these shells are fairly thin, but often, they are thick and sturdy, providing effective protection from many predators. Spewing out something toxic or foul may save the life of some macroinvertebrates, or it may at least stop the predator from eating a relative in the future (effective for some beetles). Sometimes, the best defense is a good offense, as with some true bugs that can pack a wallop with their piercing mouthpart. The caddisflies probably take the ingenuity prize for complicated security against some predators. Many build remarkable cases out of silk with rocks, twigs, or leaves that can sometimes withstand a great deal of pressure. If nothing else, the case forces small predators to get inside the case to get their prey, and forces large predators to eat the indigestible case. Habitat Reproduction Freshwater mussels can live for a very long time and reproduce many times during their life. When it's time to reproduce, the males spew sperm into the water, and nearby females filter the sperm out of the water to fertilize their eggs. The eggs hatch into glochidia which are the dispersing larval stage of freshwater mussels. The glochidia attach to the gills of a fish where they developing into a tiny mussel, and fall off the fish and onto the substrate where they burrow in and begin their life as a full fledged freshwater mussel. Aquatic insects typically spend their adult lives out of the water, where they mate, then lay eggs in their preferred habitat. The timing of these mating events can sometimes result in spectacular mass emergences where adults emerge together in mind blowing numbers. Mayflies are best known for this, but blackflies may be the most notorious for it. Burrowing mayflies can emerge in such great numbers that they blanket roads, buildings, beaches, and whatever else may be near their favorite water body. Female black flies require blood before their eggs can mature and sometimes emerge in large enough numbers to seriously affect herds of livestock. Small male midges can be commonly seen in mating swarms that look like little pulsating clouds. A few other aquatic insects skip the male altogether and reproduce parthenogenetically (the female can fertilize herself) and males are rare if present at all. Horsehair worms form something similar to a swarm, gathering in masses of up to a couple dozen worms, of which only two are actually involved in mating. MetamorphosisMacroinvertebrates become mature in a variety of ways. For example, the Crustacea and Mollusca tend to have a planktonic stage that floats around a while before settling. Once they settle, they just get larger and larger either through molting (crustaceans) or by adding growth to their shell (mollusks). Insects also have some variation in the way they grow from egg to adult, but it can be broken down into two major groups: complete metamorphosis and incomplete metamorphosis. Complete metamorphosis involves 4 major stages (see image below). The egg hatches. Larvae are the primary feeding and growth stage and may go through a number of molts. Once the larva is mature, it forms a non-feeding pupal case in which it changes dramatically. The adult then emerges from the pupal case to reproduce. Insects that go through complete metamorphosis include flies (Diptera), beetles (Coleoptera), caddisflies (Trichoptera), and hellgrammites (Megaloptera). Incomplete metamorphosis involves 3 major stages (see image below). Insects that go through incomplete metamorphosis may be referred to as nymphs during their immature stages. The egg hatches. Larvae are the primary feeding and growth stage and may go through a number of molts. As the larva becomes more mature, the wing pads develop on top of the middle and usually hind thoracic segments. Once the larva is mature, it usually crawls from the water; splits its exoskeleton along the upper middle; then the adult crawls out of the old skin (called exuvia once the adult is out). Insects that go through incomplete metamorphosis include mayflies (Ephemeroptera - which have an extra winged stage called sub-imago before they can reproduce), stoneflies (Plecoptera), dragon and damselflies (Odonata), and true bugs (Hemiptera). The adult mayfly in the image below is the sub-imago stage and must molt one more time before it will be sexually mature. Life Expectancy - Voltinism Generally speaking, voltinism is the number of life cycles (generations) that a species may go through during a given year. Below are several types of voltinism that are used to describe the longevity of freshwater macroinvertebrates and examples of each.
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