Introduction to Freshwater Macroinvertebrate Life Histories

Abundance and Diversity

If you walk through a healthy stream, hundreds of macroinvertebrates that represent scores of species are likely to be under each footfall. Every one of these organisms has particular needs for survival and has adapted over sometimes 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 a half a billion years.

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Respiration

Respiration involves the exchange of gases in a process that is, for us, breathing. All animals need the oxygen supplied to the body through respiration to survive. Since aquatic macroinvertebrates spend the majority of, if not their entire life under water, they have evolved a number of adaptations for surviving where the oxygen must be extracted from water (which averages about 0.8% oxygen but varies widely) rather than from the air (about 30.0% oxygen).

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 stoneflies (Plecoptera) and 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.

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Just as we can see lots of different food sources and habitats on land (forest, prairie, desert), aquatic habitats have a similar range of possibilities for food sources and a similar range of diversity in feeding habitats to take advantage of the available food. There are a few main types of food though: dead organic material that has fungi and bacteria growing on it; living plant material, like diatoms and plants; and other organisms that serve as prey.

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).

• collector-filterers - a common feeding type in which organic material suspended in the water is filtered out while the water passes; this is accomplished by a variety of adaptations: some caddisflies use nets to fish out the organics, blackflies have modified head parts, and clams and mussels pump it into and out of their body through siphons
• collector-gatherers - common in mayflies, caddisflies, and crustaceans; sift through substrate and debris in search of organic material worth eating
• parasites - not many macroinvertebrates are parasitic, but some nematodes, crustaceans, and leeches parasitize other macroinvertebrates or vertebrates; parasites are organisms that live in association with another organism (called a host) and hurt the host; symbionts and commensals also live in association with another organism, but neither harms its host
• predators - hellgrammites and many stoneflies, beetles, and flies survive by eating other living organisms; predators typically eat smaller macroinvertebrates, but some of the larger and more aggressive predators will feed on amphibians and fish
• scrapers - also called grazers; common among snails and some mayflies; have adaptations of scraping or rasping tiny plants called diatoms off the surface of substrates to which they tend to cling to very tightly
• shredders - many caddisflies and stoneflies; tear large plant (sometimes dead animal) material into smaller pieces, generally gaining their nutrition from the fungi and bacteria that have colonized the large pieces of organic matter; it's the slop and waste from the shredders that is often utilized by the collectors downstream

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Size is a very useful character in understanding the life history and learning the identification of aquatic macroinvertebrates. It's very difficult to give just one size to categorize a macroinvertebrate because nearly all macroinvertebrates start out very small and wind up somewhere along about a 6" spectrum of size. The insect larvae, for instance, go through several instars (growth stages that require molting of the skin much like a snake) before reaching their mature size. Also, different species in a genus will often be different sizes when they are mature. For this Guide, the sizes are given for full-grown macroinvertebrates. The categories below and the scale on the right are used for rough visual references to the size of these organisms.

• super tiny: 0.0-0.5mm (~1/64") = microscopic (shouldn't be captured by a standard 500 µm mesh net)
• tiny: 0.5-1.0mm (~1/32") = about the thickness of a dime
• small: 1.0-5.0mm (~1/8") = pinhead sized
• medium: 5-10mm (~1/4") = about the size of a pea
• large: 10-30mm (~3/4") = range of sized from the width of dime to a quarter
• super size: >30mm (>1") = half dollar and larger

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Macroinvertebrates have to be able to position themselves where food is most readily available and be able to escape when threatened. Some have very elaborate adaptations for holding onto solid substrates in moving waters. A very effective way of hanging on that is used by flatheaded mayflies and by water penny beetles is to be so flat that you live within the no-flow zone at the surface of the substrate, where friction has slowed turbulent water down to still water.

Suction is a good way to hang on to rocks, and can be accomplished in a variety of ways. Net-winged midges simply have fleshy suckers on the underside of their body, while some mayflies have flat gills that all overlap to form a circular suction disk, and other mayflies have a circle of dense, fine hairs around some of their abdominal segments to create a strong adhesion.

Having strong arms and claws can also help for hanging on in currents. Also, choosing where to locate yourself on the substrate can influence how much you need to hang on. For instance, burrowers simply live between the substrate particles. Being a strong swimmer is a tradeoff, making you more susceptible to the pressures of flowing water, but also allowing you to more effectively travel quickly to a different location when threatened or when food is scarce.

In this guide, each macroinvertebrate is assigned a behavioral habit category that describes 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.

• burrower - moves between substrate particles; often in finer substrates; includes clams and mussels, worms, and some mayflies
• climber - climb up live plants, woody debris, or streamside vegetation; applies mostly to macroinvertebrates that live in slow or still waters, including dragon and damselflies and many beetles
• clinger - adapted for hanging onto the surface of substrates in moving water (riffles or wave swept shorelines); includes many mayflies, stoneflies, flies, and snails
• diver - the main difference between a diver and a swimmer is that the diver goes to the surface for air and doesn't hang onto bottom substrates for any length of time; includes some adult beetles and many true bugs; organisms that exhibit this behavior are not technically benthic and are usually not included in counts of standard riffle samples of benthic macroinvertebrates
• skater - this most applies to the water striders and their kin who live on the surface of the water using surface tension to keep from sinking; organisms that exhibit this behavior are not technically benthic and are usually not included in standard riffle samples of benthic macroinvertebrates
• sprawler - live on top of fine substrates or on floating leaves of still water plants; particularly those living on fine sediment have adaptations for remaining on the surface and keeping gills or other breathing surfaces from being rendered ineffective by silt; includes representatives from many different groups
• swimmer - the main difference between a swimmer and a diver is that the swimmer swims around in the water (usually in quick bursts) then hangs on to the bottom substrate; includes many mayflies

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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.

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Tolerance

Macroinvertebrates must live the majority, if not all, of their life under water. Some are very tolerant to a variety of environmental conditions, but most have adapted to living under a particular set of environmental conditions and are very sensitive to changes in their environment. As human activities in a watershed change the chemistry, temperature, and physical characteristics of streams, the macroinvertebrates sensitive to these changes may not be able to survive.

Changes in water temperature and the amount of fine sediment in a stream are common challenges for macroinvertebrates, particularly when riparian cover or soil stability is affected by the removal of native vegetation by development, timber harvest, or agriculture. Organic enrichment from hatcheries, livestock, and sewage can lower the oxygen levels in water, severely affecting most macroinvertebrates. Some macroinvertebrate are particularly sensitive to heavy metals in the water from mining, industry, and urban runoff. Other forms of pollution come from a variety of different sources including toxic oils from dams and boats; fertilizers and herbicides from pest control, agriculture, forestry, golf courses, and urban yards; acid rain (more of a problem in the East); and many other types of industry from paper manufacturing to dentistry.

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Habitat

Freshwater macroinvertebrates live in nearly every conceivable aquatic environment. From the surface of the middle of the ocean, to warm springs, to a dog's water bowl. Macroinvertebrates described in detail in this Guide are primarily from flowing waters (streams and rivers), but many that live in still waters (ponds, lakes, marshes, and wetlands) are also mentioned. The range of habitats in flowing waters alone is pretty remarkable. In general, they can be broken down into pools, riffles, and glides, but also include margins, cascades, springs, seeps, thin films of water, and hyporheic water.

Riffle habitats are targeted for standard macroinvertebrate sampling. They can be described as parts of the stream where the stream becomes steeper, and the relatively shallow water moves quickly over larger substrates (sand, gravel, cobbles, small boulders). The water in riffles usually has a rough surface and some white bubbles.

A cascade is a even steeper than a riffle and is usually flowing over larger boulders and is mostly white water. The surface of a glide or run is more smooth than a riffle, but you can still tell that the water is moving. A pool is an area where the water is relatively still and can often be found above or below a riffle or run.

Seeps, springs, and thin films of water are habitats that have fairly specialized macroinvertebrates living in them. The seeps and springs are sometimes used interchangeably and are both related to groundwater (usually cold) coming to the surface. Springs are usually thought of as having deeper, more quickly moving water, while seeps are generally areas where water keeps the soils or substrate soggy for at least part of the year. The thin films can be found in springs or seeps or can be commonly seen keeping rocks wet along roadside cuts.

Hyporheic habitats are also mentioned in a few places in this Guide. The hyporheic zone is the river under and to the sides of the river. What we see on the surface is often only a small fraction of the water flowing down a valley. In some larger river valleys, there may be hyporheic flow under the fields or forests more than a mile away from the river that we see.

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Reproduction

Macroinvertebrates reproduce in a wide variety of ways, from sexual reproduction to budding to complicated life cycles that include fish or invertebrate hosts. Hydra for example can reproduce by forming buds on their stalk, then the buds can separate and become a separate individual.

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.

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Macroinvertebrates 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). 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.

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Life Expectancy - Voltinism

Macroinvertebrates live in the water for anywhere from a few weeks to 100+ years! They are also typically unable to migrate only a short distance in their lifetime. Because of these facts, macroinvertebrates are good at integrating the effects of human activities on a stream over time. In the case of a temporary pulse of a pollutant, you may miss the window of time during which the pollutant was measurable in the stream if all you're measuring is the concentration of that pollutant. Certain components of the macroinvertebrate population, however, may be heavily depleted for months or years after such an event. Granted, they won't tell you exactly what pollutant was present, but a change in their population will at least tell you there is a problem that needs to be further investigated and prevented in the future.

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.

• multivoltine - more than one generation each year; life expectancy from egg to post-reproducing adult is 1/2 of a year or less - midges, blackflies, small minnow mayflies
  • bivoltine - may also be used in describing life cycles with 2 generations/year
• univoltine - one generation each year; life expectancy from egg to post-reproducing adult is 1 year - many mayflies, caddisflies, and stoneflies
• semivoltine - more than one year for each generation; life expectancy from egg to post-reproducing adult is more than one year - freshwater mussels, many dragonflies and stoneflies

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