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  • Hannah L. Stout, Independent Researcher

Common Aquatic Invertebrates in the Spring Creek Watershed

Updated: Dec 7, 2020

A female Callibaetis mayfly from Millbrook Marsh.

Aquatic invertebrates are animals without backbones that live in water. Invertebrates that are too small to be seen with the naked eye are “microinvertebrates,” while those that can be seen without the help of a microscope are “macroinvertebrates.” Most macroinvertebrates are larger than 1 mm in size, and freshwater macroinvertebrates include thousands of species of insects, crustaceans, mites, molluscs (clams, mussels and snails), worms, and leeches.

Freshwater macroinvertebrates play a number of important ecological roles: some are predators, others graze on algae and other live plants. Many act as decomposers, consuming dead animal and plant material. One of the most important ecological roles of macroinvertebrates is their place in the food web. Mayflies alone are consumed by at least 224 species of vertebrate and invertebrate predators—exclusively so by a species of freshwater stingray!

Macroinvertebrate communities often spend their entire lives in the same place, and therefore can provide significant insight into the areas and habitats in which they live. The prevalence of the various ecological roles (e.g., predator, grazer, decomposer) within a macroinvertebrate community, and the relative number of different macroinvertebrates can be used to estimate water quality. For example, an apparently diverse community that includes species of large stoneflies is associated with cool, clean, oxygenated water, while a less diverse community dominated by worms and lacking in aquatic insects tends to raise red flags.

Macroinvertebrate sampling of Spring Creek and its tributaries has been conducted since at least the 1940s, when Stuart Frost and Isaac Aurelio began a twelve-year survey of the mayflies of these waters. More recently, surveys conducted from 1991 to 2012 have yielded approximately 100 different taxa of macroinvertebrates (See "Checklist of Spring Creek Macroinvertebrates" or "Spring Creek Macroinvertebrates by Site" for a comprehensive list). The density and diversity of macroinvertebrates found in these streams are related to surrounding land uses and water quality (Map 1).

High-diversity sites

Cedar Run (Cedar 0.1), upper Spring Creek (e.g., Spring 18.6), and upper Slab Cabin Run (Slab 3.36) are predominantly upstream from the rapidly growing urban influence of State College. (The number following the stream name is the distance in miles upstream from the confluence of its receiving stream). These locations and a number of nearby restoration and conservation projects contribute to the macroinvertebrate diversity typical of this section.

Examples of macroinvertebrates collected from sampling sites within these waters:

Fig 1. Drunella larva

  • Drunella is a “stout crawler” genus of mayfly (Family: Ephemerellidae). Larvae typically have spines on their thorax, legs, and abdomen and horn-like projections on their heads [Fig 1. Drunella larva]. Drunella mayflies are commonly called “Blue-Winged Olives,” due to their dark-colored cloudy wings in the subimago stage, also known as the “subadult,” or “dun” stage [Fig 2. Drunella subimago]. Mayflies are unique in that they are the only insect that molts after reaching a winged stage. After the final molt to the imago stage (“adult” or “spinner” stage) [Fig 3. Drunella imago], mayflies are able to use the last of their energy reserves and brief adult lives to reproduce.

Fig 2 (left). Drunella subimago, Fig 3 (right). Drunella imago

Fig 4. Nigronia larva

  • Insects in the genus Nigronia (“dark fishflies”) are in the same insect Family as dobsonflies/hellgrammites (Corydalidae) [Fig 4. Nigronia larva], but are noticeably smaller, and the wing patterns of adults are distinctive [Fig 5. Nigronia adult]. Fishflies are predatory and the larvae are long-lived, taking up to three years to reach the adult stage. Larvae of fishflies and dobsonflies require well-oxygenated water, which they leave to pupate, typically taking several weeks to reach the adult stage. Because of their high oxygen demands and long lifespan, larval fishflies and dobsonflies are considered to be indicators of good water quality.

Fig 5 (right). Nigronia adult

  • Agnetina stoneflies are in the “common stonefly” (Perlidae) family of Plecoptera. Like many other stoneflies in this family, larvae of this genus are large and highly-patterned [Fig 6. Agnetina larva]. These are predators that tend to inhabit cold, well-oxygenated streams. Contrary to their reputation, perlid stoneflies exhibit a wide range of habitat preferences and tolerances in Pennsylvania: Perlinella species are only found in large, warm rivers, and Hansonoperla species are acid-tolerant. The common name used by anglers for Agnetina is “golden stone” [Fig 7. Agnetina adult].

Fig 6 (left). Agnetina larva, Fig 7 (right). Agnetina adult

Low-diversity sites

Thompson Run (Thompson 0.1), middle Spring Creek (e.g., Spring 12.4), and lower Slab Cabin Run (Slab 0.2) flow within and immediately downstream of State College, and have borne the brunt of urbanization—especially in the last century. The macroinvertebrate communities of these sections are not as diverse as they were before catastrophic events such as the sodium cyanide-induced “Spring Creek Massacre” of 1958, or the infamous decades of kepone/mirex groundwater contamination, but since the enactment of water quality protections, as well as extensive rehabilitation and restoration projects, stream conditions have improved significantly. Today, the macroinvertebrate communities of middle Spring Creek are typical of those commonly found in “true” limestone streams.

These “typical” macroinvertebrates include:

  • Optioservus riffle beetles (Family: Elmidae) are aquatic beetles that are can be found crawling along the bottom and surfaces of fast-moving streams. Both the larvae [Fig 8. Optioservus larvae] and adults [Fig 9. Optioservus adult] are fully aquatic, equipped with strong claws and oxygen-collecting hydrofuge hairs for living in swift currents

Fig 8 (right). Optioservus larvae

Fig 9. Optioservus adult

  • Hydropsyche caddisflies are net-spinning caddisflies (Family: Hydropsychidae), which do not construct cases; instead, they construct silken nets. Hydropsyche larva use their anal claws like grappling hooks to hold onto surfaces within a current, then consume the algae, detritus and other small particles that drift into its net [Fig 10. Hydropsyche larva]. Caddisflies are the closest insect relatives of Lepidoptera, and the adults can be mistaken for moths [Fig 11. Hydropsyche adult]. A common name used by anglers for adult Hydropsyche is “spotted sedge.”

Fig 10 (left). Hydropsyche larva, Fig 11 (right). Hydropsyche adult

  • Isopods, or “sowbugs” in the family Asellidae are freshwater crustaceans that are ubiquitous throughout Spring Creek [Fig 12. Lirceus]. Freshwater isopods can generally only be found in springs, streams and caves, and yet are detritivores that are fairly tolerant of organic and thermal pollution, and therefore these crustaceans are ecologically important as “decomposers.” Like some other crustaceans, such as amphipods (“scuds”), females exhibit some degree of maternal care, incubating their eggs and retaining their young in their marsupium for up to three weeks after hatching.

Fig 12. Lirceus

Note: Because of their specialized habitat and dependence on the quality and quantity of groundwater, many species of cave isopods are rare or endangered—including Franz’s cave isopod (Caecidotea franzi). This isopod species is critically imperiled in Pennsylvania, and an aquifer in Centre County is one of the only four aquifers in this state in which this species can be found.

Moderate-diversity sites

The lower section of Spring Creek (e.g., Spring 0.2) represents a middle ground between the rural upper and urban middle sections. Input from Logan Branch (Logan 1.5) and Buffalo Run (Buffalo 3.0), as well as a number of restoration and conservation projects, contribute to the diversity of the macroinvertebrates communities found here. These macroinvertebrate communities include:

  • Brachycentrus is a genus of case-making caddisflies in the Family Brachycentridae. Larvae construct their cases from woody materials. Their square cases and their bright green bodies make Brachycentrus larvae easy to identify [Fig 13. Brachycentrus larva]. Adult Brachycentrus are known as the “grannom caddis” [Fig 14. Brachycentrus adult], and their spectacular swarms are treasured by anglers, predators (and entomologists!).

Fig 13 (left). Brachycentrus larva, Fig 14 (right). Brachycentrus adult

  • Larvae of the Ameletus mayfly (Family: Ameletidae) are “minnow-like,” in that their bodies are streamlined and their “tails” are adapted for rapid and efficient darting movements through water to avoid predators and to feed on plant and mineral particles. Ameletus larvae can be differentiated from other minnow-like mayflies by their shorter antennae and comb-like mouthparts [Fig 15. Ameletus larva]. This genus is considered sensitive to organic pollution, but also tolerant of low pH waters. Ameletus mayflies are deemed as unimportant to Pennsylvania anglers, but the imagos are stunning nevertheless [Fig 16. Ameletus imago].

Fig 15 (left). Ameletus larva, Fig 16 (right). Ameletus imago

  • Dicranota crane fly larvae (Family: Tipulidae) are predaceous burrowers. Larval crane flies are identified according to characteristics on their posterior ends—Dicranota have two long anal lobes below two dark spiracles [Fig 17. Dicranota larva]. Adult crane flies are often mistaken as very large mosquitoes [Fig 18. Dicranota adult], but are in fact harmless nectar-feeders and comically clumsy fliers.

Fig 17 (left). Dicranota larva, Fig 18 (right). Dicranota adult

From May 1941 through August 1953, Stuart Frost and Isaac Aurelio surveyed the mayflies of Spring Creek and its tributaries. Since their last survey in August 1953, a number of macroinvertebrate species—including both green and brown drake mayflies—have been extirpated from Spring Creek. With increased public awareness and environmental stewardship, and with the relentless efforts of an array of restoration projects, it is within our reach to conserve, and perhaps even restore, the macroinvertebrate diversity of these treasured waters.

Photo credits: Agnetina (larva, adult), Brachycentrus (larva), Dicranota (adult), Hydropsyche (larva, adult), Nigronia (larva, adult), Optioservus (adult) Ameletus (imago), Drunella (subimago, imago) Brachycentrus (adult) Lirceus

personal collection: Ameletus (larva), Callibaetis (imago), Dicranota (larva), Drunella (larva), Optioservus (larvae)

Map credit: S. Arnold

Hannah Stout is a freelance Entomologist and Certified Taxonomist of freshwater insects. Her graduate research involved designing a reintroduction of a mayfly species to a Pennsylvania stream, for which she received the Society for Freshwater Science Graduate Student Conservation Research Award in 2009, and her Ph.D. in 2012. Her current project is a study of how different vegetation management methods at power line rights-of-way may affect native bee density and diversity in Pennsylvania.

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