Freshwater Scuds Facts (Gammaridae)

Gammaridae is a large and highly diverse family of marine and freshwater crustaceans. It is classified under the order Amphipoda, which includes some of the most ecologically important benthic crustaceans known to man. Also known as scuds, sideswimmers, or gammarids, most of these creatures are very small, yet they play vital roles wherever they are found. They serve as decomposers and highly nutritious food sources for a wide range of freshwater species.

There are hundreds of freshwater gammarids belonging to 29 – 52 genera! The higher taxa are constantly being redefined, so the exact number of species remains very vague. Note that many species which were once listed under Gammaridae have recently been re-classified into their own families. If you’re struggling to identify those in your own nearby lake or pond system, you’re not alone. It’s best to start by narrowing down which species are typically found in your region.

That these animals are found in most freshwater biotopes is an understatement. Many of them will also have communities that exist in a broad range of environments across the globe. Freshwater gammarids are largely represented in western temperate zones by the Gammarus genus.

Under a stereoscope or magnifying glass, scuds can resemble miniature-sized shrimp. They are dominant macroinvertebrates in healthy freshwater systems, numbering in the thousands per square meter of benthic space. Thanks to advances in the field of freshwater ecology, many of these are now well-described and have updated distribution data.

What Do Scuds Look Like?

Due to their segmented morphology, multiple appendages, and small size, gammarids may look quite similar to many aquatic insects. They are white, cream, or grey-colored, crustaceans that lack a carapace and appear distinctly flattened along their sides. They have a clear exoskeleton made of chitin and calcium carbonate. The scud anatomy can be quite complex as each major segment is further subdivided into multiple parts.

Generally, gammarids consist of a distinct head with a pair of eyes, 2 segmented antennae, and ventral mouthparts. The thoracic segment is split into the pereonites, which are each associated with segmented pereopods (swimmerets). The abdominal region is distinctly shorter than the thoracic one and is subdivided into pleonites and pleopods. Finally, the posterior region is split into uronites, uropods, and a telson.

Depending on the species, environment, and sex, some segments may be reduced or wholly absent. For example, scuds that reside in subterranean environments may lack compound eyes. Also, female scuds may lack some of the sensory structures that are present in males. Classification at the species level requires the use of highly detailed taxonomic keys. Something as small as a singularly humped segment or a few hooked spines may set apart entire species.

Newborn and juvenile scuds look just like miniature versions of their parents. They are born with all the major segments and accessories and have tiny, kidney-shaped eyes just like their parents. Once they reach sexual maturity, they develop morphological features that are necessary for breeding and the development of their young.

Scud Habitats – Where Do They Live?

Freshwater gammarids reside in clean bodies of water with ample structural diversity. They favor conditions in natural ponds, lakes, streams, springs, pools, and rivers. They tend to be quite sensitive to pollutants, so their densities are usually highest in pristine waters, particularly those with submerged vegetation or benthic debris for food and coverage.

Some freshwater gammarids are equipped with adaptations to live in extreme environments. These may be found in subterranean caves with virtually no light, in hot springs, or in estuarine environments where salinity levels fluctuate regularly. One species, Gammarus wilkitzkii, has even been shown to thrive in Arctic Sea ice! This scud is definitely the exception, however, as most species prefer mild temperatures. Gammarus fasciatus, one of the most common species in North America, struggles to survive in temperatures outside 10 – 24˚C (50 – 75˚F).

Scuds tend to dominate microhabitats that are created by fallen leaf litter and benthic components. They favor shaded areas where they can remain hidden from potential predators. Populations can usually reach their peak sizes when a reduced number of fish are present. They tend to be found in clusters in highly oxygenated, shallow areas with natural coverage. They scurry through mazes of organic matter or in the gaps between large substrate particles, looking for food.  

Scuds have been introduced into areas outside of their native range due to anthropogenic activities, such as shipping. Gammarus tigrinus is a classic example of an amphipod that has become successfully invasive in European waters. Native to Atlantic North America, this scud is able to outcompete native gammarids and significantly alter the food web structure wherever its largest populations occur.

What Do Scuds Eat?

Scuds are experts at utilizing whatever resources are available as food. They are generalist foragers, skilled at breaking down a diverse array of organic materials. Tiny, decaying pieces of detritus or animal tissues are ideal sources of nutrients for them. Their impartial nature towards food is largely what has allowed them to develop successful populations in most bodies of water.

Organic matter within the water column, on settled material, and in between substrates can be consumed by scuds. While most may appear to be scavengers, some have developed a taste for their own kind or for live meat. G. tigrinus may predate on smaller amphipods, and larger individuals may even attempt to consume their own conspecifics.

Other gammarids will rarely ever attack living animals for food. In the event that they do so, extenuating circumstances may be at play. G. fasciatus favors zooplankton and microscopic plants. It may also feed on the waste of other macroinvertebrates, like freshwater mussels. Food preferences may change throughout the life cycle, with food options becoming more diverse in older gammarids.

The Life Cycle of Scuds

Scuds are born with a morphology that is largely similar to those of their adult counterparts. They are simply much smaller and lack reproductive paraphernalia. They undergo several molting phases to reach their mature size. Some species go through eight or nine instars (molting phases) prior to becoming sexually mature. Scuds must feed throughout their lives to sustain their growth needs.

In G. fasciatus, males must wait until their 9^th instar to look for a mate. Once he is ready, he conducts a mating dance to attract a female in her 8^th instar. It’s crucial that he attracts one just before she has molted as the ovulation stage occurs right after the molting period. The uropods of the male must come into contact with the brood pouch (oostegites of the female), after which he releases sperm into the water. The female has to collect and shuffle the sperm into her pouch, where eggs lay in wait for fertilization.

A successful fertilization event allows for the development of dozens of eggs at a given time. These hatch and remain in the brood pouch until they have grown the necessary appendages to survive independently of the female. The juveniles are then released and must feed to make it through multiple instars.

Adults can persist until they have undergone about 15 – 20 molting phases. In some species, such as G. fossarum, the females can produce up to 10 broods in her life. Broods may even develop successively, with females producing hundreds of offspring within a timeframe of just 2 – 3 years.

How to Attract Scuds to Freshwater Bodies

Natural bodies of freshwater are likely to have their own populations of scuds. Due to their small size, they may be very difficult to spot unless substrate or detritus samples are obtained and examined. Fully naturalized wildlife ponds will likely have their own scud populations as well, and these will have been introduced into the pond via the movement of wild animals from one body of water to another.

Scuds may sometimes get trapped in a filter, so it’s worth checking for them there while performing regular maintenance. Ornamental ponds that are fully closed off to wild animals are less likely to have their own scud populations. Note that amphipods don’t have a terrestrial phase, so they can’t move between bodies of water without a vector. If you’d like to have populations in a fish pond, perhaps to aid in the decomposition of materials, it would be best to introduce a store-bought population.

Avoid collecting scuds from natural bodies of water as these may contaminate your pond with pathogens. Scud starter cultures can often be purchased from aquarium shops or online portals because they are bred as food for some ornamental fish. Their populations can grow at an exponential rate, so be wary about adding these to ponds or aquariums that are not stocked with their natural predators.

Are Scuds Beneficial to Ecosystems?

Gammaridae is an extremely important taxon to have in most bodies of freshwater. They play several vital roles in the maintenance of a functional aquatic ecosystem. As detritivores and foragers, they aid in the breakdown of waste, freeing up important nutrients. Their feeding preferences help shape microbial communities, particularly those that settle onto the surfaces of organic materials.  

Apart from being nutrient recyclers, they collectively serve as a high-quality type of food for secondary consumers. They aren’t just consumed by fish; they are also a favorable prey type for larger crustaceans, wetland birds, amphibians, semi-aquatic reptiles, and riparian mammals. Gammarids are often considered ‘keystone’ species wherever they occur because their presence helps shape and establish working patterns for the existing ecosystem.

Freshwater ecologists use gammarids as bioindicators or bio-monitors for assessing water quality. Due to their sensitivity to ecotoxins and pollutants, gammarids are also utilized as test subjects. They are often used in experiments that evaluate the effect of chemical inputs and climate change on macroinvertebrate communities.