July 2025 Critter of the Month: Gammarus

At the PhytoChop Observatory, one of our key activities is analyzing DNA signatures of the plants, animals, and microbes in the water, which gives us an unprecedented view into the ecosystem’s biological diversity. For example, this molecular approach enables us to identify the variety of bacteria that are present, which would otherwise appear as just dots under a standard microscope. It also allows us to detect the presence of larger organisms like zooplankton and fish through remnants they leave behind—scales, sloughed-off mucilage, and other cellular debris. However, what DNA datasets don't provide is the visual connection that can make us appreciate these organisms in their full biological context. For that perspective, we turn to microscopy, where critters like Gammarus reveal themselves not just as genetic sequences, but as beautiful, complex and intriguing animals.

This month we spotlight Gammarus, a genus of small crustaceans belonging to a large group, known as amphipods. Amphipods, which go by several common names including scuds and sideswimmers (sand fleas are another familiar type), are distantly related to crabs and shrimp. The amphipods comprise thousands of species found in lake and ocean waters and especially in association with their bottom habitats. Most species are a few millimeters to less than half an inch in length, though exceptions exist. Amphipods are generally characterised as having laterally compressed bodies, giving rise to their characteristic ‘side-swimming’ behavior. Gammarus and its close relatives are primarily epibenthic, meaning they tend to live on rather than in the seafloor, where they associate with various surfaces including salt marshes, intertidal areas, and floating material such as seaweed. Their common name "scuds" also hints at their swimming style, with many species observed darting in rapid movements.

These small crustaceans play important ecological roles in many aquatic systems, including Chesapeake Bay, where they serve as both consumers and prey. Gammarus species are typically generalist feeders, usually characterized as detritivores or scavengers, though some are omnivorous and feed on living algae and small insects. In turn, they constitute important prey for small fish, making them an important link in aquatic food webs. Many species demonstrate wide environmental tolerance and can inhabit a variety of habitats. While we could not identify the exact species in our images due to the close morphological similarity between related forms, the Maryland Biodiversity Project reports at least three different Gammarus species in Maryland's marine waters based on morphological characteristics: Gammarus daiberi, G. mucronatus, and G. tigrinus. Interestingly, while these species are native to this region on the west side of the Atlantic, some have crossed the ocean, perhaps hitchhiking in ship ballast waters, and become established as non-native species in European waters. 

Two of the most outstanding features visible in this Gammarus watercolor are its compound eyes and its brood pouch. Like other crustaceans such as crabs, and similar to insects, Gammarus possesses compound eyes composed of many individual facets that combine to form an overall image. While these compound eyes provide poor image resolution compared to our own vision, they are evolutionarily selected for high sensitivity to movement and offer a much wider field of vision. This visual system makes them especially adept at detecting motion in their aquatic environment, a crucial adaptation for both finding food and avoiding predators.

This Gammarus is a female, and the greenish-colored spheres in her abdomen area are eggs. Female amphipods carry their fertilized eggs externally in a specialized chamber called a marsupium, which is formed by the arrangement of their gills. The size of this egg brood depends directly on the female's size, which increases with age, meaning these amphipods can carry anywhere from less than a dozen eggs in younger individuals to several hundred in mature specimens. This external brooding strategy allows the developing embryos to receive oxygenated water while remaining protected within the mother's specialized brood chamber. In Gammarus mucronatus, brood development lasts about 1 month. Since amphipods are relatively short-lived, she is likely to produce several cohorts this summer.