Amphipods, belonging to the order Amphipoda, are a diverse group of crustaceans within the class Malacostraca, which also includes crabs, lobsters, and shrimp. These small, shrimp-like organisms are distinguished by their laterally compressed bodies, which consist of three distinct regions: the head, thorax, and abdomen.
They possess notably varying appendages, including three pairs of pleopods and three pairs of uropods. Unlike their relatives, the isopods (order Isopoda) have uniform legs. The name ‘Amphipoda,’ which means ‘different feet,’ reflects this variation in limb structure.
While the majority of amphipods are marine, inhabiting at a depth of almost 30,000 ft, some species, such as sandhoppers from the family Talitridae, are terrestrial and live on sandy beaches. Of over 10,500 recognized species, around 1,900 inhabit freshwater environments, such as rivers and lakes.
On average, amphipods measure less than 0.4 in (10 mm) in length; however, the largest species ever observed is Alicella gigantea, which reaches up to 13 in (340 mm). Some specimens of this species, collected from the Kermadec Trench, have also measured around 13.7 in. In contrast, the smallest amphipods are not more than 0.04 in.
The size of an amphipod is closely linked to the availability of dissolved oxygen in its environment. For instance, amphipods residing at high altitudes, such as 12,500 ft in Lake Titicaca, typically grow up to 0.87 in length. In contrast, those living at a lower altitude of 1,500 ft in Lake Baikal can reach lengths of up to 3.5 in.
Although males are typically larger than females, it is the opposite in members of the genus Crangonyx.
There are 13 segments in an amphipod’s body, grouped into three broad parts: head, thorax, and abdomen.
The head is the most anterior part of the body and is fused to the thorax. It houses two pairs of sensory antennae and a single pair of fixed, compound eyes. The mouthparts are also found in this region, though they mostly remain hidden.
The second region following the head is the thorax, which features eight pairs of uniramous appendages, each consisting of a single series of segments attached end-to-end. The first pair of these appendages is equipped with claws and serves as accessory mouthparts. The next four pairs are oriented forward, while the last three pairs point backward. The thorax also contains the gills and heart, which function within an open circulatory system where oxygen is transported using the pigment hemocyanin.
This region is again subdivided into two units: pleosome and urosome. While the former bears swimming legs, the latter constitutes a telson and three pairs of uropods (posterior appendages). However, unlike in true shrimps (infraorder Caridea), the uropods are not fan-shaped in these crustaceans.
Amphipods derive their name from the Neo-Latin amphipoda, which translates to ‘foot on both or all sides.’ Over 10,500 amphipod species were traditionally classified under four suborders, Gammaridea, Caprellidea, Hyperiidea, and Ingolfiellidea, with the first suborder having the most taxa (including all marine and freshwater species), while Ingolfiellidea only had 40 species.
However, Gammaridea was considered a problematic group, and scientists felt the urgent need for a taxonomic revision.
In 2003, J. K. Lowry and A. A. Myers resorted to cladistics of morphological characters and replaced the suborder. A new suborder, Corophiidea, was formed, including members of Gammaridea and Caprellidea. Again, in 2013, a new suborder, Senticaudata, was split off from the older Gammaridea, including more than half of all known amphipod species.
Gammaridea was finally dismembered from the classification scheme in 2017, with the revised scheme having six suborders: Pseudingolfiellidea, Hyperiidea, Colomastigidea, Hyperiopsidea, Senticaudata, and Amphilochidea.
The only difference between the two schemes shown above is the replacement of the suborder Caprellidea with Senticaudata in the revised classification.
Amphipods are believed to have first appeared during the Lower Carboniferous Period. Despite their long history on Earth, fossil evidence of these arthropods is scarce. To date, fossils of only 13 species, spread across six genera, have been discovered. These fossils have been unearthed from Weald Clay in the United Kingdom, dating back to the Lower Cretaceous Period, and from Baltic amber, dating to the Upper Eocene Epoch.
They are found in a wide range of environments, from the deepest parts of the ocean, over 30,000 feet below the surface, to freshwater habitats like rivers and lakes. About 1900 species, accounting for around 20% of the total amphipod diversity, inhabit freshwater or other non-marine habitats. However, some members, like those in the family Talitridae, live in moist terrestrial environments, where they are commonly referred to as ‘landhoppers’ or ‘sandhoppers.’ These members have also recently colonized parts of Europe and North America.
Around 750 species of amphipods are troglobitic and live in caves around the Mediterranean Basin, southeastern North America, and the Caribbean countries. The most generic diversity of these animals is found in cool waters, with more than 290 genera found in the Siberian Lake Baikal. The Caspian Sea basin is also home to a large number of endemic amphipods.
A few amphipods, such as most members of the suborder Hyperiidea, live in association with gelatinous animals, like salps, medusae, siphonophores, colonial radiolarians, and ctenophores.
They typically consume algae, tiny insects, other crustaceans, like copepods, and sometimes even dead and decaying matter (detritus).
Amphipods use the claws on their first pair of thoracic legs to grasp food. They are often found to engage in compensatory feeding, where less mobile species consume larger quantities of less nutritious food, avoiding the high energy expenditure and risk of predation associated with searching for more nutrient-rich options. However, species like Gammarus mucronatus and Elasmopus levis, which are better equipped to evade predators, tend to explore a wider range of food choices.
Some amphipods, such as whale lice (family Cyamidae), are parasitic and feed on the tissues of their baleen whale hosts, while a few others exhibit cannibalism by feeding on juvenile amphipods.
Although amphipods have an average lifespan of one year, they may sometimes survive over two years.
Amphipods have distinct male and female sexes, with males typically distinguished by their enlarged gnathopods, or claws, on the second thoracic segment. Mating begins with a behavior known as amplexus, where the male clasps the female with his gnathopod, holding her against the ventral side of his body. This precopulatory guarding can last from two to over fifteen days, depending on water temperature, until the female molts and her eggs are ready for external fertilization.
During fertilization, the female carries her eggs in a brood pouch, or marsupium, where they remain until they are ready to hatch. A single clutch can contain anywhere from one to 250 eggs. In some cases, the young may stay in the pouch for 2 to 35 days. As the female ages, she continues to produce more eggs in each brood. The eggs hatch directly into juveniles, skipping the larval stage, and these juveniles undergo six to nine molts over about six months before reaching sexual maturity.
They are preyed upon by invertebrates, fish, waterbirds, and marine mammals.
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