Copepods are an amazing group of crusteceans, smaller than amphipods. Copepods & Amphipods are essential to many fish, both in the ocean as well as in the home aquarium.
Copepods are found in a wide range of aquatic environments and are often grouped together to serve complementary purposes within the controlled conditions of marine aquaria -- saltwater reef aquaria being an especially common aquarium environment -- there are many striking differences that serve to clearly distinguish one from the other. Amphipods and copepods naturally share quite a few characteristics and behavioral traits since they are both classified under the Crustacea subphylum.
In order to delineate the unique and contrasting traits and characteristics of the amphipod and copepod while also highlighting the commonalities between them, it is necessary to discuss each of these members of the Crustacea subphylum within the context of the following subcategories:
●Anatomical and behavioral characteristics
●Classification (specifically the orders, suborders and species associated with each)
●History: Origins and fossil record
●Practical behaviors and characteristics relevant to marine aquaria
The diversity of species falling within the Copepoda subclass and the Amphipoda order is especially noteworthy, and the presence of copepods is often used as an ecological indicator of biodiversity given its prevalence in so many different aquatic environments. Underscoring this diversity is the fact that, between copepods and amphipods, the size of the more than 20,000 different species can be anywhere from 1 millimeter to 340 millimeters. Copepods tend to occupy the smaller end of the range while amphipods are represented at both extremes.
Anatomical and Behavioral Characteristics
The anatomical characteristics of the amphipod feature three distinct groupings (abdomen, thorax and head) in which 13 segments are present. With its head fused to its thorax, the amphipod anatomy includes concealed mouthparts to go along with a single pair of sessile compound eyes and two pairs of antennae, the latter of which includes glands responsible for controlling the uptake and excretion of salts.
Amphipods lack the carapace typically found on a number of other crustaceans, and, despite the fact that many refer to amphipods as “freshwater shrimp,” its rear legs (uropods occurring in pairs of three, which, along with the telson, make up the urosome) do not create the tail fan common to shrimp. The pleosome complements the urosome as the other half of the amphipod’s abdomen and features the legs used for swimming purposes.
Eight pairs of uniramous appendages are requisite to the thorax, with the first pair serving as the accessory mouthparts. The four pairs that follow are directed toward the amphipod’s head while the remaining three pairs are directed toward the amphipod’s abdomen. The thorax also features an open circulatory system through which haemocyanin is utilized to deliver oxygen to the amphipod’s tissues. The thoracic segments of the amphipod also reveal the presence of gills.
Copepods, on the other hand, feature anatomical characteristics that vary quite considerably among the 10 orders that make up the Copepod subclass. The diminutive stature of the copepod (most fall within a range of 1 to 2 millimeters) causes its armored exoskeleton to be entirely transparent in most cases. The majority of copepods feature a compound eye in the center of its head -- with the eye typically colored bright red -- but there are copepods that also have two cuticular lenses as well as subterranean copepods that have no eyes at all.
Like the amphipod, copepods have two pairs of antennae and its head is typically fused with the thorax, which is typically divided into segments of three or five. While amphipods take in oxygen through an open circulatory system, most copepods absorb oxygen into their bodies without a heart or circulatory system. The small stature of the copepod also means that it is quite common for the anatomy to lack gills as well.
The behavior of the copepod is aided by the presence of highly organized myelin that surrounds neurons to increase conduction speed, thereby enabling the copepod to quickly react to and escape from predators. This is a very rare characteristic in any kind of invertebrate, but it does not make the copepod immune to predation: Seahorses approach copepods so deliberately that they avoid generating any turbulence at all, leaving the targeted copepod completely unaware of the presence of a predator.
Copepod food consists of phytoplankton, detritus and bacteria, but the specific dietary habits of each will depend on the species in question. The copepods typically found in marine aquaria usually favor a diet comprised of detritus as well as the bacteria often found within the detritus, utilizing the mouthparts that enable them to scrape and bite at the organic material. Copepods found in colder environments store energy within their tiny bodies by converting the food into oil droplets, with these droplets sometimes making up half of their bodily volume.
The majority of amphipods share the dietary characteristics of copepods, with most amphipods using the large claws located on the front pairs of legs to grasp and feed on organic detritus as well as algae and even smaller crustaceans or insects. The amphipods most skilled at predator avoidance are also those most likely to have access to a high-quality diet in which algae is the principal focus, though it should be noted that all amphipods are considered scavengers or, more specifically, detritivores. Amphipods found in Benthic ecosystems are critical to the suppression of brown algae growth and seem to prefer this type of algae to the green or red species found in this specific kind of ecosystem.
Amphipod females carry their eggs in a brood pouch (also known as a marsupium) until they are fertilized and ready to hatch, with the total number of eggs produced by the female increasing with each subsequent brood. Amphipods bypass the larval stage and are hatched as juveniles, reaching maturity after, on average, six total molts.
Copepods, on the other hand, do have a larval stage and are thus hatched into the naupilius larvae that were once believed to be an entirely different species due to the drastic difference in appearance when contrasted with a mature copepod. The copepod reaches maturity after a total of approximately 10 molts, passing from the naupilius larval stage to the copepodid larval stage before achieving adulthood. Depending on the specific type of copepod, the entire process may take just a single week or may last as long as an entire year.
Classification: Orders, Suborders and Species
Amphipods and copepods are both members of the Crustacea subphylum. Amphipods, however, belong to the Malacostraca class while copepods belong to the Maxillopoda class. The diversity of species classified as amphipod or copepod is impressively vast, with well over 20,000 total species currently classified as either amphipod or copepod.
Copepods occupy a subclass composed of 10 orders with a total exceeding 13,000 different species. Of these 13,000 species, approximately 2,800 can be found within freshwater environments. The 10 orders falling under the Copepoda subclass include each of the following:
With 9,500 total species falling under the Amphipoda order, amphipods are also quite diverse and include approximately 1,900 species inhabiting freshwater or non-marine environments. This includes some terrestrial species that are capable of survival in damp environments such as the various layers of decomposing organic material found in a forest floor.
Amphipods belong to the superorder of Peracarida and are typically divided among four suborders, although the classification has undergone revision in recent years with the Senticaudata suborder replacing the Caprellidea suborder. Following this 2013 revision, the suborders that make up the Amphipoda order are as follows:
Although there are 9,500 species classified within the Amphipoda order, only 40 of those species fall beneath the Ingolfiellidea suborder.
Origins and Fossil Record
The temporal range of both copepods and amphipods is thought to precede the available fossil record, but it is currently the case that both amphipods and copepods are classified as belonging to relatively recent epochs. The amphipod fossil record includes just 12 species found in Baltic amber and dated to the Upper Eocene, but it has been nonetheless asserted that amphipods originated long before the Upper Eocene during the Lower Carboniferous period of the geologic timescale. The same is true of copepods, with the origin being assigned as the Early Cretaceous despite the widespread belief that copepods originated during an earlier period on the geologic timescale.
Ecological Conditions: The Relationship Between Amphipods, Copepods, and the Environment
Amphipods and copepods both play essential ecological roles in the environments they inhabit, including marine (saltwater and freshwater) and non-marine ecological systems. Amphipods and copepods maintain a wide variety of symbiotic relationships and remain a critical component of the carbon cycle, not to mention the fact that they serve as a major source of food for all manner of marine and non-marine life. Planktonic copepods, for example, are a principal form of nourishment for whales, seabirds, Alaska Pollock, dragonets and krill.
The amphipod’s usual contribution to the ecosystem is the result of its status as a mesograzer, enabling it to serve as a control on the growth of certain kinds of algae. Since amphipods are found in marine environments ranging from freshwater to water sources with a salinity doubling the level found in seawater, they are a critical component of just about every ecosystem and contribute to a healthy level of biodiversity.
Copepods are similarly critical from an ecological perspective, with many experts believing that copepods form the greatest animal biomass on the planet. This volume is not only important given the role of copepods as a food source to both marine and non-marine life, but also because of how copepods function within the carbon cycle. Planktonic copepods contribute to the absorption of as much as one-third of human carbon emissions, and the manner in which they feed is responsible for making the surface layers of the ocean to be what many consider the world’s largest carbon sink.
Since copepods feed near the surface of the ocean during the nighttime hours when they are least visible to prey and then sink down to and occupy the lower depths during the daylight hours, planktonic copepods deliver carbon to the deep sea through respiration as well as through their molted exoskeletons and fecal pellets. The productivity of copepods in this regard is further enhanced by its small stature as well as its ability to quickly reproduce.
Practical Behaviors and Marine Aquaria
Copepods and amphipods are often included within marine aquaria, with hobbyists featuring these crustaceans for a wide variety of reasons. The scavenging and foraging behaviors exhibited by copepods and amphipods frequently serve as a primary reason cited by marine aquaria hobbyists, but copepods and amphipods often contribute a great deal more beyond their scavenging and foraging habits.
Saltwater aquaria hobbyists often include both amphipods and copepods within their tanks to add to the overall biodiversity of the contained marine ecosystem, and the amphipods and copepods will contribute to the cleanliness of the tank as they scavenge and forage for the detritus left behind by the other tank inhabitants. Since algae is a common source of food for amphipods and copepods, many hobbyists enduring issues with algae growth will introduce amphipods and copepods as a biological control.
Copepods and amphipods are also utilized as a source of food in marine aquaria, especially for saltwater aquaria featuring species known for their difficult dietary preferences. Seahorses and mandarin dragonets, for example, are known to feed on copepods and amphipods, and most saltwater aquaria hobbyists consider the addition of copepods and amphipods entirely beneficial from any number of other perspectives.