Marine
Guides
Cyclopoida
Cyclopoid Copepods
Cyclopoida is an order of small crustaceans within the class Copepoda, comprising approximately 30 families. Members are primarily planktonic, inhabiting both marine and freshwater environments, and are distinguished by morphological features including antennae shorter than the head and thorax combined. The order exhibits metamorphic larval development with embryos carried in paired or single sacs attached to the first abdominal somite. Molecular phylogenetic studies have reclassified the former order Poecilostomatoida as a lineage nested within Cyclopoida.
Cymothoida
Predaceous and Parasitic Isopods
Cymothoida is a suborder of isopod crustaceans comprising more than 2,700 described species across four superfamilies. Members are predominantly carnivorous or parasitic, distinguished by specialized mouthparts including a mandible with a tooth-like process adapted for cutting or slicing. The group includes diverse lifestyles ranging from free-living scavengers to obligate parasites of fish and crustaceans.
Decapoda
decapods, ten-footed crustaceans
Decapoda is the most species-rich order of Crustacea, with over 14,500 described extant species worldwide. Members include crayfish, crabs, lobsters, prawns, and shrimp—collectively known as decapods or "ten-footed" crustaceans. The order exhibits extraordinary morphological diversity, ranging from tiny symbiotic shrimps under one centimetre to large crabs and lobsters. Decapods occupy virtually every aquatic habitat on Earth, from deep-sea trenches exceeding 5,000 metres depth to terrestrial environments, with nearly half of all species being crabs. The order includes several infraorders with distinct body plans: Brachyura (true crabs), Caridea (shrimps), Anomura (hermit crabs and allies), and others.
Diaulota
Diaulota is a genus of flightless intertidal rove beetles in the family Staphylinidae, subfamily Aleocharinae. The genus comprises approximately eight described species distributed across Pacific coasts of the Northern Hemisphere. Despite being flightless, Diaulota species have achieved a trans-Pacific distribution through coastal dispersal, with some lineages crossing the Pacific Ocean directly via sea surface currents. The genus is notable for its remarkable biogeographic history and specialized intertidal habitat.
Diogenidae
Left-handed Hermit Crabs
Diogenidae is the second-largest family of marine hermit crabs, comprising 429 extant and 47 extinct species. Members are distinguished by an enlarged left chela (claw), earning them the common name 'left-handed hermit crabs'—the reverse of the typical pattern in other hermit crab families. The family includes both conventional shell-dwelling species and the unusual 'blanket-hermit crabs' of the genus Paguropsis, which live in symbiosis with sea anemones rather than occupying gastropod shells.
Dissodactylus mellitae
Sand Dollar Pea Crab, sand-dollar pea crab
Dissodactylus mellitae is a small pea crab in the family Pinnotheridae that lives as an obligate symbiont on sand dollars. It inhabits the western Atlantic Ocean and has been documented specifically on the host Mellita quinquiesperforata. The species exhibits a close association with its echinoderm host, living on the aboral surface.
Dyspanopeus sayi
Say's mud crab, small mud crab
Dyspanopeus sayi is a small mud crab native to the western Atlantic coast of North America, ranging from eastern Canada to Florida. It has become a successful invasive species in European waters, first detected in Wales in 1960 and subsequently spreading to the Mediterranean Sea, North Sea, Black Sea, and most recently the Sea of Azov. The species inhabits muddy bottoms from the intertidal zone to depths of 46 meters and is an active predator of bivalve molluscs and barnacles. It reaches sexual maturity within one year and has a maximum lifespan of approximately two years.
Elthusa
Elthusa is a genus of cymothoid isopods comprising approximately 40 described species of obligate fish parasites. Species in this genus are characterized by branchial parasitism, attaching to the gill filaments of marine fish hosts where they feed on blood and tissue fluids. The genus has a worldwide distribution with records from the Atlantic, Indian, and Pacific Oceans. Several species have been described from South Africa, New Caledonia, Taiwan, and Japan, with some exhibiting host specificity while others parasitize multiple fish species.
Elthusa californica
Elthusa californica is a marine parasitic isopod in the family Cymothoidae. It attaches to the gill chambers of fish hosts and feeds on blood and tissue. The species occurs along the Pacific coast from California to Peru. Originally described as Lironeca californica, it was later transferred to Elthusa following taxonomic revision.
Epicaridea
crustacean isopods
Epicaridea is a suborder of parasitic marine isopods now classified within Cymothoida. Members are obligate ectoparasites of other crustaceans, including ostracods, copepods, barnacles, and malacostracans. The group exhibits extreme sexual dimorphism, with females typically becoming asymmetrical and losing segmentation while males remain small and morphologically distinct. Development proceeds through regressive metamorphosis involving two or three larval stages, including the distinctive cryptoniscus stage. Approximately 704 species have been described globally, though the group remains less studied than other isopod lineages.
Ericthonius
Ericthonius is a genus of marine amphipod crustaceans in the family Ischyroceridae, first described by H. Milne Edwards in 1830. The genus contains at least 20 described species, with records from marine coastal waters of northern Europe. These small crustaceans are part of the diverse benthic communities inhabiting shallow marine environments.
Euceramus praelongus
Euceramus praelongus is a species of porcelain crab in the family Porcellanidae, described by Stimpson in 1860. It belongs to the infraorder Anomura, which includes hermit crabs and related groups. The species is known from limited records in the western Atlantic Ocean and Gulf of Mexico. Available information is sparse, with only nine observations documented on iNaturalist.
Eurypterida
sea scorpions, eurypterids
Eurypterida is an extinct order of aquatic chelicerate arthropods commonly known as sea scorpions. They ranged from the Cambrian through the Permian periods, with their main diversification occurring during the Paleozoic. The group includes both marine and freshwater species, with body sizes ranging from a few centimeters to over two meters in some species. Eurypterids are characterized by a segmented body with a prosoma and opisthosoma, paired swimming paddles, and a terminal telson spine. They represent one of the largest arthropod groups in the fossil record and are significant for understanding early chelicerate evolution.
Eurypteridae
sea scorpions
Eurypteridae is a family of extinct aquatic arthropods known as sea scorpions, representing some of the largest arthropods to have ever lived. These chelicerates inhabited Paleozoic seas from the Silurian to the Permian periods, approximately 467 to 251 million years ago. Members of this family are characterized by their large size, streamlined bodies, and well-developed swimming paddles. Eurypteridae includes the genus Jaekelopterus, which contains the largest known eurypterid species.
Excirolana
Excirolana is a genus of marine isopod crustaceans in the family Cirolanidae, comprising approximately 15 described species. Members are specialized inhabitants of exposed sandy beaches in tropical and warm temperate regions worldwide. The genus is characterized by ovoviviparous reproduction, in which embryos develop internally within paired uteri rather than in an external brood pouch. This reproductive mode is considered an adaptation to the harsh environmental conditions of intertidal sandy habitats.
Exosphaeroma
Spherical Isopods, marine pillbugs
Exosphaeroma is a genus of marine isopods in the family Sphaeromatidae, found in shallow ocean waters worldwide. The genus exhibits highest diversity in the Southern Hemisphere and is notably one of few sphaeromatid genera occurring in the southern reaches of the Southern Ocean. Recent taxonomic work has described new species from the northeastern Pacific, including one discovered in urban Los Angeles and another from 142-year-old Alaskan collections.
Exosphaeroma diminutum
Exosphaeroma diminutum is a small marine isopod in the family Sphaeromatidae, described by Menzies and Frankenberg in 1966. The species epithet 'diminutum' reflects its notably small body size relative to congeners. Like other Exosphaeroma species, it belongs to a group of crustaceans commonly known as marine pillbugs or rolly pollies, which are relatives of terrestrial isopods. The species has been recorded from Saint Thomas in the Caribbean region.
Exosphaeroma inornata
Exosphaeroma inornata is a sphaeromatid isopod originally described by Dow in 1958 from California. The species was later synonymized with E. media George and Stromberg 1968 from San Juan Island, Puget Sound, with differences between original descriptions attributed to author errors or phenotypic variations. It is a wide-ranging intertidal species with documented occurrence from San Diego, California north to San Juan Island, Washington, though a significant distribution gap exists between Humboldt Bay and Puget Sound.
Fucellia
seaweed flies
Fucellia is a genus of true flies (Diptera) in the family Anthomyiidae, commonly known as seaweed flies. The genus comprises at least 20 described species that inhabit marine and coastal environments, particularly in association with seaweed. Several species have been studied for their potential use in aquaculture feed due to their fatty acid profiles and protein content.
Gammaridae
gammarids, scuds
Gammaridae is a family of amphipod crustaceans with a distribution centered on Eurasia. The family exhibits euryhaline tolerance as a lineage, inhabiting environments from freshwater to marine waters. Historically, Gammaridae served as a wastebin taxon for numerous gammaridean amphipods, many of which have since been reassigned to separate families including Anisogammaridae, Melitidae, and Niphargidae. In North America, members are commonly referred to as scuds.
Gammaridea
Gammaridea was historically recognized as a suborder of Amphipoda encompassing approximately 7,275 species (92% of described amphipods) across ~1,000 genera and ~125 families. The group included nearly all freshwater amphipods alongside numerous marine species. Taxonomic revisions by Lowry and Myers (2003–2017) demonstrated that Gammaridea was paraphyletic, leading to its deconstruction into new suborders: Corophiidea (2003), Senticaudata (2013), and Amphilochoidea (2017). The name Gammaridea is no longer recognized as a valid taxon in current amphipod classification.
Gammarus locusta
scud
Gammarus locusta is a marine amphipod crustacean in the family Gammaridae, commonly known as a scud. It inhabits coastal and estuarine environments with a cosmopolitan distribution in European waters. The species has been extensively studied as a model organism for ecotoxicology, particularly regarding responses to ocean acidification, warming, deoxygenation, and pharmaceutical contaminants. Research indicates it possesses the ability to biosynthesize or retain essential long-chain polyunsaturated fatty acids, making it of interest for aquaculture applications.
Gerroidea
Water Striders and Ripple Bugs
Gerroidea is a superfamily of semiaquatic true bugs (Hemiptera: Heteroptera) comprising at least three families—Gerridae (water striders), Veliidae (smaller water striders or riffle bugs), and Hermatobatidae—with over 2,000 described species. Members are specialized for life on the water surface, utilizing water-repellent legs and surface tension to remain afloat. The group occupies diverse aquatic habitats ranging from freshwater ponds and streams to marine coastal environments including mangroves and open ocean. Recent research has documented significant undescribed diversity in tropical regions and novel bacterial symbiont associations.
Gnorimosphaeroma noblei
Gnorimosphaeroma noblei is a marine isopod in the family Sphaeromatidae, described by Menzies in 1954. It is a small crustacean capable of conglobation (rolling into a ball), a defensive behavior common in pill isopods. The species occurs in the temperate North Pacific region. Like other sphaeromatids, it inhabits marine intertidal and shallow subtidal zones.
Gnorimosphaeroma oregonense
Oregon pill bug, Oregon pillbug
Gnorimosphaeroma oregonense is a small intertidal isopod crustacean commonly known as the Oregon pill bug. It inhabits tidal pools and intertidal zones along the Pacific coast from California to Alaska. The species has been documented at depths up to 24 meters.
Hadziidae
Hadziidae is a family of amphipod crustaceans established by S. Karaman in 1943. Members are taxonomically difficult to distinguish from the closely related family Melitidae. The family includes both marine and freshwater-dwelling species, with some lineages having colonized subterranean habitats.
Halacaridae
halacarid mites, marine mites
Halacaridae is a family of meiobenthic mites comprising over 1,100 described species in 64 genera, representing the largest marine radiation of arachnids. Members occupy marine, brackish, and freshwater habitats worldwide, from intertidal zones to deep-sea hydrothermal vents. The family exhibits diverse feeding strategies including predation, algivory, scavenging, and parasitism.
Halocoryza
Halocoryza is a genus of small, intertidal ground beetles (Carabidae) restricted to tropical and subtropical coastal habitats. The genus comprises four described species distributed across shorelines of the Atlantic, Pacific, and Indian Oceans, including Caribbean islands and the Gulf of Mexico. All species are nocturnal predators associated with sandy or coralline substrates near mangroves, coral reefs, or seaweed accumulations. The genus is characterized by reduced flight capability, slow running speed, and adaptations for burrowing in coastal sediments.
Harpacticoida
Harpacticoid Copepods
Harpacticoida is an order of benthic copepods comprising approximately 463 genera and 3,000 species. Members are predominantly marine but include freshwater families (Ameiridae, Parastenocarididae, Canthocamptidae). They represent the second-largest meiofaunal group in marine sediments after nematodes and are also common in Arctic and Antarctic sea ice. A few species are planktonic or live in association with other organisms.
Hepatidae
Hepatidae is a family of marine decapod crustaceans within the order Decapoda. The family was historically recognized to contain certain crab-like taxa, though its current taxonomic status and composition remain subjects of revision in modern crustacean systematics. Members of this group are marine organisms with characteristic decapod features including ten walking legs and a segmented body plan. The family's validity and scope have been debated, with some authorities synonymizing it with other families or reclassifying its constituent genera.
Hepatus epheliticus
Dolly Varden Crab, calico crab, calico box crab, Gulf calico crab
Hepatus epheliticus is a small crab species reaching 76 mm in carapace width, distinguished by its distinctive red-spotted coloration. It inhabits shallow coastal waters of the western Atlantic Ocean from Chesapeake Bay to the Dominican Republic, including the entire Gulf of Mexico. The species is notable for its association with the sea anemone Calliactis tricolor, which it frequently carries on its carapace.
Heterocheilidae
half-bridge flies
Heterocheilidae is a family of parasitic nematodes (phylum Nematoda) within the order Rhabditida and infraorder Ascaridomorpha. Members are intestinal parasites primarily infecting fish hosts, including elasmobranchs and teleosts. The family includes genera such as Brevimulticaecum, Ortleppascaris, and Dujardinascaris. Notably, the name 'Heterocheilidae' has also been applied to a family of dipteran insects (true flies), but this usage is taxonomically distinct and not accepted in current nematode classification.
Hippolytidae
broken-back shrimp, cleaner shrimp, anemone shrimp, hump-backed shrimps
Hippolytidae is a family of marine caridean shrimp commonly known as broken-back shrimp, cleaner shrimp, or anemone shrimp. The family was redefined in 2023, with several former genera transferred to other families (Bythocariidae, Lysmatidae, Merguiidae, Thoridae), leaving 16 genera in Hippolytidae sensu stricto. Some members engage in cleaning symbiosis with fish, removing parasites and debris from host bodies. The family shows both diurnal and nocturnal activity patterns, with behavioral differences in cleaning site preferences.
Homarus americanus
American lobster, Atlantic lobster, Canadian lobster, true lobster, northern lobster, Canadian Reds, Maine lobster
Homarus americanus is a large marine crustacean found on the Atlantic coast of North America. It is the heaviest crustacean in the world, capable of exceeding 20 kg and 64 cm body length. The species is commercially important, supporting major fisheries from Labrador to New Jersey. It inhabits benthic environments from shallow coastal waters to depths exceeding 700 meters.
Hyperia
Hyperia is a genus of amphipod crustaceans in the family Hyperiidae, established by Latreille in 1823. Members of this genus are known for their parasitic or commensal associations with gelatinous zooplankton, particularly jellyfish (Cnidaria). The genus includes species such as Hyperia medusarum, which has been documented as a parasite of scyphozoan and hydrozoan medusae in the southeastern Atlantic Ocean.
Hyperiidae
Hyperiidae is a family of amphipod crustaceans established by Dana in 1852. Members of this family are exclusively marine and are characterized by their obligate associations with gelatinous zooplankton, including jellyfish (Scyphozoa) and ctenophores. The family contains eight recognized genera: Euthemisto, Hyperia, Hyperiella, Hyperoche, Laxohyperia, Parathemisto, Pegohyperia, and Themisto. These amphipods are frequently documented as parasites or associates of their gelatinous hosts in pelagic marine environments.
Hyperiidea
hyperiid amphipods
Hyperiidea is a suborder of exclusively marine amphipod crustaceans characterized by large eyes and a planktonic lifestyle. Unlike other amphipod suborders, they do not occur in freshwater. The group comprises approximately 284 species across 20-23 families. Most species are associated with gelatinous zooplankton as parasites or predators of salps and jellyfish, though some members such as Themisto gaudichaudii are free-swimming predators of copepods and other small planktonic animals.
Hypocharassus
Hypocharassus is a genus of long-legged flies (Dolichopodidae) comprising five described species distributed across the Nearctic, Oriental, and Palearctic regions. The genus is the sole member of the tribe Hypocharassini within subfamily Hydrophorinae. Species are associated with marine and coastal habitats, with adults flying low over wet sand and larvae inhabiting intertidal zones. The Korean species H. cavitarsus represents the first Palearctic record, expanding the genus's known range.
Hypocharassus pruinosus
A marine dolichopodid fly inhabiting the intertidal zone. Larvae develop in decaying seaweed and algal mats in the supralittoral zone, with four larval instars before pupation. Adults are associated with the same marine coastal habitat. The species shows physiological adaptations to tolerate salinity and periodic submersion.
Idotea
Idotea is a genus of marine isopod crustaceans inhabiting cold temperate waters. The taxonomy remains unresolved, with many currently recognized species potentially representing synonyms or candidates for reassignment to other genera. Members are associated with seaweed habitats and exhibit flexible habitat selection behaviors. The genus includes species with distinct physiological adaptations to different oxygen regimes.
Idotea metallica
metallic isopod
Idotea metallica is a neustonic marine isopod in the family Idoteidae, commonly known as the metallic isopod. It is recognized by its metallic blue coloration and blunt, squared-off telson. The species inhabits drifting objects at the sea surface, forming persistent local populations on floating patches of debris. It has a worldwide distribution in open ocean waters but is competitively inferior to coastal congeners such as Idotea baltica, limiting its persistence in nearshore environments.
Idotea urotoma
blunt-tailed isopod
Idotea urotoma, the blunt-tailed isopod, is a marine isopod species inhabiting low intertidal and shallow subtidal zones along the northeastern Pacific coast. It exhibits color polymorphism that matches its algal or seagrass substrate, providing camouflage. The species is distinguished by a broadly triangular telson margin lacking a distinct median projection.
Idoteidae
Common Valvetails
Idoteidae is a family of aquatic isopod crustaceans in the suborder Valvifera, distributed globally in marine and freshwater habitats. The family includes approximately 20 genera and numerous species, with highest diversity in temperate coastal waters. Members range from free-living forms in macroalgae and seagrass beds to commensal species associated with other marine organisms. The family has been extensively studied in Australia, New Zealand, the northeastern Pacific, and the North Atlantic.
Inachidae
spider crabs
Inachidae is a family of marine crabs commonly known as spider crabs, characterized by long, slender legs and often elaborate camouflage behaviors. The family contains approximately 39 genera distributed primarily in Atlantic, Mediterranean, and Indo-Pacific waters. Members of this family are known for their association with sessile organisms including sea anemones, sponges, and algae. Some species exhibit specialized epibiosis, deliberately accumulating algae and other materials on their carapace for camouflage.
Janiridae
Janiridae is a globally distributed family of marine isopods in the suborder Asellota, comprising over 170 species across approximately 23 genera. The family exhibits remarkable bathymetric range, from intertidal zones to hadal depths exceeding 6,000 meters. Most species inhabit shallow shelf waters within 100 meters depth, though several genera have colonized deep-sea environments including whale falls, hydrothermal vents, and abyssal plains. The genus *Jaera*, predominantly northern hemisphere in distribution, includes the notable deep-sea specialist *Jaera tyleri*, discovered on whale bones in the Southern Ocean at 1,445 meters depth—the first *Jaera* species documented in the southern hemisphere. Janiridae demonstrates broad environmental tolerance to salinity, temperature, and oxygen stress.
Janiroidea
Witchers
Janiroidea is a superfamily of marine isopods in the suborder Asellota, established by G. O. Sars in 1897. It comprises 25 families, with Paramunnidae and Munnidae as the two largest and most diverse. Members occupy a remarkable depth range from shallow littoral zones to hadal trenches exceeding 8,000 meters. The superfamily exhibits substantial morphological diversity, including adaptations for interstitial, benthic, and deep-sea habitats.
Lepadiformes
stalked barnacles, goose barnacles
Lepadiformes is an order of stalked barnacles within the class Maxillopoda. Members possess a fleshy peduncle that attaches the capitulum (shell) to substrates, distinguishing them from sessile acorn barnacles. The order includes approximately nine extant families, with Lepadidae being the most species-rich. Many species are epibiotic, attaching to mobile marine hosts including crustaceans, elasmobranchs, and marine debris.
Lepas
Goose Barnacles
Lepas is a genus of goose barnacles in the family Lepadidae, comprising pelagic crustaceans that attach to floating substrates using a flexible stalk. The genus includes at least eight described species, with Lepas anatifera being among the most widely distributed and studied. Members of this genus are characterized by their stalked morphology and calcareous shell plates, representing a distinctive lineage within the barnacle group.
Lepas anserifera
Goose Barnacle
Lepas anserifera is a pedunculate barnacle that attaches to floating substrates including driftwood, ships' hulls, and marine debris. It possesses a capitulum of six white calcareous plates supported by an orange, flexible stalk. The species exhibits rapid growth and early maturation, with individuals reaching reproductive size within approximately two weeks under favorable conditions. As a hermaphroditic filter feeder, it plays a role in marine neustonic communities and has a cosmopolitan distribution in temperate and tropical seas.
Leptocheliidae
Leptocheliidae is a family of small, low-mobility crustaceans in the order Tanaidacea, comprising over 30 genera and 140 described species. Members are abundant in shallow marine waters and show high sensitivity to environmental shifts, particularly dissolved oxygen and temperature changes. Global diversity patterns reveal a bimodal latitudinal distribution with peaks in lower latitudes and decline at the equator, with biodiversity hotspots in the Indo-Australian region, Central Indo-Pacific, Gulf of Mexico, and Caribbean. The family serves as an important indicator group for monitoring environmental change in threatened coastal habitats.