Marine
Guides
Lernaeopodidae
Lernaeopodidae is a family of parasitic copepods in the order Siphonostomatoida. Females are typically large and fleshy, attaching permanently to fish hosts using a chitinous plug called the bulla. Males are smaller and cling to females using their antennae. Members parasitize both marine and freshwater fishes, with some species causing significant problems in aquaculture.
Leucosiidae
Purse Crabs
Leucosiidae is a family of marine crabs commonly known as purse crabs. The family contains three subfamilies and several genera of uncertain placement. Members are characterized by their rounded, often smooth carapaces that resemble small purses or pebbles. The family was established by George Samouelle in 1819.
Libinia emarginata
portly spider crab, common spider crab, nine-spined spider crab
Libinia emarginata is a heavily calcified spider crab native to the western Atlantic coast of North America. It is characterized by its triangular carapace, khaki coloration, and distinctive spiny appearance. The species exhibits unusual forward-walking locomotion rather than the typical sideways movement of most crabs. As a decorator crab, it frequently covers itself with debris and small invertebrates for camouflage.
Limulus
Atlantic Horseshoe Crabs
Limulus is a genus of horseshoe crabs in the family Limulidae, containing one extant species, Limulus polyphemus, commonly known as the Atlantic horseshoe crab. The genus also includes at least one confirmed fossil species, Limulus coffini, from the Late Cretaceous of the United States. Several other fossil species have been assigned to Limulus historically, though many have since been reclassified to other genera. The genus is distinguished from other horseshoe crab genera primarily by geographic distribution and subtle morphological features.
Limulus polyphemus
Atlantic horseshoe crab, American horseshoe crab
Limulus polyphemus is a marine and brackish chelicerate arthropod commonly known as the Atlantic or American horseshoe crab. Despite its common name, it is more closely related to spiders, ticks, and scorpions than to true crabs. The species has remained morphologically unchanged for approximately 200 million years, earning it the designation of a 'living fossil.' It is ecologically and medically significant, with populations distributed along the Atlantic coast of North America and in the Gulf of Mexico.
Lophopanopeus bellus
black-clawed crab, Blackclaw Crestleg Crab
Lophopanopeus bellus is a small crab native to the Pacific coast of North America, ranging from Alaska to California. The species is characterized by its rounded carapace with low tubercles, black claws, and highly variable coloration. Two subspecies are recognized: L. b. bellus in the northern portion of the range and L. b. diegensis in the southern portion. The species is notable for being parasitized by the barnacle Loxothylacus panopaei.
Lysmata californica
red rock shrimp, lined shrimp
Lysmata californica is a caridean shrimp species native to the East Pacific, commonly known as the red rock shrimp or lined shrimp. It inhabits shallow coastal ecosystems of Southern California where it plays a role in benthic communities. The species has been studied for its behavioral responses to environmental stressors including ocean acidification and warming.
Merostomata
Horseshoe Crabs
Merostomata is a class of chelicerate arthropods containing the extinct Eurypterida (sea scorpions) and the extant Xiphosura (horseshoe crabs). The group is characterized by appendages that function as mouthparts proximally and swimming legs distally, reflected in the etymology from Greek μηρός ('thigh') and στόμα ('mouth'). Phylogenetic status remains debated: some analyses suggest Merostomata is not monophyletic, with Xiphosura basal to a clade comprising Eurypterida and Arachnida, while a 2022 analysis recovered monophyly with Xiphosura in a derived position.
Micralymma
Micralymma is a genus of rove beetles (Staphylinidae: Omaliinae) established by Westwood in 1837. The genus contains marine intertidal species, with M. marinum being the most documented member. Species in this genus are among the few beetles adapted to regular saltwater submersion. The genus has been recorded in both Europe and North America.
Microcerberidea
Cerberuses and Pillslaters
Microcerberidea is a suborder of minute isopod crustaceans established by Lang in 1961. Members are among the smallest isopods, measuring less than 2 mm in length. They inhabit interstitial spaces in marine sediments across tropical and subtropical coastal regions.
Oncaeidae
Oncaeidae is a family of microcopepods characterized by small body size, high abundance, and high species diversity across all ocean basins. The family was established by Giesbrecht in 1893 and comprises approximately 115 described species across seven genera. Members occur from surface waters to bathypelagic depths, exhibiting a life strategy fundamentally different from most other pelagic microcopepod families. Their ecological role in marine ecosystems remains incompletely understood despite their numerical importance.
Ovalipes ocellatus
Lady Crab, Ocellated Crab, Calico Crab, Leopard Crab, Atlantic Leopard Crab
Ovalipes ocellatus, commonly known as the Lady Crab, is a medium-sized crab species native to the western Atlantic coast of North America. The species is distinguished by its yellow-grey to light purplish carapace adorned with distinctive leopard-like clusters of purple spots, which provide limited iridescence for signaling. It is primarily nocturnal and often buries itself in sandy substrates. The species has been described as notably aggressive toward humans, with documented instances of pinching waders.
Pachycheles
porcelain crabs
Pachycheles is a genus of porcelain crabs (family Porcellanidae) comprising more than 40 described species. These anomuran decapods are distributed across tropical and temperate marine regions, with origins in the Indo-Pacific and subsequent spread to the Americas. The genus is characterized by fragmented lateral carapace walls and exhibits considerable morphological diversity. Several species are subject to parasitism by rhizocephalan barnacles and bopyrid isopods, which can significantly affect host growth and reproduction.
Pachygrapsus
shore crabs, lined shore crabs
Pachygrapsus is a genus of small shore crabs in the family Grapsidae. The genus comprises approximately 14 species distributed across coastal regions, with Pachygrapsus crassipes (lined shore crab) being among the best-studied members. Recent genetic data suggest the genus may be polyphyletic, indicating potential taxonomic revision. Species in this genus occupy intertidal habitats and exhibit complex behavioral adaptations for life in dynamic coastal environments.
Pacifastacus leniusculus
Signal Crayfish
The signal crayfish is a large North American freshwater crayfish introduced to Europe in the 1960s to replace declining native fisheries. It has become one of the most ecologically impactful invasive decapods in Europe, outcompeting native crayfish and serving as a carrier of crayfish plague (Aphanomyces astaci). The species establishes self-sustaining populations rapidly, with six age cohorts documented within a few years of introduction. It shows sex- and size-specific coloration patterns, with males displaying more saturated and brighter ventral claw surfaces than females, suggesting a role in intraspecific communication.
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Pagurus is a genus of marine hermit crabs comprising approximately 170 described species. Members possess an uncalcified, asymmetrical abdomen that they protect by inhabiting empty gastropod shells. These decapod crustaceans occupy diverse marine habitats from intertidal zones to deep continental slopes. The genus exhibits broad geographic distribution across temperate and tropical waters worldwide.
Pagurus acadianus
Acadian hermit crab
Pagurus acadianus is a marine hermit crab species in the family Paguridae, first described by J.E. Benedict in 1901 from specimens in the western Atlantic. It is distinguished from the closely related Pagurus bernhardus by morphological features including larger eyestalks, shorter chelae fingers, and sharper chelipeds. The species inhabits rocky intertidal zones and exhibits seasonal population fluctuations, with peak abundance in June and reduced activity from November through March. It has been documented as the most abundant hermit crab in some Maine localities, though 95.4% of museum records represent preserved specimens rather than living observations.
Pagurus annulipes
Brown Banded Hermit
Pagurus annulipes is a hermit crab species in the family Paguridae, commonly known as the brown banded hermit. It inhabits the western Atlantic Ocean and is notable as the only hermit crab with distinctly banded legs on the northeastern United States coast. This species occupies empty gastropod shells for protection, typical of hermit crab behavior.
Pagurus hirsutiusculus
Pacific Hairy Hermit Crab, Hairy Hermit Crab
Pagurus hirsutiusculus is a small marine hermit crab found along the North Pacific coast from Alaska to California and Japan. Adults reach up to 70 mm body length in northern populations, with southern populations being smaller and less hairy. The species is distinguished by dense body hair, white and blue bands on walking legs, and grayish-brown antennae with white bands. It inhabits the intertidal zone to depths of 110 m, commonly occupying empty gastropod shells for protection.
Pagurus longicarpus
long-clawed hermit crab, long-wristed hermit crab
Pagurus longicarpus is a small hermit crab species reaching up to 12.7 mm in shell length. It inhabits empty gastropod shells, primarily those of periwinkles, oyster drills, and mud snails. The species is distributed along the Atlantic coast of North America from Nova Scotia to Florida and westward to Texas, including the Gulf of Mexico. It occupies intertidal and subtidal habitats to depths of 200 meters, with seasonal movements between shallow tidal pools in warmer months and deeper waters in winter.
Pagurus pollicaris
flat-clawed hermit crab, flatclaw hermit crab, gray hermit crab, shield hermit crab, thumb-clawed hermit crab, broad-clawed hermit crab, warty hermit crab
Pagurus pollicaris is a marine hermit crab found along the Atlantic coast of North America. It inhabits empty gastropod shells, particularly those of shark eye snails and whelks, and reaches approximately 31 mm in length. The species is distinguished by its broad, flat claws that can lock together to form an operculum when the crab withdraws into its shell.
Palaemon
Glass Shrimps
Palaemon is a genus of caridean shrimp in the family Palaemonidae, commonly known as glass shrimps. The genus is widely distributed across marine, brackish, and freshwater habitats in temperate and tropical regions. Molecular studies suggest the conventional circumscription of Palaemon is likely paraphyletic, with related genera Palaemonetes, Exopalaemon, and Couteriella nested within it. Phylogenetic relationships in this group correspond more closely with geographic origin than with traditional genus-level taxonomy.
Palaemon intermedius
Striped Shrimp
Palaemon intermedius is a species of caridean shrimp in the family Palaemonidae. It is endemic to temperate waters of southeastern Australia. The species is characterized by its translucent body with distinctive red spots and narrow stripes. It is a marine species with limited available ecological data.
Paracerceis
Paracerceis is a genus of marine isopod crustaceans in the family Sphaeromatidae, established by Hansen in 1905. The genus comprises at least 13 described species distributed across tropical and temperate coastal regions. The best-studied species, Paracerceis sculpta, has become a model organism for research on alternative reproductive tactics and sexual selection due to its unusual system of three discrete male morphs. Several species have been introduced outside their native ranges, including P. sculpta in Australia and Türkiye.
Pentidotea
Pentidotea is a genus of marine isopods in the family Idoteidae, established by Richardson in 1905. The genus comprises approximately 13 described species of flattened, oval-shaped crustaceans found in coastal marine environments. These isopods are members of the suborder Valvifera, characterized by their ability to roll into a ball. Species in this genus are primarily associated with algae and seagrass habitats in temperate to cold waters.
Pentidotea aculeata
Pencil Isopod
Pentidotea aculeata is a marine isopod in the family Idoteidae, commonly known as the Pencil Isopod. It inhabits the intertidal zone along the coast of California. The species was described by Stafford in 1913. Like other idoteid isopods, it is adapted to life in shallow coastal waters where it likely feeds on algae and detritus.
Pentidotea montereyensis
Pentidotea montereyensis is a marine isopod in the family Idoteidae, first described by Maloney in 1933. The species is found in the temperate northern Pacific Ocean and is associated with kelp and algal habitats. Like other idoteid isopods, it is dorsoventrally flattened and adapted for clinging to macroalgae. The species has been documented through 254 iNaturalist observations, indicating moderate contemporary recording effort.
Pentidotea resecata
Eelgrass Isopod
Pentidotea resecata, commonly known as the eelgrass isopod, is a marine isopod in the family Idoteidae. It inhabits shallow intertidal zones along the Pacific coast of North America, where it is closely associated with eelgrass (Zostera) beds. The species plays a documented role in eelgrass ecology through grazing activities.
Persephona
mottled purse crab (for P. mediterranea)
Persephona is a genus of true crabs in the family Leucosiidae, established by Leach in 1817. The genus contains approximately five described species, including the mottled purse crab (P. mediterranea). These crabs belong to the purse crab family, characterized by their rounded, often box-like carapaces. Species in this genus are found in marine environments, with records from the Atlantic and Mediterranean regions.
Petrochirus diogenes
Giant Hermit Crab
Petrochirus diogenes is a large marine hermit crab native to the Caribbean Sea and western Atlantic Ocean. It is notable for its ability to occupy fully grown shells of the queen conch (Lobatus gigas), which it acquires by preying upon living conchs. Originally described by Linnaeus in 1758 as Cancer diogenes, the species epithet references the Greek philosopher Diogenes of Sinope, who famously lived in a large ceramic jar.
Petrolisthes eriomerus
flattop crab
Petrolisthes eriomerus, commonly called the flattop crab, is a small porcelain crab inhabiting the eastern Pacific coast of North America. It reaches 20 mm in carapace width and exhibits a distinctly flattened, rounded body form adapted for life under rocks and in crevices. The species employs filter feeding and deposit sweeping to consume diatoms and organic material. It displays notable social behavior, including aggregations and ritualized agonistic interactions between individuals.
Pilumnidae
Pilumnidae is a family of crabs in the superfamily Pilumnoidea, characterized by free articulation of all segments of the male abdomen and distinctive gonopod morphology. Members are common in tropical and subtropical intertidal and subtidal habitats, occurring on rocky substrates, coral rubble, and mud. The family has a broad geographic distribution across the Indo-West Pacific, western Atlantic, and other tropical regions, with species often associated with specific habitats such as mangrove mudflats, seagrass meadows, and continental shelf areas.
Pilumnus
hairy crabs
Pilumnus is a genus of true crabs in the family Pilumnidae, commonly known as hairy crabs due to their setose (bristly) exoskeletons. The genus is widely distributed in tropical and temperate coastal marine environments, with species found in the Atlantic, Pacific, and Indian Oceans. Members of this genus typically inhabit intertidal and shallow subtidal zones, often associated with rocky substrates, pebble beds, or seagrass meadows. Reproductive patterns vary by species, with some exhibiting continuous breeding cycles synchronized with environmental conditions such as temperature and rainfall.
Pinnixa
pea crabs
Pinnixa is a genus of small symbiotic crabs in the family Pinnotheridae, commonly known as pea crabs. Species in this genus are obligate or facultative symbionts of various marine invertebrates including bivalve mollusks, tube worms, ghost shrimp, and holothurians. The genus is characterized by a reduced rostrum, absence of teeth between the eyes, and a compact carapace adapted for living within host structures. Multiple species have been documented across the Pacific and Atlantic coasts of the Americas and Asia.
Pinnotheridae
pea crabs, bivalve pea crabs
Pinnotheridae is a family of minute, soft-bodied crabs commonly known as pea crabs. Members are obligate symbionts that inhabit the mantles of bivalve molluscs, with occasional associations with large gastropods (genera Strombus and Haliotis), echinoids, holothurians, and ascidians. The family exhibits extensive morphological adaptations for symbiotic life, including reduced carapace calcification and specialized feeding structures. Fossil records extend to the Danian (Early Paleocene).
Podon
Podon is a genus of marine cladocerans in the family Podonidae, characterized by onychopod morphology. The genus includes at least four described species: P. intermedius, P. leuckarti, P. leuckartii, and P. schmackeri. Members are euryhaline and eurythermal, inhabiting estuarine and coastal marine waters. Populations exhibit strong seasonal dynamics, with spring and fall peaks and summer disappearance in temperate regions.
Pycnogonida
sea spiders, pycnogonids, Pantopoda
Pycnogonida is a class of marine arthropods commonly known as sea spiders. Over 1,300 species have been described, ranging in leg span from 1 mm to over 70 cm. They occur in oceans worldwide from shallow coastal waters to deep sea and Antarctic regions. Males possess specialized ovigerous legs for carrying eggs, a distinctive trait among chelicerates.
Pycnogonidae
Small Sea Spiders
Pycnogonidae is a family of sea spiders containing two genera: Pycnogonum and Pentapycnon. Members are distinguished by their short, stout legs and reduced cephalic appendages. Most species have four pairs of legs as adults, though Pentapycnon species possess five pairs. The family has a worldwide marine distribution.
Pycnogonum stearnsi
Stearns' sea spider
Pycnogonum stearnsi is a sea spider in the family Pycnogonidae, found on the Pacific coast of North America and in Japan. It reaches about 2.5 cm in length and lacks eyes, chelicerae, and pedipalps—unusual traits among sea spiders. It feeds by inserting its barrel-shaped proboscis into soft-bodied prey and sucking fluids. Males carry egg masses on specialized ovigerous legs.
Randallia
Randallia is a genus of true crabs in the family Leucosiidae, established by Stimpson in 1857. The genus comprises approximately 17 described species. These crabs belong to the diverse brachyuran crab fauna and are classified within the subfamily Ebaliinae. Members of this genus are marine decapod crustaceans.
Rheumatobates minutus
Rheumatobates minutus is a small water strider (family Gerridae) described by Hungerford in 1936. It belongs to the subfamily Rhagodotarsinae, a group of marine and estuarine water striders adapted to life on the water surface. The species has been recorded across the Caribbean, Middle America, North America, and South America, indicating a broad Neotropical and Nearctic distribution. As with other members of its genus, it likely inhabits coastal marine environments and estuaries rather than freshwater habitats.
Sapphirina
Sea Sapphires
Sapphirina is a genus of marine planktonic copepods in the family Sapphirinidae, commonly known as sea sapphires. Males exhibit striking iridescent structural coloration ranging from gold to deep blue, produced by multilayered guanine crystal plates beneath the dorsal cuticle. Females are translucent and lack this coloration. The genus comprises specialized predators of pelagic tunicates (salps), with some species exhibiting a complex life history involving both parasitic and predatory phases.
Sapphirinidae
Sapphirinidae is a family of copepods in the order Cyclopoida, established by Thorell in 1859. The family includes three genera: Copilia, Sapphirina, and Vettoria. Members exhibit striking sexual dimorphism in coloration, with males displaying bright, iridescent colors while females remain transparent. They are found in marine environments.
Seroloidea
Seroloidea is a superfamily of marine isopod crustaceans within the suborder Sphaeromatidea. It comprises six families, four of which are extant and two extinct. The superfamily was established by Dana in 1852. Seroloidea is distinguished from other sphaeromatidean superfamilies by unique morphological characteristics of its constituent families, particularly the Serolidae.
Siphonostomatoida
Siphon-mouth Copepods
Siphonostomatoida is an order of copepods distinguished by siphon-like mandibles and a frontal filament used for host attachment. The order comprises 40 recognized families, with approximately 75% of all fish-parasitizing copepods belonging to this group. Most species are marine symbionts, though a few inhabit freshwater environments. Members exhibit diverse host associations, with 17 families parasitizing vertebrates (primarily fishes) and 23 families associated with invertebrates.
copepodparasitemarinefish-parasitesymbiontcrustaceanectoparasiteSiphonostomatoidabiodiversitySouthern-AfricaJapanBrazilsponge-associatecoral-associatedevelopmental-stageschalimusfrontal-filamentsiphon-mandibleAsterocheridaeLernaeopodidaePennellidaeHatschekiidaeSphyriidaeNotodelphyidaeBotryllophilidaeSphaeroma
pillbug, roly poly, marine pillbug
Sphaeroma is a genus of aquatic isopod crustaceans in the family Sphaeromatidae. These small crustaceans are commonly known as marine pillbugs or roly polies, though they are distinct from terrestrial isopods. The genus contains multiple species distributed across marine and estuarine environments globally. Some species, such as S. terebrans, are specialized wood-borers in mangrove habitats, while others inhabit rocky intertidal zones or construct burrows in soft sediments. The genus has been subject to recent taxonomic revision, with new species described from the northeastern Pacific and elsewhere.
Sphaeromatidae
seapills, Typical Seapills
Sphaeromatidae is a family of marine isopods commonly known as seapills, containing approximately 100 genera and 619 marine species with about 65 additional species in freshwater. Members are frequently encountered on rocky shores and in shelf waters of temperate zones. Many species exhibit dorsoventrally compressed body shapes, often with vaulted dorsums, though some are strongly flattened and scale-like. The family includes both free-living and symbiotic forms, with some genera associating with sponges or other marine organisms.
Sphaeromatidea
Seapills and allies
Sphaeromatidea is a suborder of isopod crustaceans established by Wägele in 1989, containing approximately 8587 recorded observations. The suborder comprises seven extant families across two superfamilies: Seroloidea (including Serolidae, Basserolidae, Bathynataliidae, and Plakarthriidae) and Sphaeromatoidea (including Sphaeromatidae, Ancinidae, and Tecticipitidae), plus three extinct families. Members exhibit substantial morphological diversity, with some species having colonized freshwater habitats from marine ancestors.
Sphaeromatoidea
Seapills
Sphaeromatoidea is a superfamily of isopod crustaceans commonly known as seapills. Members of this group are characterized by their ability to conglobate—roll into a ball when disturbed. The superfamily includes approximately 1,000 described species distributed across multiple families, primarily in marine and estuarine habitats. These isopods are distinguished from related groups by specific morphological features of the pleon and uropods.
Squilla
mantis shrimp, spearer mantis shrimp
Squilla is a genus of stomatopod crustaceans commonly known as mantis shrimp or spearer mantis shrimp. Members of this genus are small marine predators, typically 40–70 mm in length, characterized by raptorial appendages adapted for spearing soft-bodied prey rather than smashing hard shells. The genus has a fossil record dating to 145 million years ago and has diversified into over a dozen extant species. Squilla species are distributed in tropical and subtropical marine waters worldwide, including the Indian Ocean and Mediterranean Sea.