Arapaima (Arapaima gigas): The Metazoa Explorer

Image Credit: Scientific Frontline

Taxonomic Definition

Arapaima gigas, colloquially known as the pirarucu, is a giant neotropical freshwater teleost belonging to the family Arapaimidae within the order Osteoglossiformes (bonytongues). It is endemic to the Amazon Basin, predominantly inhabiting the floodplains (várzea) and slow-moving tributaries of Northern South America, including Brazil, Peru, and Guyana. This species represents one of the largest extant freshwater fishes, morphologically characterized by a broad, bony head and a streamlined, sub-cylindrical body.

Horned lizards (Phrynosoma): The Metazoa Explorer

Texas Horned Lizard
Phrynosoma cornutum
(Public Domain)

Taxonomic Definition

The genus Phrynosoma (from the Greek phrynos, "toad," and soma, "body") comprises a distinct group of squamate reptiles within the family Phrynosomatidae, order Squamata. Characterized by a dorsoventrally flattened body plan, short tail, and prominent occipital and temporal spines (horns), these lizards are endemic to North and Central America. Their range extends from southern Canada through the western United States and Mexico into Guatemala, inhabiting diverse arid and semi-arid biomes including deserts, grasslands, and pine-oak woodlands.

Roadrunner (Geococcyx): The Metazoa Explorer

Roadrunner (Geococcyx)
Left:Lesser Roadrunner (Geococcyx velox) Right:Greater Roadrunner (Geococcyx californianus)
Image Credits: (Lesser):Francesco Veronesi (Greater): Jessie Eastland
(CC BY-SA 4.0)

Taxonomic Definition

The genus Geococcyx comprises two species of fast-running ground cuckoos within the family Cuculidae and the order Cuculiformes. Native to the arid and semi-arid scrublands of the Southwestern United States, Mexico, and Central America, these birds are morphologically distinct from their arboreal relatives due to their terrestrial specializations. They are defined by their zygodactyl feet, elongated tails used for balance, and high-speed cursorial locomotion.

Silky Shark (Carcharhinus falciformis): The Metazoa Explorer

The Silky Shark (Carcharhinus falciformis)
Photo Credit: Alex Chernikh
(CC BY-SA 4.0)

Taxonomic Definition

The Silky Shark (Carcharhinus falciformis) is a large, migratory pelagic shark belonging to the family Carcharhinidae (Requiem Sharks) and the order Carcharhiniformes. It is a circumtropical species, ubiquitously distributed across the Atlantic, Pacific, and Indian Oceans, typically inhabiting the epipelagic zone from the surface down to at least 500 meters, often near continental shelf edges.

Spix's macaw (Cyanopsitta spixii): The Metazoa Explorer

Spix's macaw (Cyanopsitta spixii)
Image Credit: Scientific Frontline / stock image

Taxonomic Definition

Spix's macaw is a medium-sized Neotropical parrot belonging to the family Psittacidae and the order Psittaciformes. It is the sole representative of the monotypic genus Cyanopsitta, distinguished by its cyan-blue plumage and facial skin that fades from grey to white as the bird matures. The species is endemic to the Caatinga dry forest biome of northeastern Brazil, specifically restricted to riparian gallery woodlands dominated by Caraibeira trees (Tabebuia aurea) along seasonal creek systems in the São Francisco River basin.

Caiman (Caimaninae): The Metazoa Explorer

Caiman (Yacare)
Photo Credit: Rodrigovigil
(CC BY-SA 4.0)

Taxonomic Definition

The Caimaninae constitute a subfamily of crocodilians within the family Alligatoridae, distinguished morphologically from true alligators by the absence of a bony septum between the nostrils and the presence of composite ventral osteoderms. Endemic to Central and South America, this clade occupies a diverse array of freshwater habitats ranging from the Amazon basin to the Pantanal wetlands. The subfamily comprises three extant genera: Caiman, Melanosuchus, and Paleosuchus.

Humboldt marten (Martes caurina humboldtensis): The Metazoa Explorer

Humboldt marten (Martes caurina humboldtensis)
Image Credit: Scientific Frontline / stock image

Taxonomic Definition

The Humboldt marten is a critically imperiled subspecies of the Pacific marten (Martes caurina), belonging to the family Mustelidae and order Carnivora. It is biologically distinct from the American marten (Martes americana) and is historically endemic to the humid, coastal coniferous forests of Northern California and Oregon. Currently, the taxon is restricted to four fragmented, isolated population areas (extant population areas or EPAs) along the Pacific coast, relying heavily on dense shrub understories in old-growth redwood and Douglas-fir ecosystems.

Wall lizard (Podarcis muralis): The Metazoa Explorer

Wall lizard on the Lavagna side of the Entella river
Photo Credit: Mariomassone
(CC BY-SA 4.0)

Taxonomic Definition

Podarcis muralis, commonly known as the common wall lizard, is a lacertid lizard within the order Squamata and family Lacertidae. The species exhibits a widespread distribution across Central and Southern Europe, extending into Asia Minor, and has established significant invasive populations in North America and the United Kingdom. It is morphologically variable and serves as a model organism for studying phenotypic plasticity and reptilian polymorphism.

Sea anemones (Actiniaria): The Metazoa Explorer

Bubble-tip anemones
Photo Credit: David Clode

Taxonomic Definition

The Actiniaria are an order of soft-bodied, predatory marine invertebrates within the class Anthozoa and the phylum Cnidaria. Exclusively polypoid in structure, lacking a medusa stage, they attach primarily to hard benthic substrates via an adhesive pedal disc, though some species burrow in soft sediment or float pelagically. Distributed across all marine environments from the intertidal zone to the hadal trenches, Actiniaria represent one of the most diverse groups of hexacorallians.

Muntjac (Muntiacus): The Metazoa Explorer

Red Muntjac female, Muntiacus vaginalis in Khao Yai national park, Thailand
Photo Credit: Tontantravel
(CC BY-SA 4.0)

Taxonomic Definition

The Muntjac (Muntiacus) constitutes a genus of small-to-medium-sized ungulates within the family Cervidae, specifically placed in the tribe Muntiacini. Often recognized as the oldest lineage of extant deer, they are endemic to South and Southeast Asia, ranging from Pakistan and India through China, Vietnam, and the Indonesian archipelago, with introduced populations establishing in the United Kingdom and Japan.

Timber rattlesnake (Crotalus horridus): The Metazoa Explorer

Timber rattlesnake (Crotalus horridus)
Photo Credit: Peter Paplanus
(CC BY 4.0)

Taxonomic Definition

The Timber rattlesnake (Crotalus horridus) is a venomous pit viper belonging to the family Viperidae and the subfamily Crotalinae. It is the sole member of its genus found in the populous northeastern United States, though its range extends south to northern Florida and west to eastern Texas and Minnesota. As a sexually dimorphic species, it is characterized by dorsal chevron patterns and a distinct rattle structure, occupying diverse habitats from deciduous forests to cane thickets.

Honey Bees (Apis mellifera): The Metazoa Explorer

Photo Credit: Sarah Damen

Taxonomic Definition

Apis mellifera, commonly referred to as the Western honey bee, is a eusocial insect belonging to the family Apidae and the order Hymenoptera. Originally native to Europe, Africa, and Western Asia, the species has achieved a near-global distribution due to anthropogenic domestication for pollination services and honey production. It is the type species of the genus Apis and is distinguished by strict caste differentiation and perennial colony structures.

Octopus (Octopoda): The Metazoa Explorer

Photo Credit: Diane Picchiottino

Taxonomic Definition

The Octopoda is an order of soft-bodied, eight-limbed mollusks within the class Cephalopoda. Characterized by bilateral symmetry, a ganglionic nervous system, and the complete reduction of the internal shell (gladius), members of this order occupy a ubiquitous range across the global ocean, extending from intertidal coral reefs to abyssal hydrothermal vents. They are taxonomically distinct from Decapodiformes (squids and cuttlefish) by their lack of feeding tentacles and specific arm morphology.

Aardvark (Orycteropus afer): The Metazoa Explorer

Aardvark (Orycteropus afer)
Image Credit: Scientific Frontline / stock image

Taxonomic Definition

The Aardvark (Orycteropus afer) is the sole extant species of the order Tubulidentata and family Orycteropodidae, representing a distinct phylogenetic lineage within the superorder Afrotheria. Endemic to sub-Saharan Africa, its geographical range encompasses savanna, grassland, and semi-arid bushland, excluding only the true deserts (Sahara, Namib) and dense equatorial rainforests where soil composition prevents fossorial activity. It is characterized as a nocturnal, myrmecophagous (ant and termite-eating) mammal with unique dental morphology that defines its taxonomic order.

Glass sponges (Hexactinellida): The Metazoa Explorer

Euplectella aspergillum
Photo Credit: National Oceanic and Atmospheric Administration

Taxonomic Definition

The Hexactinellida, commonly known as glass sponges, constitute a class within the phylum Porifera characterized by a skeleton composed of siliceous spicules typically exhibiting triaxon (six-rayed) symmetry. Exclusively marine and predominantly deep-sea organisms, they are found globally at depths ranging from 200 to over 6,000 meters, with significant concentrations in the Antarctic and North Pacific waters. Unlike other sponges, their soft tissue consists largely of a syncytium, a continuous multinucleated cytoplasm, rather than discrete cellular units.

Koala (Phascolarctos cinereus): The Metazoa Explorer

Photo Credit: David Clode

Taxonomic Definition

The Koala (Phascolarctos cinereus) is an arboreal herbivorous marsupial native to Australia and the sole extant representative of the family Phascolarctidae. Taxonomically situated within the order Diprotodontia, it is most closely related to the Vombatidae (wombats). Its range extends along the eastern and southern distincts of Australia, spanning Queensland, New South Wales, Victoria, and South Australia, characterized by a dependence on sclerophyll forests dominated by the genus Eucalyptus.

Tigers (Panthera tigris): The Metazoa Explorer

Taxonomic Definition

Panthera tigris constitutes the largest extant species within the family Felidae and the genus Panthera. Taxonomically situated within the Order Carnivora, this obligate carnivore is historically distributed across much of Asia, ranging from the temperate forests of the Russian Far East to the tropical mangroves of the Sundarbans and the rainforests of Sumatra. It is defined by its distinct dark vertical stripes on orange-brown fur with a lighter underside, a phenotype resulting from specific expression of the Agouti and Tabby signaling pathways.

Herons (Ardeidae) : The Metazoa Explorer

Photo Credit: Gerhard Crous

Taxonomic Definition

The Ardeidae are a cosmopolitan family of wading birds within the order Pelecaniformes, comprising herons, egrets, and bitterns. This clade is characterized by long legs, elongated necks, and a distinctive spearing feeding mechanism, inhabiting a vast array of freshwater, brackish, and coastal wetland ecosystems globally, with the exception of the poles and extreme deserts. Recent molecular phylogenetic studies have firmly placed Ardeidae within Pelecaniformes, distinct from the Ciconiiformes (storks) with which they were historically grouped.