Round 1 - Phylum Chordata
(Sources - 1, 2, 3, 4)
Chordata is a phylum consisting of bilaterial animals that have, at at least some point in their development, a notochord and a dorsal nerve chord. It consists of the Cephalochordates (lancelets), Tunicates (sea squirts, salps, and larvaceans), and Vertebrates.
Vertebrates replace their notochord with a spine in early development, while tunicates only retain their notochord and dorsal nerve chord during their larval stage. Vertebrates and tunicates are more closely related to each other than they are to lancelets (first image), which are fish-shaped filter-feeders. They typically inhabit the sea floor, burrowing the bottom halves of their bodies into soft substrate. They have gill slits, but these are used for feeding on plankton rather than respiration. They have light-sensing organs, and one frontal eye. They do not have hearts or brains. They have two sexes, though hermaphroditism has been observed, as well as at least one instance of a lab-raised female transforming into a male. They breed by releasing eggs and sperm into the water synchronously. Larvae are asymmetrical, with the mouth and anus on the left side, and the gill slits on the right side.
Most tunicates are also filter-feeders, ranging from the sessile, sponge-like sea quirts (second image) to the planktonic salps. All tunicates start life as free-swimming, tadpole-like larvae with rudimentary brains and light sensors, before they metamorphize into their adult forms. Meanwhile, the larvaceans retain tadpole-like shapes and active swimming all their lives. Salps move by contracting, similarly to jellyfish, straining phytoplankton from the water. They have a complex life cycle, in which one generation of solitary individuals reproduces asexually by producing a chain of tens to hundreds of individuals, which are released from the parent at a small size. The next generation consists of a colony of salps (called blastozooids) remaining attached together while swimming, feeding, and growing. This generation reproduces sexually, first maturing as females and later transforming into males. Older chains of male blastozooids will fertilize the eggs of younger female chains. Growing embryos are called oozooids, and eventually detach from their parent blastozoids, to feed and grow as the next solitary, asexual generation. Meanwhile, some species of sea squirt live as solitary individuals, while others replicate by budding and become colonies of zooids. They are filter feeders with two tubular openings, called siphons, through which they draw in and expel water.
The most simple vertebrates are hagfish, which have a skull but no vertebral column. They are marine predators and scavengers who can defend themselves against larger predators by releasing copious amounts of slime from mucous glands in their skin.
Lampreys have an ambiguous position in the vertebrate tree of life, bearing a complete braincase and rudimentary vertebrae. They spend the majority of their life as filter-feeders. A small handful of species are known to be carnivorous as adults, boring into other fish to consume flesh and/or blood.
Chondrichthyes (“cartilaginous fish”) have skeletons composed mainly of cartilage. They breath through gills but lack opercula (gill coverings). They have internal fertilization and some species lay eggs while others give live birth. Chondrichthyans have tooth-like scales called dermal denticles or placoid scales. These usually provide protection, and in most cases, streamlining. Today, chondrichthyans are represented by sharks, rays, skates, sawfish, and chimaeras. All species are carnivores, though at least one species is omnivorous.
The majority of chordate species are Actinopterygians (“Ray-finned Fishes”). They are so called because of their lightly built fins made of skin webbings supported by thin bony spines. They are the most abundant free-swimming aquatic animals and can be found almost anywhere there is water. They come in a vast majority of sizes, shapes, colors, and behaviors, from the 8 mm (0.3 in) long Paedocypris to the 11 m (36 ft) long Giant Oarfish (Regalecus glesne). In most actinopterygians, males and females exist and reproduce through external fertilization. However, some species utilize sequential hermaphroditism, in which they start life as females and convert to males at some point. In a few species, they start life as males and convert to females. Some species give live birth, and some species self-fertilise. Actinopterygians have feeding strategies ranging from predatory to grazing to filter-feeding.
And lastly, the Sarcopterygians (“Lobe-finned Fishes”), named for the prominent muscular limb buds (lobes) within their fins. They are represented by the coelacanths, lungfish, and tetrapods. The vast majority of the rest of chordate species are tetrapods, a terrestrial clade of sarcopterygians who evolved air-breathing using lungs. They are highly diverse, with a large variety of forms, biological strategies, and ecological roles. Along with arthropods, they are the only other group of animals to have adapted to life in dry environments, and the majority of them live on land.
Chordata is one of the largest phyla of animals when it comes to species and is also one of the oldest phyla, known from as early as the Cambrian explosion.
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