The Hidden Biology of Sponges and Ctenophores

Flashcards covering the terminology, phylogenetic hypotheses, and unique biological traits of sponges (Porifera) and comb jellies (Ctenophora) as discussed in the lecture notes.

Hidden biology

Blind spots to complex traits in non-model species, often caused by viewing all animals through a bilaterian lens which distorts the view of animal evolution.

Aboral end

The end of an animal that is opposite its mouth.

Choanoflagellates

Eukaryotic organisms that are the closest known relatives of animals, consisting of collar cells that live singly or in small colonies.

Choanocyte

Collar cells of sponges; whereas their feeding role is identical to choanoflagellates, their cell morphology, function, and development differ significantly.

Clade

A monophyletic group of organisms.

Collar cell

A cell with a flagellum surrounded by a collar of microvilli where the motion of the flagellum draws water past the microvilli for feeding or sensory functions.

Mesenchymal cells

Unpolarized cells dispersed in an extracellular matrix.

Mesoglea

A thick extracellular matrix found in many animals, including ctenophores and cnidarian medusae, often referred to as the jelly in jellyfish.

Monophyletic

A group of organisms comprising all the descendants of their most recent common ancestor and no other organisms.

Picoplankton

Plankton on the order of $0.2-2\,mm$; including many organisms such as bacteria and small eukaryotes.

Synapomorphy

Evolutionarily derived traits that are shared by multiple organisms and were present in their most recent common ancestor.

Coelenterata

A hypothesized clade where Ctenophora is the sister group to Cnidaria, often based on superficial similarities like radial symmetry and two germ layers.

Acrosomata

A hypothesized clade where Ctenophora is the sister group to Bilateria, proposed based on sperm and muscle structure.

Parahoxozoa

A clade comprising Placozoa, Cnidaria, and Bilateria.

Combs

Eight longitudinal rows of ciliary paddles used for locomotion in ctenophores, giving them the name 'comb jellies'.

Colloblasts

Specialized prey-capturing adhesive cells (glue cells) found only in ctenophores, usually located on side branches of contractile tentacles.

Apical organ

A domed structure at the aboral end of a ctenophore containing bundles of cilia supporting carbonate secretions, serving primarily as a gravity-sensing organ.

Rotational symmetry

A symmetry where half of the animal must be rotated $180^{\circ}$ about the oral-aboral axis for matches, rather than being reflected as in bilateral symmetry.

Porifera

Sessile benthic animals, commonly known as sponges, that filter bacteria and picoplankton through tiny holes called porocytes.

Choanocyte chambers

Pumping and feeding structures in sponges formed by choanocytes, connected to an extensively branched water canal system.

Spicules

Articulated inorganic skeletons built of silica or calcium found in sponges; deep-sea carnivorous sponges use hook-shaped versions to ensnare prey.

Osculum

The excurrent opening in a sponge; it acts as a sensory organ containing ciliated sensory cells that provide feedback on water flow.

Hexactinellid sponges

Also known as glass sponges, these have syncytial bodies allowing electrical signals to travel unimpeded to rapidly arrest feeding currents.

Sponge signaling speeds

Signaling in cellular sponges is slow, traveling between $2 \times 10^{-6}$ and $3.7 \times 10^{-4}\,m\,s^{-1}$, whereas hexactinellid syncytia travel at $2.9 \times 10^{-3}\,m\,s^{-1}$.

Homoplasy

When a trait, such as the nervous system, has arisen more than once or has been lost in one or more lineages across the animal tree.

Scala naturae

The linear 'ladder of nature' misconception that organisms evolve from 'lower' to 'higher' states, rather than gaining and losing complex traits across lineages.

Triploblastic

Animals described as having three tissue layers: ectoderm, endoderm, and a middle mesoderm unique to Bilateria.

Diploblastic

A catch-all term for animals with two tissue layers, though it is not a single well-defined character state given the unique middle layers in ctenophores and sponges.