Testactinaridae

Testactinaridae is a family of ostracophytes that inhabit a diverse array of habitats on Wallace II, from rivers to coral reefs to the deep sea. They are primarily filter feeders, but some may feed upon larger prey. These "shell anemones" all possess six tentacles and a test, or ​​​​​​shell, during at least one part of their life cycle.

**Etymology**

  The name Testactinaridae is derived from the type species *Testactinaria ubiquita.* The genus name is a reference to its superficial resemblance to the sea anemone clade Actinaria present on Earth, with the test- prefix in reference to the test, or shell, present in all species within the clade\*.

* *The species* Atesta nektos *bears a cartilaginous test in the larval stage, but the test dissolves via apoptosis as it grows, leaving the subadult and adult morphs completely shelless.*

**Phylogeny**

  Testactinaridae is, according to the Linnaean taxonomic system, a family containing a number of subfamilies, within the “coralic” branch of order Ostracophyta and suborder Ostracanthes. Below (right) is a cladogram including most polyspeculate clades so far described, according to Haefoc et al. (2019).

  Within Testactinaridae, multiple groups of related genera can be defined. Classified by morphology and habitat, these relationships have yet to be tested genetically, and the cladogram does not include all 118 known species, but a large variety of unique and common genera that have been thoroughly researched so far.

  When using the tern “xenocorals” for most reef-building Ostracanthes, the word itself is not necessarily monophyletic. Other ostracanths do form reefs, and many ostracanths also don’t, and remain solitary. Still others are completely different orders of marine polyspeculate, yet are still analogous to soft corals of Earth’s reefs. Therefore, the term xenocoral must apply to either the whole of Ostracanthes and only that clade, or remain an unscientific, polyphyletic general classification of Wallace II’s reef builders. Unless future taxonomists begin referring to the ‘coralic’ clade of Ostracanthes as something in particular, like Xenocorallia or a similar name, such a conflict between vernacular and scientific language can be perhaps solved.

**Gross Anatomy**

  The basic form of any testactinarid is quite similar, with minor differences among genera. The presence of six tentacles, a shell at some stage, and a ciliated membrane covering said shell are traits shared in all species in some stage in their life cycle.

  This cross-sectional view of *Vadumiotestactinus lobotentaculata,* a relative of *Hydropyra,* displays the internal and external organs of the polyspeculate.

**Organs**

**Stoma:** A sphincter located at the center of the polyspeculate’s dorsal face, between the tentacles. The stoma can open and close, widen to allow for larger food, and expels waste from the stomach when digestion is complete.

**Pharynx:** The “throat” of the organism, housing large cilia-like structures that trap in smaller prey and food particles when filter feeding. These can also trap and recognize larvae, which are transferred to the gonads.

**Stomach:** The largest organ, where digestion occurs. The stomach-lining tissue is moldable to form temporary mesenteries, where digestion at different stages can occur simultaneously. Small vessels excrete enzymes that break down food, and then absorb the remaining nutrients into the nutrient vascular system. The rest of the waste material is excreted back out through the stoma.

**Gonads:** The reproductive organs of testactinarids, where larvae and sporozoites are born into. See Reproduction for greater detail.

**Digestive Glands:** The glands that produce the various digestive enzymes for the stomach, transported via enzymatic vessels.

**Endocrine Glands:** Hormones and other bodily chemicals are produced here, to regulate growth, behavior, sexual maturity, and more.

**Nutrient Vascular Pump:** The digested food is brought into this organ and transported through nutrient vessels to each cell. This system also brings in the calcium carbonate that the test is built from.

**Hydrovascular Pump:** Cellular respiration and water are regulated through this pump and vascular system. Cell waste is excreted into the stomach, and oxygenated water is brought in through it as well in repeated strokes.

**Nervous Core:** The center for cellular communication, stimulus reaction, and other related functions. Serves as a primitive brain for an otherwise non-encephalized organism.

**Tentacles:** Used to trap food. Bear stinging cells that fire venomous needles on contact with pressure, and cilia to catch small particles and funnel them down into the stoma.

**Ciliated Membrane:** The outer membrane, covering the test. Bears cilia that draw food up into the stoma and has muscles that can move the testactinarid around by crude swimming or by crawling on the seafloor.

**Mucophore:** The organ at the bottom of the ciliated membrane, used to attach to substrate and hold the testactinarid in place.

**Test:** The calcium carbonate shell of the organism. Can be shed and regrown if necessary or retracted into in the case of danger or if the testactinarid is out of the water.

**Reproduction**

  Reproduction amongst Testactinaridae can be both sexual or asexual. Most sessile members of the family can reproduce rhizomatically, by growing a new sporozoite from a tentacle, and splitting off to form a new polyp nearby. However, such buds are clones of the parent polyp, and in order to recombine genes sexual reproduction is necessary.

  Sexual reproduction is done in a manner similar, yet distinct, to many plants on Earth- with alternating generations. The polyp is the sporozoite, producing haploid larvae that are expelled into the current en masse. These larvae have limited motility and feeding abilities, and are softer, simpler, semi-planktonic versions of the adults, bearing cartilaginous tests instead of calcium carbonate. They may end up being caught in a sporozoite, wherein they remain in the gonads to absorb spores. These spores merge with the larval gonadal cells, and radiate out to form a sporozoite larva that is then expelled to grow into a polyp and continue the process.

**Species Profiles**

This section features 20 of the 118 known and described
species within Testactinaridae. Though diverse in their own right, this is only
a sampling of the diversity that is truly present in the waters of Wallace II.