Which of the following is a hydrozoan coral ?

Question

Accepted Answer

The correct option is 3: Stylaster

The term 'coral' is a common name that broadly describes several groups of marine invertebrates in the phylum Cnidaria that secrete a hard skeleton. To answer this question correctly, one must recall the specific taxonomic classification of these organisms. *Stylaster*, commonly known as **lace coral** or **hydrocoral**, is the correct answer because it belongs to the Class **Hydrozoa**, whereas the majority of well-known, reef-building corals belong to the Class Anthozoa. Hydrozoan corals are unique because their life cycle typically includes a larval planula, a sessile polyp colony, and in some species, a free-swimming medusa (jellyfish) stage, unlike Anthozoans which lack the medusa stage entirely. *Stylaster* is part of the Order Anthoathecata and forms delicate, often brightly colored, calcareous skeletons that superficially resemble the true stony corals, but they are developmentally and structurally distinct. They possess specialized polyps, including defensive dactylozooids with powerful stinging cells, which are a characteristic feature of Hydrozoans.

The incorrect option is 1: Alcyonium

*Alcyonium* is commonly referred to as **dead man's fingers** or a **soft coral**. It belongs to the Class **Anthozoa**, specifically the Subclass **Octocorallia** (or Alcyonaria). The Anthozoa are distinguished by the complete absence of a medusa stage in their life cycle. Octocorals, such as *Alcyonium*, are further characterized by polyps that always have exactly eight feathery or pinnate tentacles and eight internal partitions called mesenteries. Crucially, they do not produce a massive, continuous calcium carbonate skeleton like the true stony or hydrozoan corals. Instead, they secrete microscopic, calcareous skeletal elements called sclerites or spicules, which are embedded within the fleshy tissue of the colony.

The incorrect option is 2: Anthipathes

*Anthipathes* is known as a **black coral** or **wire coral**. Like *Alcyonium*, it is classified in the Class **Anthozoa**, but it belongs to the Subclass **Hexacorallia** (or Zoantharia). Black corals are unique for the composition of their skeleton, which is made of a tough, black or dark-brown proteinaceous material called spongin or chitin. This distinguishes them chemically from the calcium carbonate skeletons of both stony and hydrozoan corals. Furthermore, the polyps of *Anthipathes* have tentacles in multiples of six, which is characteristic of the Hexacorallia, but they lack the distinct structural features and the medusa stage found in the Hydrozoa.

The incorrect option is 4: Madrepora

*Madrepora* is a classic example of a **true stony coral** and belongs to the Class **Anthozoa**, Subclass **Hexacorallia**, and the Order **Scleractinia**. The Scleractinian corals are the principal reef-builders worldwide, often referred to as "hard corals." Their defining characteristic is the creation of a massive, rigid, external skeleton composed primarily of aragonite, a crystalline form of calcium carbonate. Since *Madrepora* is an Anthozoan, it entirely lacks the medusa stage that is typical of Hydrozoans. Its complex six-fold symmetry and calcium carbonate skeleton place it firmly in a different class from the hydrozoan corals.