Not Corals! But Still Stinging-Celled Animals: The Hydrozoans, Part 1

To: Part 2, Part 3, Part 4

by Bob Fenner

Distichopora violacea Red Sea

Notable to aquarists and divers alike as being beautiful, fragile and slightly to highly toxic to the touch! Watch your hands underwater... and don't touch your mucus membranes w/o washing your hands after diving or working in your tanks. Ouch!

Hydrozoans are the "other" Class of cnidarians (Anthozoans making up the mainly Polypoid corals, anemones, sea fans... and Scyphozoans the "real" Jellyfishes that live most of their lives as medusas, inverted bell-shapes). Most of the Hydrozoans are small, obscure not-so funny to touch Christmas tree sort of affairs, but they include such notables as Portuguese Man of War, Fire Corals (Millepora), and the beautiful delicate Stylasterines (Lace Corals) amongst their ranks.

These are colonial animals, sometimes a branch per colony, other times a specialized part (like the "Sail" in the Man 'O War...).

Order Hydroida: Hydroids, the most common Hydrozoans. Most are "tree-like" in shape with their stinging cells much like small leaves or ornaments. Most reproduce sexually, with mature attached colonies releasing small medusae that form at the base of their "branches"... these swim off producing either eggs or sperm, that if joined, metamorphose into a planula larval form that if fortunate, is blown by currents to a suitable reef surface and attaches, becoming a new branched colony.

Genus Aglaophenia:

Aglaophenia sp. Fiji.

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Aglaophenia cupressina Lamouroux 1812, Feather Hydroid. Indo-Pacific. To about two feet in height. Zooplankton filter feeder. Occurs in whitish and tan varieties. A colony in Australia and one showing reproductive structures in N. Sulawesi.

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Aglaophenia latecarinata Feather plume hydroid. Branches out from a dark central stalk in one plane; 1-3". Here in Cozumel 2019.

Genus Antennellopsis:

Antennellopsis sp. Very narrow single strand colonies. N. Sulawesi images.

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Genus Cnidoscyphus

Cnidoscyphus marginatus. Unbranched Hydroid. Yellow, alternating polyps. To four inches. Cozumel 2017

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Genus Dendritica:

Dendritica dendritica Feather bush hydroid. Central stalk accompanied by angular side branching and sub-branches that bear polyps. W. Atlantic. Cozumel 2012

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Genus Gymnangium:

Gymnangium eximium, (Allman 1874), Indo-Pacific; Sri Lanka, Fr. Polynesia... Here in the upper Red Sea (?!). 2019.

Gymnangium hians (Busk 1852), Feather Hydroid. Found in areas of good current on underhangs, in caves (pukas). Gray to light brown in color. 2-3 inches in length. Indo-Pacific. Hawaii pix.

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Gymnangium longicauda, Feather Hydroid. 3 1/2 to 12" tall. Thin, close-spaced individual branches with whitish branchlets. Found worldwide in tropical seas. Cozumel

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Gymnangium speciosum, Slender Feather Hydroid. Found on reef tops in areas of good water quality and current. 2.5-5" tall. Tropical W. Atlantic. Here in Cozumel 2016

To: Part 2, Part 3, Part 4

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