Stinging Hydroid

The Stinging Hydroid (Aglaophenia cupressina) is a captivating macro species belonging to the family Aglaopheniidae. Often referred to by divers as the "Fireweed Hydroid" or "Sea Cypress" due to its distinctive appearance and potent defensive capabilities, this colonial hydroid is a common sight in many Indo-Pacific dive sites. While its conservation status is currently listed as Data Deficient, it is generally considered to be fairly common across its range.

These fascinating invertebrates are not plants, despite their fern-like appearance, but rather colonies of tiny animals (polyps) that work together. They are well-known for their powerful sting, delivered by specialized cells called nematocysts, which can cause a painful sensation similar to an electric shock upon contact.

Identification

• Colony Structure: The Stinging Hydroid forms erect, stiff, and highly branched colonies that resemble miniature cypress trees or delicate ferns. These colonies can reach a maximum height of up to 30 cm (12 in), though some reports indicate they can grow taller.
• Branches and Polyps: Its feather-like projections feature plumose (feathery) branches and numerous side-branches. Each tiny individual polyp within the colony resides in a cup-like structure called a hydrotheca, which has a toothed margin, allowing the polyp to retract for protection.
• Coloration: The coloration of Aglaophenia cupressina can vary, typically ranging from green to brown/grey, or yellowish to brownish. This subtle coloring often allows it to blend seamlessly with its surroundings.
• Habitat and Distribution: Divers will commonly encounter this species attached to hard structures such as rocks, sponges, and corals. It thrives in shallow reef environments with moderate to strong currents or surge, often found on the upper portions of outer reefs. While primarily observed in shallow waters, typically between 1-43 m, it has been reported from depths up to 564 m. Its geographical distribution spans the Western Indian Ocean and the wider Indo-Pacific region, including locations from East Africa to the West Pacific, with observations in areas like the Andaman and Nicobar Islands, the Philippines, and Indonesia.
• Reproductive Structures: The reproductive structures, known as gonophores, are often enclosed within basket-like structures called corbulae, which can sometimes be visible as small clusters on the branches.

The Stinging Hydroid, Aglaophenia cupressina, is a striking colonial hydroid commonly encountered by divers across the tropical and subtropical waters of the Indo-Pacific and Western Indian Ocean. Its distribution spans a vast area, including the Andaman and Nicobar Islands, Okinawa Islands, French Polynesia, and the Philippines, with specific observations in dive sites like Cabilao Island in Bohol.

This species typically thrives in shallow reef environments, often found at depths of 1–18 m. While some reports indicate occurrences at much greater depths, its prevalence for recreational divers is within these shallower zones. Aglaophenia cupressina prefers areas with moderate to strong currents, where it can efficiently filter feed.

• Substrate Preference: Divers will often spot Aglaophenia cupressina colonies attached to hard substrates such as coral and rocks. It is particularly common on the upper sections of outer reefs that are exposed to currents or surge. Beyond natural formations, it can also colonize current-swept walls, the entrances of swim-throughs, and even the rigging of shipwrecks, where water flow is consistent.
• Ecosystem Associations: This hydroid is a significant component of shallow reef ecosystems, where its colonies can grow to form "forests" that provide habitat and complexity for other marine organisms. It is notably present in scleractinian-dominated shallow reefs. Interestingly, Aglaophenia cupressina has been observed to host zooxanthellae, symbiotic algae that contribute to its nutrition, which may explain its success in well-lit, shallow waters. Several species of nudibranchs are known to prey on these hydroids.

The Stinging Hydroid (Aglaophenia cupressina) is a fascinating colonial organism commonly encountered by divers in the Indo-Pacific and Western Indian Ocean, particularly on shallow reefs with moderate current or surge.

• Underwater Encounter Behavior: Divers will observe Aglaophenia cupressina as sessile, feather-like colonies, typically ranging in color from green to brown/grey. These colonies can reach a maximum height of 30 cm (12 inches) and are often found attached to rocks, sponges, and algae. They thrive in rocky subtidal habitats and are commonly observed at dive sites like Hole in the Wall, at depths ranging from 1 to 43 meters, though frequently encountered between 5–20 m. A crucial aspect for divers is its potent sting, which can resemble an electric shock upon contact. Divers should exercise caution and maintain appropriate buoyancy control to avoid accidental contact.
• Feeding Habits: As a colonial filter feeder, Aglaophenia cupressina captures food particles from the water column using its specialized feeding polyps, known as gastrozooids. These polyps possess a protective cup-like structure, or hydrotheca, into which they can retract when threatened. Additionally, the yellowish to brownish coloration of some colonies indicates the presence of symbiotic zooxanthellae, suggesting that the hydroid can also derive nutrients from light through photosynthesis.
• Social Structure: Aglaophenia cupressina exhibits a colonial social structure, meaning it is composed of numerous individual polyps (zooids) that are physically connected and function as a single organism. Within the colony, different zooids are specialized for specific tasks, such as feeding (gastrozooids) and reproduction (gonozooids). The reproductive structures, known as corbulae or rice-like capsules, can sometimes be seen on the branches of the colony.
• Notable Seasonal Patterns: While specific seasonal patterns for Aglaophenia cupressina are not extensively documented, reproduction occurs sexually. Mature colonies release small medusae that produce eggs or sperm. After fertilization, these develop into planula larvae that settle onto a suitable reef surface to form new colonies. The species is found in waters with temperatures typically ranging from 24.6°C to 28.9°C.