On a current-swept wall in Komodo or Palau, a giant sea fan is the thing your eye lands on first: a red-brown net as wide as a doorway, set squarely across the flow.

What is Subergorgia mollis?

It is a large gorgonian octocoral now called Annella mollis. WoRMS lists Subergorgia mollis as a superseded combination and places the species in the genus Annella, family Subergorgiidae. Earlier versions of this page placed it in the Gorgoniidae; that is the family of the Caribbean common sea fan, which is only distantly related. Divers usually just call it the giant sea fan.

How do you identify the giant sea fan?

• Size: very large, flat fans up to about 2 m.
• Mesh: a fine, regular net of fused branches, denser than many other Indo-Pacific fans.
• Color: usually reddish, orange-brown or tan; polyps often paler.
• Orientation: always side-on to the prevailing current.

Similar large Indo-Pacific fans include Annella reticulata and species of Melithaea and Muricella; separating them reliably often needs sclerite examination under a microscope.

Where does Annella mollis live?

Across the Indo-West Pacific, including Indonesia, Palau, the Philippines and northern Australia. It grows on lower reef slopes, walls and channels, anchored on rock, mostly at about 10–40 m. Some species accounts report it most commonly around 12–18 m on lower reef slopes, on rock and sand-covered hard bottom.

How does the giant sea fan feed?

Its polyps catch plankton and organic particles from the passing current. Growing as a single wide plane across the flow maximizes how much water passes through the mesh, which is why giant fans are tied to channels, points and walls with reliable current. Like many Indo-Pacific fans, it relies mainly on captured food rather than symbiotic algae.

What lives on giant sea fans?

Large Indo-Pacific fans host a whole community: whip gobies, small shrimps and crabs, brittle stars, feather stars and, on certain fan species, pygmy seahorses. Hippocampus bargibanti lives on Muricella fans, which divers often confuse with Annella; see pygmy seahorse.

Where can divers see giant sea fans?

Look for them wherever Indo-Pacific reefs meet steady current: channel walls, outer slopes and pinnacles. Field guides illustrate Annella mollis from sites such as Palau and Komodo in Indonesia, and similar giant fans are a feature of walls across the Coral Triangle. The best views come on a gentle drift along a wall at 15–30 m, when fans stand out in silhouette against blue water. Bring a light to see their true red and orange colors, which fade to brown at depth.

Are giant sea fans threatened?

They are vulnerable to physical damage because of their size and slow growth. Anchors, fishing lines, nets and divers who grab fans in current all break them. Warming and bleaching events also stress reef octocorals. Use a reef hook on rock, never on a fan, and keep fins well clear.

Can you keep Annella mollis in an aquarium?

No. It grows far too large, needs strong, plankton-rich current, and is not photosynthetic. Aquarists should look at aquacultured photosynthetic gorgonians instead; see gorgonian care.

How is a giant sea fan built?

Its skeleton is made differently from Caribbean sea fans, which is why the two belong to separate families.

All gorgonians are colonies of eight-tentacled polyps sharing a common tissue, called the coenenchyme, wrapped around a supporting axis. In the Caribbean common sea fan and other holaxonian gorgonians, that axis is a continuous rod of horn-like gorgonin. In the Subergorgiidae, the family of Annella, the axis is built largely from calcareous sclerites bound together with gorgonin. Taxonomists traditionally grouped these sclerite-axis octocorals separately, and although octocoral classification has been substantially revised with genetic data, giant Indo-Pacific fans and Caribbean sea fans remain only distantly related despite their similar shape.

The fine mesh of an Annella fan forms as branches grow and fuse wherever they meet, producing the regular net that gives the fan its strength. The tissue is studded with tiny sclerites whose shapes and sizes are the main features specialists use to separate species, which is why a photograph alone is rarely enough for a confident identification.

Why do sea fans grow so big and so flat?

A wide, flat net across the current is the most efficient way for a fixed animal to strain food from moving water.

A polyp can only catch what passes within reach of its tentacles. By growing as a single broad plane set at right angles to the prevailing current, a giant fan exposes every polyp to fresh, food-carrying water. The mesh slows the flow slightly as it passes through, giving the polyps a better chance to grab plankton. Where currents reverse with the tide, as they do in many Indonesian channels, a fan still works well because water passes through it in both directions.

Size is a trade-off. A bigger fan intercepts more food but also faces more drag, and in a storm surge or exceptionally strong current, large colonies can tear or be ripped from the rock. That is why the biggest fans are usually found in places with strong but steady flow, such as deeper walls and channels, rather than on shallow reef tops exposed to breaking waves.

How do giant sea fans reproduce?

Detailed studies of reproduction in Annella mollis are limited. Like other octocorals, it reproduces sexually, with colonies producing eggs and sperm, and the resulting larvae settle on hard rock to found new colonies. Growth from a tiny recruit to a fan a metre or more across is likely to take many years, which is why damaged or lost fans are slow to be replaced. Broken pieces may survive for a while, but unlike shallow sea rods, a giant fan does not readily regrow from fragments.

Tips for photographing giant sea fans

Giant fans are classic wide-angle subjects. Shoot slightly upward so the fan stands out against blue water, and use strobes or a video light to bring back the reds and oranges that disappear at depth. A buddy positioned well behind the fan adds scale without anyone touching it. For macro, scan the edges and the base slowly: crabs, gobies and shrimps often sit where branches fuse. Keep fins tucked and stay off the fan entirely; even a light grab can strip tissue, and broken branches rarely recover.