Sabellaria alveolata | Honeycomb Worm

Introduction

The honeycomb worm (Sabellaria alveolata) is a small tube-building polychaete that constructs one of the most extraordinary structures on the shore: great masses of sandy tubes cemented together into solid, honeycomb-like reefs. Each worm lives in its own tube with a golden crown of bristles and a lid of bristles to seal the door. Where conditions suit, their reefs can stretch for hundreds of metres and reshape the shoreline itself.

Taxonomy

Sabellaria alveolata is a polychaete worm in the family Sabellariidae, the reef-building or honeycomb worms. It is a close relative of the terebellids but belongs to the same annelid phylum as the earthworm, Lumbricus terrestris, sharing the segmented body, the bristle-bearing appendages, and the basic annelid plan, while living a life utterly unlike a burrower in soil: cemented to rock, building upward and outward in colonies of millions.

Physical Characteristics

The body is short and stout, usually three to five centimetres long, pale yellow to greyish, and is divided into distinct regions. The head end carries a golden crown made of three rows of stiff, toothed bristles used to gather sand, and a flattened area that forms an operculum—a plug of bristles used to close the tube opening. The rest of the body bears bundles of fine bristles and small paddles along the sides. The worm itself is almost never seen: only the tube opening and the crown are visible.

Habitat

Honeycomb worms build on hard surfaces in the lower to middle shore—rocks, boulders, and stone structures—where there is a good supply of coarse sand and shell fragments and a reasonable amount of wave action. They occur on Atlantic coasts from Britain and Ireland south to Morocco and into the Mediterranean. Their reefs range from thin crusts to massive platforms and can form extensive, raised banks that hold water in pools at low tide.

Diet

They are suspension feeders. The worm extends its crown of bristles from the tube mouth, catching plankton and organic particles from the moving water; the bristles are also used to sort through sediment for sand grains to build with. Trapped food is passed along grooves to the mouth. Because so many worms feed together, a large reef strips a great deal of food from the water passing over it.

Reproduction

The sexes are separate, and spawning occurs mainly in spring and summer, with eggs and sperm released into the water. The larvae drift in the plankton for several weeks, and they settle preferentially on surfaces already colonised by adults—a chemical cue draws them to existing reefs, which is why the structures keep growing in the same place. Settlement often happens in huge pulses, so an entire reef surface can be colonised at once.

Conservation

The species itself is not threatened, but its reefs are a priority habitat in European conservation law because they are slow to form, easily destroyed, and support a rich community of other species. Physical damage from trampling, bait-digging, coastal works, and dredging can flatten a reef that took decades to build, and recovery is patchy and slow. Several European sites are designated specially to protect honeycomb worm reef.

Fun Facts

• Its reefs look like solid honeycomb, and can cover hundreds of square metres.
• Each worm builds its tube from carefully selected sand grains and shell fragments.
• A bristly plug, the operculum, is used to seal the tube door when the tide is out.
• Larvae are chemically attracted to existing reefs, which is why the structures keep growing.
• The reefs create pools and crevices that shelter hundreds of other species.