Calcareous tubeworms

Hard, rocky substrates tend to be the preferred sites for attachment of the calcareous tubeworms (Family Serpulidae), although various species also affix to algae and seagrasses, mollusc shells and arthropod exoskeletons. All calcareous tubeworms have tubes formed primarily of calcium carbonate and a matrix of mucus and carbohydrates. Specialized glands near their heads are responsible for tube production. When the mucus/carbohydrate mixture is secreted within the confined space of these glands, calcium carbonate is precipitated from the surrounding seawater. This mixture is then applied to the leading edges of the tubes. Consequently, as the worms grow, so do their encasements. The calcareous tubes immediately identify these worms and readily distinguish them from their close relatives, the feather-duster tubeworms, whose shelters consist of mucus/sediment combinations. As with the feather-duster tubeworms (AN60–67), the calcareous tubeworms have tentacular crowns that function in both feeding and respiratory activities. The tentacular crown is composed of two halves or lobes, each carrying a number of feather-like radioles (spoke-like branches), which have tiny lateral branches called pinnules. Calcareous tubeworms have as few as three pairs of radioles to 20 pairs or more. Most, but not all, calcareous tubeworms have the added benefit of an operculum (trap door) to seal off the tube entrance—presumably an anti-predator device. Each operculum is a transformed radiole (feather-like branch) of the tentacular crown and can be ornate, symmetrical or irregularly shaped, calcified or chitinized (tough, protective). Some species of calcareous tubeworms have achieved an economic status as significant fouling organisms on pilings, docks and boat hulls.