Abarenicola pacifica

Like other lugworms, A. pacifica lives under the sand in a J-shaped burrow with its head at the bottom and its tail near the surface. It is a deposit feeder and the head is constantly swallowing muddy sand. This creates a shallow depression on the surface. The cleaner sand, with nutrients removed, is expelled through the posterior end of the worm and ends up as a neat pile of coiled sand on the surface. Bacteria and microfauna may colonise the worm's gut and enrich the faecal casts. Some beaches may have fifty to a hundred lugworms per square metre and the sand is consequently being perpetually recycled by the worms. The lugworm maintains a constant stream of water through the burrow by the alternate contraction and dilatation of its body segments. Gas exchange is facilitated by the bushy tufts of gills which project from its middle region and which continually turn and flex. These adaptations to life under the sediment provide protection for the worm from desiccation and predation while providing a plentiful supply of food and oxygen.^[5] At low tide, when the sediment in which the lugworm is living is no longer covered by water, aerial respiration takes place.^[6]

When feeding, A. pacifica everts its oesophagus (which then resembles a mushroom) and engulfs a "mouthful" of sand before restoring the oesophagus to its rightful position.^[2] Organic detritus and organisms such as nematodes, diatoms, bacteria and microphytes ^[7] are ingested with the sand and digested in the gut. Periodically, the worm reverses up its burrow and defecates near the surface, creating a mound of coiled cast.^[2]

Individuals of A. pacifica are either male or female. Reproduction occurs once a year in the spring at a time when the worm has a high level of stored fats.^[8] The embryos are brooded in the burrow. The larvae do not feed and only spend a short time drifting in the water, or possibly none at all.^[9] When they have developed to five or six segments, they leave their mother's burrow and start living independently in the top few millimetres of substrate. They grow rapidly and become mature in one to two years.^[10]

Pinnixa schmitti, the Schmitt pea crab, is a commensal of A. pacifica and is often found cohabiting in its burrow. The crab may benefit from the protective environment but it is unclear whether the worm gains from the arrangement.^[11]

A. pacifica may be useful in the recovery of beaches from the effects of oil pollution. The worm helps transport sediment from lower depths to the surface where aerobic microbes can break down the pollutant hydrocarbons.^[12] It is considered to be useful as an indicator species for monitoring ecological conditions after an oil spillage, being the second most useful organism after the Baltic clam (Macoma balthica). This is because of the abundance of the lugworm and its widespread distribution near oil facilities.^[13]

The burrowing activities of A. pacifica may also cause the release of methylmercury and other heavy metals from underlying sediments into the water above. The breeding success of clams such as Macoma and Mya spp. is reduced when A. pacifica is present. Disturbance to the sediment has also been associated with a decline in the abundance of the polychaete Pygospio elegans and the crustacean Cumella vulgaris, both of which happen to be predators on juvenile lugworms.^[9] The rock sandpiper (Calidris ptilocnemis), the shrimp (Crangon spp.) and the isopod, Saduria entomon prey on both adults and young.^[13]