Capitellid thread worms: Heteromastus filiformis and Mediomastus ambiseta

Description

Capitellid thread worms (Heteromastus filiformis and Mediomastus ambiseta) are segmented marine polychaetes (family Capitellidae) with vestigial parapodia; slightly swollen, dark red thoracic regions; and long, slender, paler abdomens (Weiss 1995). In general, they look like earthworms and grow to approximately 10 to 15 centimeters in length (Gosner 1978). Both species build tubes of fine sediment grains and mucus, and they orient themselves in a head-down body position within the sediment.

Background

Distribution. H. filiformis and M. ambiseta are found in most marine environments on the Atlantic and Gulf coasts of the U.S. (Hartman 1947), and they range into estuaries at shallow depths (Gosner 1978). Their distribution is strongly correlated with fine sediments, and they are particularly known for their tolerance of polluted environments. In NY/NJ Harbor, H. filiformis was found throughout the entire region, with highest densities in the Raritan Bay area; M. ambiseta was similarly ubiquitous in silty sediments (this study).

Feeding. Capitellids eat through the substrate, consuming detritus and sediment grains, similar to earthworms (Weiss 1995).

Life Cycle

Polychaete gonads appear as swellings during the breeding season, and gametes are shed into the coelom (epithelium-lined body cavity between the body wall and the digestive tract) and expelled through the nephridia or via the rupturing of the body wall. Fertilization is external, and development proceeds through larval and juvenile stages (Myers 1997, Speer 1997).

Environmental Influences

Salinity. Capitellids are marine species, and their distributions in estuaries are correlated to the salinity structure. Abundance decreases as salinity lowers, and even high amounts of rain can temporarily affect capitellid distributions (Reish 1979).

Predation. Like other polychaetes, capitellids are a food source for various bottom-feeding species of fish, such as spot (Leiostomus xanthurus) and winter flounder (Pleuronectes americanus) in NY/NJ Harbor (Grosslein and Azarovitz 1982).

Pollution. Polychaete distributions and dominance patterns are used as indicators of pollution, and capitellids are particularly tolerant to pollution (Gosner 1978, Weiss 1995). Often capitellids are one of the first groups to recolonize an area impacted by dredge spoil dumping or by an oil spill (Weiss 1995).

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