A new look at bristle worms living in the deep sea reveals how these creatures brood their young.
Bristle worms (polychaetes) are segmented marine worms that occur in all oceans from the intertidal to the greatest depths. While most bristle worms are relatively poorly known, those that live in the sediments of the deep, dark sea are among the most mysterious. We have discovered just how some deep-sea bristle worms manage to protect their eggs and developing offspring to ensure the survival of their populations, an important part of deep-sea communities.
Most bristle worms of the family Onuphidae build tubes in which they live and thus in ecology are termed as “ecosystem engineers”. Some live submerged in muddy or sandy bottoms but others (quill worms and allies) carry their tubes around. The tubes can be parchment-like or formed of bits of shells, seaweeds, plant debris and/or sand stuck together with mucus.
Knowledge of the reproductive biology of these deep-sea bristle worms is scant. Because of the depth of their habitat, research vessels can only collect these worms using dredges and trawls or by using a ROV (remotely operated vehicle) operated from the mother ship. Only on the fortuitous occasions when reproductively active animals are captured can we get a glimpse into their reproductive biology.
Although it might seem strange that worms look after their young, onuphid worms are well known for their brooding behaviour. Many species lay eggs that are incubated within their tubes or attached to them. These eggs can hatch as swimming larvae that are planktonic before becoming sessile (fixed on the seafloor) adults (“indirect development”), or as small baby worms, which are like miniature adults (“direct development’’).
While re-examining specimens collected during a project in 1987/88 from the the Bay of Biscay (SW Europe), we came across three species of bristle-worms collected in depths of 468–1186 m with eggs and/or developing juveniles inside their tubes.
Two of these worms, Paradiopatra hispanica and Rhamphobrachium brevibrachiatum, build round tubes that have an external covering of mud, sand and broken shells. Only the upper part of the tube is above the surface and the lower part is embedded in the sediment.
The third species, Nothria maremontana, has a very distinctive flattened tube with bivalve shells on its upper surface and shell fragments on the lower part (see related photos), all held together by a secreted glue-like substance. This worm does not live in the sediment but pulls itself along the surface in what is called “epibenthic crawling”, by protruding its head and anterior part of the body from the tube, similar to a case moth caterpillar.
The eggs of the three species are few in number but relatively large (up to 0.5 mm in diameter, which is huge considering the size of the worms themselves) and thus contain enough nutrients to feed the developing young. They develop into miniature worms while still in the tube and before leaving the parental tube and heading to the bottom to build their own tube.
By having few young, and spending a lot of energy and care on their maintenance, these bristle worms help ensure that their offspring survive the challenging deep-sea conditions. Understanding just how these deep-sea marine worms are able to exist in such a tough environment is important for understanding deep-sea communities as a whole.
Dr. Hannelore Paxton
Research Associate, AMRI
University of Oviedo, Spain
Paxton H. & Arias A. 2014. Brooding deep-water onuphid polychaetes (Annelida) from the Bay of Biscay. Marine Biology Research 10, 892–905.
Arias A., Richter A. Anadón N. & Paxton H. 2013. Evidence of simultaneous hermaphroditism in the brooding Diopatra marocensis (Annelida: Onuphidae) from northern Spain. Journal of the Marine Biological Association of the United Kingdom 93, 1533–42.