Lurking in the shallow muddy bottom of Darwin Harbour are hundreds of undescribed species of polychaete worms. Polychaetes are the (mostly marine) cousins of oligochaetes, or earthworms; they comprise Annelida, together with leeches and a few lesser known worm-like groups.
The polychaete specimen shown here, about 50 mm in length and only 1 mm wide, belongs to the genus Dendronereis and has recently been recognised as being new to science. It will shortly become the holotype (representative specimen) of a new species being described by the Museum and Art Gallery of the Northern Territory in conjunction with a Taiwanese taxonomist.
It is the first time that the genus Dendronereis has been reported from Australian waters, so this discovery adds significantly to knowledge of native marine biodiversity.
Furthermore, this species and other mud-dwelling polychaetes are an important food resource for prawns and wading birds.
On the flip side however, Dendronereis species have been identified as potential carriers of the White-spot virus, which is detrimental to many crustaceans including farmed prawns.
Fig. 1. Photo of a living specimens of a new species of Dendronereis from Darwin Harbour. Photo: C. Glasby, MAGNT
Dendronereis is perhaps the most widely known genus in the most recognisable family (Nereididae) of all polychaetes – nereidids are often portrayed in textbooks as the “typical polychaete”. Most members of this family have four pairs of tendril-like appendages on either side of the head, two antennae, a pair of large, two-jointed mouth parts, and four eyes. The other notable feature of this family is the mouth, which when turned outward, exposes a pair of black jaws and many rows of soft teeth.
With an armature like this, nereidids like Dendronereis are capable of eating a diverse range of foods, including algae, organic debris, and other small invertebrates on the sea floor.
Fig. 2. Close up of head and upper body segments of the new Dendronereis species. Photo: C. Glasby
Of the 600 or so nereidid species worldwide, only a handful have gills. Most likely these have evolved as a means of increasing oxygen uptake in poorly-aerated estuarine sediments. Unlike the internal gills of fish, which have likely evolved only once, the gills of nereidid polychaetes have appeared independently several times. In Dendronereis the gills are the most developed and resemble a well-branched tree, with the main stem and side branches being fed by blood vessels and capillaries. These tree-like branchiae are so unique that it was used to name the genus, which is derived from the Greek words for tree, Dendron, and a sea-nymph, Nereis.
Fig. 3. Line drawing of key diagnostic features of the closely similar species, Dendronereis aestuarina from India, showing in particular the unique branched structure of the gills on the front body segments (4F, G, H). Illustration: Southern, R. 1921, Fauna of the Chilka Lake. Memoirs of the Indian Museum Vol. 5, Plate 1. Plate prepared from lithograph by D. Bagchi.