|General Biology Additional Information||Protobranchs
Nucula nitidosa is a protobranch, a primitive form of bivalve that lacks the extensive gills typical of most bivalves and is, therefore, an obligate deposit feeder (Davis & Wilson, 1985).
Nucula nitidosa is a selective deposit feeder that feeds on a variety of microzooplankton, organic and inorganic matter, and microbes including bacteria and fungi. Protobranchs maintain contact with the substratum by a pair of tentacles, elongations of the margins of the mouth. Each tentacle is associated with a large fold composed of two flaps, called a labial palp, one located to either side of the mouth (Ruppert & Barnes, 1994). During feeding, the palp probosci are extended between the ventral side immediately posterior to the head of the foot (Yonge, 1939). The probosci extend beyond the confines of the shell and actively search for food particles in the sediment. Material is collected by the tip and passes, by way of a ciliated groove, to the base of the proboscis. Here the food particles are transferred to the inner surface of the pouch which conveys them between the palp lamellae where food is sorted out by the action of complex series of ciliary tracts (Yonge, 1939).
It was reported that Nucula nitidosa was also able to feed from inhaled suspensions (Caspers, 1940; cited in Rachor, 1976). This filter feeding ability was demonstrated but shown to be of little importance for Nucula nitidosa (Trevallion, 1965; cited in Rachor, 1976). Nucula nitidosa may assist in the incorporation of organic material into the ecosystem in two ways. Firstly Nucula nitidosa may eat the organic matter present and convert it into flesh, providing food for predators such as flatfish (Blegvad, 1928; cited in Davis & Wilson, 1985). Secondly, Nucula nitidosa may alter the character of the organic matter, for example by producing faeces.
It has proved difficult to get a clear idea of the growth rate of Nucula nitidosa from shell ring analysis (Ford, 1925, Allen, 1953b). It has been suggested that this is due to great variability in reproductive behaviour and possibly growth (Rachor, 1976). In the German Bight, the annual growth rate of young Nucula nitidosa was at least 3.5 mm in the first year of life, while older Nucula nitidosa grew more slowly (1 mm and less during subsequent years) (Rachor & Salzwedel, 1976). Allen (1953b, 1954) calculated a maximum age of 12 years for individuals of Nucula nitidosa that were 12 mm long. This value was revised because Allen (1954) did not take the faster growth of juveniles into consideration (Rachor & Salzwedel, 1976). Rachor (1976) assumed that an individual of 3-4 mm in length was 1 year old with a further growth of 1 mm per year, and revised estimates showed that larger individuals around 12 mm in length were 9 years old rather than 12 years. It was also reported that weight increments decrease with age when Nucula nitidosa reach a length of 6.5 mm (Rachor, 1976).
Populations of Nucula nitidosa can increase markedly when the bottom sediments are suitable. Petersen (1977) reported that the density of Nucula nitidosa was highest at depths shallower than 50 m (Petersen, 1977). For example:
- In the German Bight between 1969 and 1974 the average density of Nucula nitidosa was 498 ind/m² (Rachor, 1976);
- in Aberdeen Bay the species makes up only 6% of the bivalve population on a bottom of coarse sand but 74% when the bottom consisted of fine sand (Tebble, 1976); and
- in Dublin Bay, a mean density of 350 ind/m² was reported for Nucula nitidosa (Wilson, 1983b).
Biomass and Production
According to Stripp (1969; cited in Rachor & Salzwedel, 1976), Nucula nitidosa is the dominant species of the Abra alba community in the German Bight and was found to contribute 23% to the mean macrobenthic biomass of this community. The production of Nucula nitidosa in the German Bight was estimated by Rachor (1976) from seasonal differences in body weights. Trevallion (1965 cited in Davis & Wilson, 1985) calculated some of the components of the energy budget for the closely related Nucula sulcata from British waters. Both studies suggested that about 50% of the total production was allocated to gonad output, a figure considerably in excess of that shown by other bivalves. In Dublin Bay, the production of Nucula nitidosa was 20 KJ m²/yr, which accounted for about 23% of the total benthic productivity of the Bay (Davis & Wilson, 1985).
Nucula nitidosa can be found beneath the surface of the sediments from a few millimetres to a few centimetres deep, where it can dig and creep amongst the sediments. It has been suggested that Nucula nitidosa can move a few to several centimetres per day (Rachor, 1976).
Nucula nitidosa is a representative of the primitive bivalve condition (Purchon, 1968; cited in Holmes et al., 2002). Nucula nitidosa has a comparatively small underdeveloped gill functioning solely as a respiratory organ (Yonge, 1939).
Supports which species
Rachor (1976) reported that epizoic ciliates and hydroid polyps can sometimes be found on the ventral margins of the shells of Nucula nitidosa. Edwards (1965) also reported that the hydroid Neoturris pileata was found living commensally on Nucula nitidosa. Neoturris pileata may impair mobility and interfere with shell growth (Edwards, 1965).