INTRASPECIFIC DENSITY EFFECT ON GROWTH OF Marphysa sp.JUVENILES
The territorial tube-building Marphysa sp. (Eunicidae:Polychaete), commonly known in Portugal by "goose", is largely used as bait for sport and professional fishing, due to the high resistance in the hook and to its ability to attract high-value fish species such as Sparus aurata and Diplodus sargus. As such, it is a high-valued resource, attaining prices of 500 000€/year (unpublished data). Effects of intraspecific density on juvenile growth were studied over a four-month period in laboratory facilities. Three polychaete densities (50, 150 and 250 worms in 0.25 m2 aquarium, containing sandy sediment and recirculating water) were used. Total length, wet weight and number of segments were recorded for 30, 75 and 150 worms, (60% of the initial population).
A two factor ANOVA showed significant density and time effects (p<0.001), and a significant density x time interaction (p<0.001) for all the growth variables in every month. For all the densities used, estimated daily growth was higher in the first month, having progressively decreased over time (Fig. 1).
The high aggressiveness and territorial behavior of Marphysa sp. juveniles was well evidenced by the highest mortality (35%) and greater incidence of broken and regenerating posterior segments (19.3%) observed under high density in the 1st month and by the presence of worms with lesions under low density (30%) (Fig. 2).
In conclusion, the results obtained reveal that densities higher than 1000 juveniles per m2 cause high mortality (50% or more) and Marphysa sp. juveniles have a very territorial and aggressive behavior that should be considered if the species is used for aquaculture production. Additional studies are required to determine the density effects for different developmental stages.
