The effect of the decorator crab Notomithrax minor on cultivated greenshell mussel spat (Perna canaliculus) survival, growth, and byssal characteristics

Abstract

The decorator crab Notomithrax minor is common on Greenshell mussel (Perna canaliculus) farms in the Marlborough Sounds, New Zealand. Individuals in the Greenshell mussel industry have suggested that the presence of N. minor, found on mussel lines, is related to substantial losses of Greenshell mussel spat. Laboratory and field investigations were used to assess the effect of N. minor presence on the retention and productivity of Greenshell musselTM spat. Specific consideration was given to predation pressure and induced anti-predator defenses, both of which can cause financial losses to mussel farmers. High (12 crabs/cage-1) and low (3 crabs/cage-1) densities of large (males: >20mm, females: >15mm TCW) and medium (males: 15-20mm, females: 10-15mm TCW) decorator crabs were placed in cages on commercial Greenshell mussel farm droppers at two sites in the Pelorus Sound. The byssal characteristics, spat retention rate and spat shell length were assessed at 8 and 11 weeks after trial initiation. Greenshell mussel density on the experimental droppers decreased significantly when medium and high densities of the decorator crabs (N. minor) were introduced. N. minor presence induced the remaining Greenshell mussel spat to produce more and thicker byssus threads which consequently lead to increased mussel attachment. The decrease in retention rate and the increase in mussel attachment strength were more pronounced in small recently seeded spat. Laboratory experiments to assess the consumption rate of small (≤5mm) Greenshell mussel spat by decorator crabs showed that mussel consumption by N. minor peaked at 56.43 (± 13.02 (95% C.I.)) crab-1 hr-1, however the rate of consumption decreased significantly over the duration of the three day trial. N. minor prey size preference was also assessed using Laboratory trials; crabs were offered 4 size classes of mussels (small (<5 mm), small-medium (5-10 mm), medium-large (10-15 mm), large (>15 mm) simultaneously. Female crabs consumed more mussels in the <5 mm and 5-10 mm size classes than in the two larger mussel size classes (10-15 mm and >15 mm), whereas the male crabs showed a numerical preference for mussel spat in the small-medium and medium-large size classes. This study provides preliminary evidence that the decorator crab N. minor is a previously overlooked and under-estimated threat to the Greenshell Mussel industry in the Marlborough Sounds that deserves closer scrutiny and experimental testing.