The Effect of the Decorator Crab Notomithrax Minor on Cultivated Greenshell Mussel Spat (Perna Canaliculus) Survival, Growth, and Byssal Characteristics

<p>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.</p>

The Effect of the Decorator Crab Notomithrax Minor on Cultivated Greenshell Mussel Spat (Perna Canaliculus) Survival, Growth, and Byssal Characteristics

<p>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.</p>

Morphological descriptions of the larval and first juvenile stages of the decorator crab Camposcia retusa (Latreille, 1829) from laboratory-reared material

The complete larval and first crab stages of the decorator crab Camposcia retusa (Latreille, 1829) are described and illustrated based on laboratory-reared material for the first time. Specimens were obtained from larvae hatched from adult crabs collected from coral reefs of Queensland, Australia. Newly hatched larvae were successfully reared to settlement as the first-stage crabs. Larval development consisted of two zoeal stages and one megalopal stage. The morphology of each larval stage was compared with those available from a previous study using material from the Red Sea. Due to substantial differences in morphology of the second zoeal and megalopal stages between the two studies, we argue that these larval stages described by the earlier report may not be that of C. retusa. Finally, the morphological characters of both larval and first crab stages of C. retusa are also compared with the corresponding stages of previously reported Inachidae.

Population dynamics and reproductive aspects of the decorator crab Microphrys bicornutus (Crustacea: Brachyura: Majidae) in an extreme shallow Thalassia complex

The decorator crab, Microphrys bicornutus, is a common inhabitant of the turtle grass Thalassia testudinum. In order to establish the behaviour of a population of this species in an extreme shallow Thalassia complex (maximum depth less than 50 cm, and often exposed to the air), size distribution, population dynamics, reproductive effort and egg size were studied in Buchuaco—Venezuela. Monthly sampling was undertaken between June 1988 and December 1990. A total of 1403 specimens of M. bicornutus were captured. In each of the sampled years, size distribution was strongly skewed to the predominance of small size crabs. Females were significantly larger than males in all the three years. Small size and large size crabs showed periodic annual fluctuations in abundance, with peaks during the last months of the year for the first, and peaks between April and June for the latter. Medium size crabs showed no periodicity at all. Number of eggs per female ranged between 40 and 4305 (mean = 1067.4), and was positively correlated with female body size. Eggs increased their size gradually during their development, and differences in egg size between development stages were significant. The biological parameters found in this study are compared with other populations of this crab, found in Venezuela and Brazil, in different ecosystems. Results acquired in this work add new insights to the behaviour of this decorator crab, specifically on density variation and egg size (among others), living in an environment that is constantly exposed to air and to other physicochemical variations.