The acorn barnacle Balanus reticulatus plays an important role in certain marine ecological systems and is one of the most dominant fouling species in tropical and subtropical waters. Its larval development proceeds through six naupliar stages and a single cyprid stage. To understand the effects of K⁺ and Cr⁶⁺ on the larvae of the barnacle B. reticulatus, the following study on different stage larvae and their survival rate to such free ions was conducted.
Adult B. reticulatus were collected from the aquaculture facilities in Dapengao of Daya Bay, Shenzen, China and egg lamellae containing late stage embryos were removed from the mantle cavities by dissection. When such lamellae were placed in a 500mL glass beaker with filtered seawater, nauplii were released within a few minutes. The nauplii were collected and transferred to 100mL beakers with 50mL filtered seawater. Larval culture was conducted over five days at the initial density of 1 larvae/mL in complete darkness at 30℃ with the green algae Platymonas subcordiformis (2.5×10⁵-3.0×10⁵ cells/mL) added.
Stock metal solutions were made up from analytical grade potassium chloride (KCl) and chromium Ⅵ oxide (CrO₃) in distilled water (concentrations of stocks were 10g/L for K⁺ and 1g/L for Cr⁶⁺, respectively). Five test concentrations of each metal were prepared by diluting the stock solutions in filtered seawater. The concentrations of the K⁺ solutions for testing were 0.1, 1, 10, 100 and 1000mg/L. For Cr⁶⁺ they were 0.01, 0.05, 0.1, 1 and 10mg/L, respectively. Five replicates were run for each concentration.
In the tests of K⁺ treatments the survival rates of larvae were 57.4% in the Control, 62.6% in the group treated by K⁺ at a concentration of 0.1mg/L, 58.9% at 1mg/L, 58.0% at 10mg/L, 58.4% at 100mg/L and 59.9% at 1000mg/L, respectively. There was no significant difference in the larval survival rates between the Control and treatment groups (P>0.05). In the Control group, Nauplii Ⅴ accounted for 52.2% and Nauplii Ⅵ 5.2%. In the group treated by K⁺ at a concentration of 0.1mg/L, Nauplii Ⅴ was 58.6% and Nauplii Ⅵ 4.0%; at 1mg/L, Nauplii Ⅴ 55.0% and Nauplii Ⅵ 3.9%; at 10mg/L, Nauplii Ⅴ 54.8% and Nauplii Ⅵ 3.2%; at 100mg/L, Nauplii Ⅴ 56.1% and Nauplii Ⅵ 2.3%; at 1000mg/L, Nauplii Ⅴ 56.3% and Nauplii Ⅵ 3.6%. There was no significant difference in the larval survival percentages of Nauplii Ⅴ and Ⅵ between the Control and treatment groups (P>0.05).
Concerning the tests using Cr⁶⁺ treatments the larval survival rates were 55.4% in Control, 49.0% treated by Cr⁶⁺ at a concentration of 0.01mg/L, 45.9% at 0.05mg/L, 44.2% at 0.1mg/L, 42.4% at 1mg/L and 0% at 10mg/L, respectively. There was a significant difference between the Control and treatment groups (P<0.05). Of them, the survival percentage of naupliar Ⅴ was 8.3%, naupliar Ⅵ 11.2%, and cyprid 35.9% in the Control. Treated by Cr⁶⁺ at 0.01mg/L, the survival percentage of naupliar Ⅴ was 8.2%, naupliar Ⅵ 9.0%, and cyprid 31.8%; at 0.05mg/L, the survival percentage of naupliar Ⅴ was 13.4%, naupliar Ⅵ 15.0%, and cyprid 17.4%; at 0.1mg/L, naupliar Ⅴ was 13.4%, naupliar Ⅵ 15.0%, and cyprid 15.7%; at 1mg/L, naupliar Ⅴ was 14.0%, naupliar Ⅵ 14.0%, and cyprid 14.4%, respectively. No larvae survived the 10mg/L Cr⁶⁺ treatment. The cyprid percentages in the groups treated by Cr⁶⁺ were significantly less than the Control (P<0.05). However, compared with the Control, more larvae at the stage of the naupliar Ⅵ occurred in the treatment groups under the influence of Cr⁶⁺ in the range from 0.05mg/L to 1mg/L (P<0.05).
According to the results gained it can be concluded that the effect of K⁺ on B. reticulatus nauplii survival rate and development was neutral, whereas there was a marked negative effect with the Cr⁶⁺ treatments, which presented a marked inhibitory effect on larval development of the acorn barnacle B. reticulatus.