Responses of Bugula neritina, an arborescent bryozoan, to the removal of growing tips at different locations within the colony

Brief description

For colonial organisms physical damage or predation can result in the loss of part of the colony. Although the colony may survive, the loss of part of the colony may alter patterns of growth and reproduction. In this study the response of Bugula neritina, an arborescent bryozoan, to physical damage was tested in three experiments: 1) short term regenerative responses to the removal of zooids from growth tips; ii) growth and reproductive responses to damage imposed at different locations within the colony; and iii) the effects of damage produced by a natural predator, the nudibranch Polycera hedgepethi. This data set gives the size and reproductive output of B.neritina colonies over a 4 week period in January and February 2000 following 3 damage treatments: i) full tip removal, ii) half tip removal and iii) branch removal and a control or 'intact colony'. Twenty replicate colonies were assigned to each treatment. The damage treatments removed 32 zooid pairs from each colony (approximately 20%) of each colony and were undertaken on colonies in the laboratory and then returned to the field site at Williamstown, Victoria. Size and reproductive output of the colonies were measured once a week, over 4 weeks in January and February 2000. There were no differences in colony size between the damage treatments but colony fecundity was reduced by 70% in the full tip removal treatment.

Lineage

Maintenance and Update Frequency: notPlanned

Statement: To prevent light exposure triggering the release of larvae, mature colonies were kept in a light-tight box for 24 hours after collection. In the laboratory, colonies were placed in continuous flow seawater systems. The release and subsequent settlement of larvae from the colonies was initiated by exposure to a halogen light source. Larvae were allowed to settle on roughened PVC plates (110 mm x 110 mm by 6 mm thick) placed in plastic trays. Damage (i.e. removal of zooids) to the colonies were done using scissors and a 20 x magnification lamp. Data was collected every week for 4 weeks between January and February 2000. Eighty immature colonies that had undergone 5 branching events (branching score of 5) were used in the experiment. Control colonies were left intact, but subjected to the same handling as other treatments. Damage treatments were established by removing 32 zooid pairs from each colony as follows: i) full tip removal, 1 zooid pair was removed from each of the 16 branch tips, ii) half tip removal treatment, 2 zooid pairs were removed from the eight branch tips on 1 half of the colony and iii) branch removal, the number of zooid pairs along each branch was counted prior to damage, and the 32 zooid pairs were removed while removing the least possible number of growing tips. This dataset gives measures of colony growth (branching score, change in average branching score and average colony growth per unit time) and reproductive output (proportion of reproductive colonies, average branching score at first reproduction and embryos per branching score) of Bugula neritina colonies (n= 18-20). All measures were taken on both sides of the colony and then averaged across the whole colony. PVC plates with colonies attached were positioned at a minimum depth of 2 m adjacent to the Breakwater Pier at Williamstown, Victoria. To reduce exposure to light and sediment accumulation, plates were installed so that the colony faced the sediment. At each measurement period plates were retrieved and immersed in trays of seawater while transported back to the laboratory and returned after measurement. Two colonies from each of the control, half tip removal and full tip removal treatments died during the 4 weeks and were excluded from the final analysis. Branching score represents the number of times a branching event had taken place and provides an index of colony growth. Immature colonies of the same age and size used in the experiment were established by inducing the release and settlement of larvae from mature colonies of Bugula neritina that had been collected from Altona Pier, Port Phillip Bay, Victoria and kept in darkness. The release of larvae from these mature colonies was triggered by exposure to a bright light source within 24 hours of collection. The released larvae were allowed to settle onto roughened PVC plates (0.11 m x 0.11 m x 0.006m thick) placed in trays of seawater. For more details on the methods and experimental design see Bone, E.K.A. & Keough, M.J. (2005) Responses to damage in an arborescent bryozoan: effects of injury location Journal of Experimental Marine Biology and Ecology 324:127-140.

Notes

Credit
Supported by a grant from the Australian Research Council to Prof M.J. Keough