naturally stratified oxic-anoxic sediment developed. Aeration was provided to the oxic-anoxic tanks using airstones to maintain dissolved oxygen levels within the water column (Table 1). Prior to stocking into experimental tanks, the juvenile sea cucumbers (n = 128) were suspended in mesh bags for twenty four hours to ensure gut contents were evacuated prior to weighing. They were then drained on a damp cloth for one minute, weighed to the nearest 0.01 gram (g) and photographed for individual to permit monitoring of individual growth rates (Raj 1998). Animals with a mean weight of 7.3 ± 0.07 g individual-1 (mean ± SE) were allocated randomly to 32 groups of four individuals per group. Each individual was gut-evacuated for 24 hours and reweighed every 27 days over the 81 day experimental period. Wet weight data were used to calculated specific growth rate (SGR), growth rate and co-efficient of variation (CV) as follows: SGR (%/day) = 100 (lnW2 − lnW1) / T Growth rate (g/day) = (W2 – W1)/T, CV (%) =100 × (SD/) where W1 and W2 are initial and final wet body weight of sea cucumbers in each experimental tank (g); T is the duration of the experiment (days); SD is the standard deviation in body weight and is the mean wet weight (g) of sea cucumbers in each experimental tank for a particular sampling period. Behavioural observations Behavioural observations were carried out over three consecutive 24 h periods in the penultimate week of the study. Observations were made at four hour intervals, commencing at noon. Red light was used to facilitate night time observations. During
Table 1: Description of the experimental treatments. Treatment no. Tank structure Aeration Factor 1: Redox regime Factor 2: Sediment depth Factor 3: Particle size 1 Plenum Airlift pump Oxic 2 cm Fine 2 Plenum Airlift pump Oxic 2 cm Medium 3 Plenum Airlift pump Oxic 4 cm Fine 4 Plenum Airlift pump Oxic 4 cm Medium 5 No plenum Airstone Oxic-anoxic 2 cm Fine 6 No plenum Airstone Oxic-anoxic 2 cm Medium 7 No plenum Airstone Oxic-anoxic 4 cm Fine 8 No plenum Airstone Oxic-anoxic 4 cm Medium Fine particle size = 125-250 μm; Medium particle size = 250-500 μm Table 2: Definition of sea cucumber behaviour categorised into burial state and activity according to Wolkenhauer (Wolkenhauer 2008). Behaviour Definition Burial state Buried Entire body under the sediment surface Semi buried >50% of the body under sediment surface On the surface Entire body on sediment’s surface Activity Resting Animal is inactive with no movement
Oxic Oxic Oxic Oxic Oxic-anoxic Oxic-anoxic Oxic-anoxic Oxic-anoxic 2 2 4 4 2 2 4 4 Fine Medium Fine Medium Fine Medium Fine Medium 21.40 ± 3.21 23.83 ± 0.11 21.91 ± 1.92 18.73 ± 2.61 18.35 ± 2.11 21.26 ± 1.61 18.69 ± 1.70 20.96 ± 2.00 0.89 ± 0.15a 0.84 ± 0.04a 0.87 ± 0.15a 0.74 ± 0.04ab 0.65 ± 0.08ab 0.77 ± 0.01ab 0.51 ± 0.03b 0.72 ± 0.07ab 0.40 ± 0.08ab 0.50 ± 0.01b 0.45 ± 0.17ab 0.23 ± 0.04a 0.22 ± 0.05a 0.34 ± 0.03ab 0.39 ± 0.05ab 0.54 ± 0.06b 13.91 ± 1.41ab 16.39 ± 1.36b 13.82 ± 0.41ab 13.78 ± 2.10ab 11.47 ± 0.70a 13.26 ± 1.30ab 11.54 ± 0.46a 12.24 ± 0.32a 63.39 ± 3.89ab 58.45 ± 1.43b 62.95 ± 1.72ab 66.51 ± 4.34ab 69.32 ± 2.84a 64.37 ± 1.81ab 68.87 ± 1.82a 65.55 ± 1.67ab 87.83 ± 1.66 88.82 ± 0.58 88.64 ± 0.53 87.07 ± 1.69 87.61 ± 0.74 90.13 ± 0.17 88.51 ± 0.65 88.92 ± 0.60 4.89 ± 0.88 5.76 ± 0.50 5.09 ± 0.23 4.07 ± 0.65 4.14 ± 0.59 4.85 ± 0.34 4.00 ± 0.44 4.89 ± 0.56