Evaluating the feasibility of measuring planar area of Sinularia penghuensis and Cladiella hartogi using 2D image analysis

Soft coral (Octocorallia: Alcyonacea) farming has been widely used to decrease the fishing pressure on wild populations harvested for use in the ornamental trade and studies related to natural products. With a soft and flexible body, only a few attempts have been made to quantify the growth rate, either in the field or in aquarium systems. In this study, the planar area measurement approach was applied and evaluated for its applicability to soft corals. All fragments of Sinularia penghuensis and Cladiella hartogi were placed on individual tiles (10 × 10 cm) and randomly distributed in the mesocosm. Two treatments were applied: (1) finger touch treatment, and (2) air exposure for one minute to avoid the influence of morphological flexibility caused by the hydroskeleton before taking photographs underwater. Two independent tests were carried out including (1) applying two treatments before taking photographs for four sides (only for S. penghuensis), and (2) comparing the planar area and linear measurement with its dry weight (for both species). Since the shape of C. hartogi is stable through time, the first test was not required for C. hartogi. The photographs were taken twice per day (11:00am and 8:00pm), and five pictures were taken for each side. The results showed that there was no significant difference between one and five pictures used (P > 0.05). Furthermore, taking photographs of two sides that are perpendicular to each other is recommended to provide sufficient statistical power. Meanwhile, the air treatment made the fragments less flexible during the photographing process which should be used in this standardized 2D image analysis. Our results also showed that planar area was strongly correlated (P < 0.001) with its dry weight (R2 = 0.929 for S. penghuensis and R2 = 0.945 for C. hartogi). In addition, the air exposure treatment was consistently similar during both times (daytime and nighttime). Based on the results, we suggest this standardized 2D method to measure the growth of soft corals by measuring planar area changes through time.