Quantitative assessment of reproductive condition of the Antarctic clam, Laternula elliptica (King & Broderip), using image analysis

Abstract

We quantitatively assessed the reproductive state of the Antarctic clam, L. elliptica, collected from a small cove on King George Island in the spawning season. Using computer-based image analysis, percentage gonad area (PGA), percentage egg area within a follicle (FI), and mean oocyte diameter (MOD) were determined from gonadal cross-sections prepared for histological studies. These indices were then compared on the basis of scores from 1 to 6 on a conventional maturity index (MI), which is scored using subjective criteria determined by microscopic examination. Clams collected during two different spawning periods were analyzed. The FI and MOD values differed significantly between the two groups, but MI and PGA values did not, suggesting that FI and MOD are the more reliable and sensitive indicators for differentiating reproductive condition. This study also showed that a planimetric technique using computer-based image analysis is fast, convenient to use, and better than the conventional MI at providing reliable quantitative information about L. elliptica reproduction. We also investigated whether these indices varied with body size. The FI and MOD values peaked in clams of 76 to 85 mm shell length, indicating that clams of this size have the highest reproductive output at spawning time. Therefore, animals of a standard size (76 to 85 mm) should be monitored seasonally or over a longer term.