Reproductive Biology, Seasonal Variation of Flat-head Lobster Thenus orientalis (Lund, 1793) in Pethalai, Sri Lanka

Abstract

The flat head lobster Thenus orientalis is an economically important species in the Indo-Pacific, supporting fisheries that yield more than 5,000 t annually in Southeast Asia. Despite its commercial value, detailed information on its reproductive biology—particularly the timing of spawning, size-related fecundity and regional reproductive variation—remains limited. This study examined the reproductive biology of T. orientalis in Pethalai, Sri Lanka, based on monthly samples collected from June 2021 to June 2024 (n = 407) from the commercial catch.  Gonadosomatic index (GSI), size at sexual maturity, sex ratio, fecundity and ovarian development stages. Results indicate a bimodal spawning pattern, with the main reproductive peak occurring during September to March (north-east monsoon period) and a secondary peak around May to June. GSI values varied seasonally (~1–6 %), with maxima in April 2022 (4.35± 1.07 %), December 2023 (4.14±1.77 %) and January 2022 (4.04± 1.64 %). The size at maturity (carapace length) was estimated at 60.0 mm. The sex ratio approximated 1:1 overall (χ² = 0.011, p = 0.917), though temporal variation was significant (χ² = 270, p < 0.001). Fecundity ranged from ~50,000 to 75,000 eggs and showed a positive—but relatively weak—relationship with carapace length. Ovarian development comprised four stages (early maturing, late maturing, ripe and spawning); late maturing females dominated June–September, whereas spawning females predominated October–March. The carapace length–fecundity relationship differed across maturity stages, with stronger correlations at early development. These findings provide critical baseline data for management of T. orientalis fisheries in Sri Lanka. It can be recommended implementing seasonal closures during peak spawning periods and a minimum size limit of 60 mm carapace length to protect spawning females. Furthermore, the size–fecundity association and environmental influence on reproductive output underscore the importance of habitat conservation and ecosystem-based management in sustaining T. orientalis stocks.