Combined Physical and Biological Cues Enhance Spawning Response in the Hard Clam Meretrix lyrata: Implications for Tropical Bivalve Hatchery Development

Abstract

Reliable hatchery seed production is essential for reducing dependance on wild seed and supporting expansion of marine bivalve aquaculture. This study evaluated three spawning-induction methods for the hard clam Meretrix lyrata under controlled tropical hatchery conditions: heat shock (HS), HS combined with gonad addition (HS + GA), and air shock followed by HS and gonad addition (AS + HS + GA). Wild broodstock were conditioned with a mixed microalgal diet before spawning induction, and embryos and larvae were reared in filtered seawater at 27–29°C. AS + HS + GA produced the highest spawning response (46.67%), significantly exceeding HS (20.00%) and HS + GA (28.89%). Fertilization rates ranged from 55.25% to 63.39% and did not differ significantly among treatments. Hatching rates ranged from 33.48% to 43.55%. The overall treatment effect was not significant (p = 0.0503), although HS + GA differed significantly from AS + HS + GA in the Tukey-adjusted pairwise comparison (p = 0.0388). Embryonic and larval development followed the typical venerid sequence, with first cleavage within 1 h after fertilization (hAF), trochophores at 14–16 h, D-veligers at 24 h, umbonate larvae by day 5, pediveligers by day 6, and settlement by day 18; spat were maintained through day 23. Fertilized eggs, D-veligers, pediveligers, and settled spat measured 70.98 ± 7.94, 100.85 ± 4.99, 119.62 ± 8.20, and 188.33 ± 20.54 µm, respectively. Developmental timing and size progression were broadly consistent with published benchmarks for other venerid clams, providing the first hatchery-based developmental reference for M. lyrata in Bangladesh. Although larval abundance appeared to decline after the pediveliger stage, the study demonstrates the feasibility of induced spawning and larval rearing through settlement under local hatchery conditions. Further optimization of broodstock conditioning, fertilization control, larval feeding, stocking density, settlement management, and microbial regulation is needed to improve hatchery seed-production reliability.