The embryonic and larval development of the long-lived, keystone mussel Modiolus modiolus: Implications for its restoration using conservation aquaculture

Abstract

Biogenic reefs formed by bivalves are biodiversity hotspots globally endangered due to anthropogenic pressures. In Strangford Lough (Northern Ireland) bottom trawling caused extensive damage to reefs formed by the horse mussel Modiolus modiolus, compromising their conservation status. Full cycle aquaculture has been proposed as a potential restorative strategy whereby hatchery grown horse mussel juveniles can be used to reseed impacted beds, but this approach requires a thorough knowledge of the species biology. For M. modiolus, a long-lived, cold-water mussel, there is no clear consensus on the reproductive strategies and duration of the larval development cycle.This study presents the results of the first successful, full cycle aquaculture hatchery for M. modiolus in Atlantic Europe, from fertilized egg to spat. The use of air exposure was the only successful method to induce spawning in M. modiolus overcoming a known obstacle in the captive breeding of this species. Spawning lasted five to eight days until broodstock mussels were spent. The duration of the larval cycle from fertilized egg to the pediveliger stage was 34 days while juvenile mussels reached a maximum length of 5.5 mm after six months. All larval stages were recorded: trochophore (t0 + 24 h), D-larva (t0 + 36 h), early umbonated veliger (t0 + 20 days), late veliger (t0 + 32 days) and pediveliger with active foot (t0 + 36 days). Although settlement trials using inert substrates were unsuccessful, juvenile spat were recorded six months after fertilization when live adult M. modiolus were introduced in the rearing tanks. Growth was significantly faster when the larvae were fed with the microalga Chaetoceros calcitans, on its own or combined with Tetraselmis suecica.The results of these breeding trials demonstrated that hatchery culture of M. modiolus is possible, with up to 2 million D-larvae obtained just 24 h after successful spawning of hatchery-kept broodstock. High mortalities and poor settlement indicate that further trials need to be undertaken to streamline protocols if captive breeding is to be considered a viable restoration option for the species. In the interim, a combination of strict protection and brood stock supplementation from healthy beds is the recommended restoration strategy.