The relevance of reproduction and recruitment to the conservation and restoration of keystone marine invertebrates: A case study of sublittoral Modiolus modiolus reefs impacted by demersal fishing

Abstract

1. Biogenic reefs formed by bivalves are biodiversity hot spots that provide important ecosystem services, but are globally endangered. In Strangford Lough (Northern Ireland) the use of mobile fishing gear (MFG) has caused extensive damage to reefs formed by the horse mussel (Modiolus modiolus L.), leading to declines in their extent and condition.2. Captive breeding and translocation have been proposed as potential restorative strategies for the remaining reefs, and thus the reproductive biology of M. modiolus as the keystone species needs to be described prior to attempting large-scale ecological restoration.3. Physiological condition and gametogenesis were studied in two sympatric M. modiolus subpopulations using direct gonad assessment methods. The size-and age-frequency structure of the whole metapopulation were described to establish present or past recruitment events. Although gonad condition remained high in eachyear for the two subpopulations studied, significant losses were recorded in one subpopulation. These observations are consistent with autumn and winter spawning, but were not recorded in reefs thought to be more damaged by MFG. 5. Length-and age-frequency histograms showed a distinct bimodal distribution in all remnant M. modiolus subpopulations, which were dominated by mussels over 50 years old. Little or no successful recruitment has occurred in some subpopulations, where low reproductive output caused by declines in broodstock numbers might be preventing natural recovery. 6. A combined approach of strict protection of the remaining larval sources and active restoration through stock supplementation is recommended to address recruitment limitations and increase connectivity between remnant subpopulations.

Genetic Connectivity and Diversity of a Protected, Habitat-Forming Species: Evidence Demonstrating the Need for Wider Environmental Protection and Integration of the Marine Protected Area Network

“…Sampling also targeted populations within and outside of MPAs with at least one protected site with M. modiolus reef as a designated feature in each region. While widely dispersed individual M. modiolus may contribute toward genetic connectivity, they were not included in the sampling strategy as reefs are considered key recruitment and settlement sites (Fariñas-Franco and Roberts, 2018). In addition to examination of genetic connectivity, the Noss Head site (Figure 1) was also selected for consideration of self-recruitment potential as it is the largest (3.85 km 2 ) and most abundant known M. modiolus reef in Scotland (Hirst et al, 2012).…”

“…M. modiolus is predominantly a sublittoral semi-infaunal species though intertidal populations are occasionally encountered in shallow pools and damp cervices along the lower reaches of the intertidal region (Seed and Brown, 1977). The species is generally long-lived (e.g., 50+ years in several North-East Atlantic populations) and has maximum shell lengths varying from 65 to 210 mm depending on location (e.g., Anwar et al, 1990;Dinesen and Morton, 2014;Brash et al, 2018;Fariñas-Franco and Roberts, 2018). M. modiolus reefs are subtidal, typically occurring at 10-35 m depth (Mair et al, 2000(Mair et al, , 2010Hirst et al, 2012) in fully saline conditions and tide-swept areas, though a smaller proportion can also be found in sheltered bays, fjords, or lochs (Rees, 2009), where temperature and salinity regimes may vary.…”

“…Marine biogenic habitats such as M. modiolus reefs comprising conglomerations of sessile individuals, for example, are maintained through the physical dispersal of larvae which dictates levels of genetic connectivity and the genetic structure of populations across space and time. The species is predominantly dioecious with occasional hermaphrodites (Wiborg, 1946;Jasim and Brand, 1989;Fariñas-Franco and Roberts, 2018) and age to sexual maturity has been shown to vary between Norwegian, Canadian, Northern Irish and Manx studies from 3 to 8 years of age (see Dinesen and Morton, 2014). Variation in spawning appears to depend on latitude and site-specific conditions with seasonal events, inactive years, prolonged 'trickle-spawning' and synchronized epidemic spawning all reported (see De Schweinitz and Lutz, 1976;Dinesen and Morton, 2014 and references therein).…”

Timing it right: Non-consumptive effects on prey recruitment magnify overtime

“…As mussels are a ubiquitous feature of benthic coastal communities worldwide, from intertidal to subtidal and rocky to soft-sediment systems (Bayne, 2009), such behaviour has the potential to cause major shifts in population dynamics. That is, a reduced settlement will reduce the potential to form mussel-reefs and in turn will have cascading effects on both community and overall ecosystem functioning (Fariñas-Franco and Roberts, 2018;O'Connor et al, 2013). M. edulis is also a commercially important species which is farmed extensively (Dolmer et al, 2012;Knights, 2012;Smaal, 2002).…”

Habitat with small inter-structural spaces promotes mussel survival and reef generation

“…It is often assumed that biogenic reefs have a self-sustaining mechanism, such that once a reef is established, its complex structure provides refuge from predation, which facilitates recruitment (Bertness and Grosholz 1985 ; Nestlerode et al 2007 ; Walles et al 2015 , 2016 ) and maintains a healthy and stable reef system. When a reef is damaged, however, this process will be diminished (Lenihan 1999 ), which could de-stabilise a reef-dominated system by reducing the establishment of new recruits and subsequent reef re-formation (Barrios-O’Neill et al 2017 ; Fariñas-Franco et al 2018 ; Fariñas‐Franco and Roberts 2018 ), potentially leading to an alternative stable state which may be represented as a ‘degraded’ system lacking in complexity (Petraitis and Dudgeon 2004 ).…”