Search

The spatial distribution and size structure of two infaunal filter‐feeding bivalves, that is, Sakhalin surf clam Spisula sachalinensis (Schrenck, 1862) and dwarf sunray surf clam Mactra crossei (Dunker, 1877), and an epibenthic deposit feeder, namely the sand dollar Scaphechinus mirabilis (A. Agassiz, 1864), were investigated on an open sandy beach in northern Japan after the tsunami caused by the 2011 Tohoku earthquake. Settlement of S. sachalinensis was relatively abundant in the 2011‐ and 2012‐year classes, but then substantially decreased until the early months of 2013. The 0+‐year‐old distribution was associated mainly with a depth gradient and weakly with the median grain size of the sediment. Mactra crossei had (1) an inverse trend in long‐shore variation in abundance at 2+ years of age and (2) later‐post‐settlement processes that regulated its population dynamics, compared to those of S. sachalinensis. The 2010‐year classes for both Mactridae species survived until 2+ years of age, although their abundance showed a high level of spatial variation. Scaphechinus mirabilis showed fine‐scale spatial variation in their recruitment dynamics. The cross‐shore distribution of S. mirabilis showed a clear ontogenetic shift from shallow to deep water. The cross‐shore distribution of small S. mirabilis (<40 mm in test diameter) overlapped that of the juvenile S. sachalinensis, but their interaction was unclear. Overall, the level of spatial and temporal recruitment variation was relatively high in each species. However, the tsunami has scarcely influenced the cross‐shore distribution and population structure of each species. [ABSTRACT FROM AUTHOR]

Mesodesma donacium is a surf clam endemic to the southeast Pacific coast, and it is an important resource species of the sandy beach artisanal fishery in Peru and Chile. Over time, the species has shown high variability in population dynamics (presence and abundance), which has been attributed to overfishing and environmental events. In this study, we assess the phylogeography of seven natural beds of M. donacium (17 to 42° S), to reveal the geographic distribution of the genetic diversity through the analysis of 278 sequences of the mitochondrial gene COI. Sequences of the nuclear genes 18S and 28S were used to evaluate the divergence of COI haplogroups. Two divergent parapatric mitochondrial haplogroups were found, which lacked divergence at nuclear markers (18S and 28S); this mito‐nuclear discordance allows inferring that there is no reproductive isolation of mitochondrial haplogroups. The North haplogroup is the only one present at the northernmost site, while only the South haplogroup was present in Cucao in the south. Between 28° and 32° S, coincident with a coastal biogeographic break at 30° S, both haplogroups are in sympatry. Haplogroups differed in their genetic structure, with the North haplogroup representing a single, highly diverse population and the South haplogroup with genetic differentiation and more restricted genetic diversity and gene flow. The divergence in mitochondrial haplogroups without nuclear divergence suggests a past scenario of geographic isolation in the northern and southern areas, without developing reproductive isolation, followed by secondary contact. Given the phylogeography, genetic information should be considered in restocking and management activities. [ABSTRACT FROM AUTHOR]

Although mitochondrial inheritance in metazoans is typically strictly maternal, doubly uniparental inheritance (DUI) is probably the major exception to this widespread rule. DUI has been found in many species of bivalve molluscs, belonging to several different families. Based on current understanding, the detection of DUI generally relies on the detection of two distinct mitochondrial DNA lineages: a female-transmitted one, that dominates somatic tissues in males and females and eggs, and a male-transmitted one, that dominates the male germline and sperm. When a new species with DUI is identified, novel data are available to make a better inference on the evolution of this phenomenon within the Bivalvia. In this study, mitochondrial heteroplasmy in Pseudocardium sachalinense (Schrenck, 1862) is described. This species belongs to the family of Mactridae, in which DUI has not been previously demonstrated: this finding allowed to upgrade the present knowledge about the distribution of DUI. [ABSTRACT FROM AUTHOR]

Post‐settlement dispersal is a key process in the recruitment of bivalves. To assess the role of such secondary migrations and to identify potential associated triggers, we conducted an in situ sampling survey during the summer recruitment period on the bivalve assemblages of a coarse‐sediment temperate tidal habitat in the Chausey archipelago (Normandy, France). The dynamics of drifters were studied using three types of settler traps (for example, bedload, pelagic, and sinking transports), and we monitored both the abiotic (hydrological and hydrodynamic conditions) and the trophic (nano‐ and pico‐sized particulate organic matter [POM]; fatty acid composition of POM and sediment organic matter [SOM]) environmental parameters. Such an approach allows the discrimination of passive migration (due to sediment erosion by tidal currents and waves) from active migration (related to recruit behavior). Secondary migrations were observed in 25 bivalve taxa, and these mainly involved decreasing abundances of Mytilidae, Nuculidae, Semelidae, Mactridae, and Lucinidae individuals on the study site, highlighting the crucial role of these processes in highly dynamic coastal benthic assemblages. Surprisingly, the intense post‐settlement dispersal observed at the end of the recruitment season was not synchronized with periods of high hydrodynamic stress but to a change in the structure of phytoplanktonic assemblages, particularly the nanoeukaryotic component. Such a response by bivalve recruits to a trophic pelagic cue—triggering secondary migrations—could result from an increased demand for energy required for active migratory behavior. [ABSTRACT FROM AUTHOR]