Spatiotemporal variations of Chlamys islandica larval shell morphometry between 2000 and 2018 in a depleted coastal scallop fishing area

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Bivalves larval shell morphometry can provide valuable information on their planktonic (PI-Prodissoconch I) and early benthic life (PII). We examined spatial (at two scales: fishing sub-areas and exploited beds) and temporal variability (between 2000 and 2018) of PI and PII sizes among 424 Icelandic scallop juveniles (<10 mm total length) from the northern Gulf of St. Lawrence. PI and PII mean sizes were not different from juveniles collected inside or outside of exploited beds. For PI sizes, there were no temporal changes, but there was a difference between the two scallop fishing sub-areas, suggesting a disparity in egg size, origin or in broodstock physiological state. Regarding PII sizes, there was no difference between sub-areas, but PII sizes in 2003 and 2004 were significantly higher, likely related to differential larval growth or delayed metamorphosis associated to synergistic effects of environmental factors. Finally, this study demonstrates the possibility to accurately follow larval morphometry variations from small juvenile bivalve shells to track potential changes of recruitment success in order to sustainably manage stocks of commercial bivalve species.