Semilunar postlarval settlement periodicity ensuing from semilunar larval release cycle in the Nihonotrypaea harmandi (Crustacea, Decapoda, Axiidea, Callianassidae) population on an intertidal sandflat in an open marine coastal embayment

Abstract

Many decapod crustaceans in marine intertidal habitats release larvae toward coastal oceans, from which postlarvae (decapodids: settling‐stage larvae) return home. Decapodid settlement processes are poorly understood. Previous studies showed that in Kyushu, Japan, the callianassid shrimp population on an intertidal sandflat of an open bay joining the coastal ocean near a large estuary released eight batches of larvae basically in a semilunar cycle from June through October and that decapodids performed diel vertical migration, occurring in the water column nocturnally. We conducted (a) frequent sampling for population density and size‐composition on the sandflat through one reproductive season, (b) planktonic and benthic sampling for decapodids around the bay mouth, and (c) current meter deployment at three points across the bay mouth for tidal harmonic analysis. On the sandflat, six batches of newly‐settled decapodids (settlers) occurred in a semilunar periodicity until October, with peaks occurring 0–3 days before syzygy dates except for the first one. For larval Batches 1–4, buoyancy‐driven shoreward subsurface currents during July to mid‐October would transport some pre‐decapodid‐stage larvae (zoeae) toward the bay. The absence of expected settler Batches 7–8 would be due to the converse subsurface currents caused by water‐column mixing and seasonal winds after mid‐October, carrying zoeae offshore. Once in the bay, phasing of night and nighttime‐averaged shoreward tidal current explained the settlement pattern for Batches 1–4. For Batches 5–6 occurring in mid‐September to mid‐October, water currents generated by seasonal wind and tidal forcings may have caused peak settlement after the time expected from tidally‐driven decapodid transport.

Applicability of the source–sink population concept to marine intertidal macro‐invertebrates with planktonic larval stages

“…The maximum LDD of N. harmandi decapodids was constant in the laboratory, and their planktonic duration seems to be unaffected by a large temperature change over their vertical distribution range from seabed to surface (Tamaki et al, 2013). The semilunar periodicity of decapodid settlement on Tomioka sandflat was well explained by the few‐day cumulative densities of settlers that were transported by nighttime flood‐tidal currents in Tomioka Bay during the time window around each mid‐tide period (Tamaki, Itoh, et al, 2020).…”

“…Thus, instead of the procedure to lump both steps together, the second step of competent larval return home would better be modeled separately. Based on the current‐meter (acoustic Doppler current profiler) data from near Tomioka Bay mouth, only half the N. harmandi decapodids there were estimated to reach the sandflat within their competent period when tidal forcing is predominant over cross‐shore wind forcing (Tamaki, Itoh, et al, 2020).…”

“…By contrast, decapodids of N. harmandi in Amakusa‐nada performed normal DVM only, ascending by several tens of meters almost exclusively in the nighttime (Tamaki et al, 2010; Figure 1a, inset). Around the Tomioka Bay mouth, decapodids stay on the seabed in the daytime (Tamaki, Itoh, et al, 2020), and normal DVM appears to be their consistent behavior regardless of water depth around them. It is suspected that the above brachyuran megalopae exhibiting FTT may not be larvae but settlers or juveniles that had settled around the estuary mouth and were further undertaking their upstream travel via a few‐meter vertical movement.…”

“…Both benthic shrimp of the Tomioka sandflat population and planktonic larvae in their nursery ground would form an almost closing loop to maintain the membership in the sense of the member–vagrant hypothesis. This point is elaborated on below, based on Tamaki et al (2010, 2013) and Tamaki, Itoh, et al (2018, 2020) including results of their own studies and review papers for the related literature therein.…”

“…By contrast, the effect of water circulation caused by cross‐shore winds in waters shallower than 30 m (see Fewings et al, 2008; Tilburg, 2003) on larval transport has received only a little concern (Hawkins & Hartnoll, 1982; Morgan et al, 2018; Pineda & Reyns, 2018). In the above‐mentioned Tomioka Bay case, when cross‐shore winds toward the sandflat occurred seasonally in autumn, causing the downwind sea‐surface residual current and subsurface counter‐residual current (Figure 1b, inset), the net transport of N. harmandi decapodids from the bay mouth to the sandflat was negatively affected (Tamaki, Itoh, et al, 2020). This is because decapodids in the lower water column would not have reached the sandflat, resulting in the disruption of the tidally driven periodicity of their settlement on the sandflat.…”