Stage-V copepodites of Calanus sinicus and Calanus jashnovi (Copepoda: Calanoida) in mesopelagic zone of Sagami Bay as identified with genetic markers, with special reference to their vertical distribution

We applied genetic makers to identify Calanus species occurring in Sagami Bay, Japan, in order to investigate their vertical distribution in the upper 1000 m. First, interspecific genetic distances of three gene loci, mitochondrial small ribosomal RNA (srRNA), nuclear internal transcribed spacers 1 (ITS1) and 2 (ITS2), were estimated from morphologically distinguishable adult females of Calanus sinicus, Calanus jashnovi and Calanus pacificus that were collected from Sagami Bay, the Kuroshio Extension and the Oyashio region, respectively. The highest levels of interspecific genetic distance were observed in srRNA, followed by ITS1 and ITS2. The intraspecific genetic distances within C. sinicus were much lower than the interspecific genetic distances, indicating that DNA sequences in these loci are consistent with the morphological differences. This information was used as a criterion for species identification based on DNA sequence variation, and allowed us to identify the fifth copepodites (CVs) or younger stages of these species. Next, the vertical distribution of Calanus species was investigated in Sagami Bay in May 2006, on the basis of a stratified sampling in the upper 1000 m. By applying the genetic markers, 23 individuals comprising all copepodite stages were allocated into either C. sinicus or C. jashnovi, and the small- and large-sized CVs were identified as C. sinicus and C. jashnovi, respectively. The total abundance of C. sinicus was highest at 0-50 m and decreased with depth. On the contrary, CV individuals of C. sinicus were abundant not only in 0-50 m but also below 200 m with minimum occurrences in 150-200 m depth. C. jashnovi was much less abundant than C. sinicus and comprised of only CIV and CV which occurred in the upper 100 m and deeper than 50 m depths, respectively. The abundance of C. sinicus in the 1000-m water column of Sagami Bay was at a level comparable to that in shelf waters, suggesting the importance of off-shelf individuals in the biological production and organic transport in the respective areas.     

Vertical distribution of diapausing Calanus pacificus (Copepoda) and implications for transport in the California undercurrent

Migration to deep water during diapause may contribute to the retention of several dominant oceanic calanoid copepod populations in eastern boundary current systems, where the mean flow of poleward undercurrents is in opposition to mean equatorward surface flow. The vertical distributions of Calanus pacificus late copepodid stages were measured at a 1200-m deep, open-ocean station in the Southern California Bight on 13 dates between April 2000 and March 2001 using a MOCNESS (multiple opening and closing net and environmental sensing system). Copepod vertical distribution was compared to the vertical position of the California Undercurrent. Diapausing C. pacificus were primarily found between 300 and 400 m at the beginning of the diapause season, in June and July, and between 250 and 350 at the end of the diapause season, in November and January. Depth distributions were broader from August to October, ranging from about 350 m to the maximum depth sampled, 1100 m, and the median depth of diapausing C. pacificus was deeper, up to 800-900 m, during this period. Maximal depths of diapausing C. pacificus, 1100-1000 m, were greater than have previously been reported. The mean depth of the California Undercurrent was 250 m, and its approximate depth range was 110-430 m. Diapausing C. pacificus CV were abundant in the California Undercurrent at the beginning and end of the diapause season, in June to July and late-November to January, suggesting that poleward transport of diapausing copepods in the California Undercurrent contributes to C. pacificus population retention in the California Current System.