Morphological characteristics, molecular species validation and spatio-temporal distribution of myctophid eggs off the Pacific coast of northeastern Japan

Abstract

Myctophids in the world's oceans have a huge biomass and act as an important link between secondary producers and upper trophic levels through their extensive diel vertical migrations from the epipelagic to mesopelagic layers. Dominance of myctophid larvae in oceanic ichthyoplankton has been reported from various parts of the major oceans; however, there are few records concerning their eggs. We established species-specific morphological characteristics for identifying myctophid eggs which were then validated by sequence analysis of mitochondrial 12S rRNA and cytochrome b genes using representative subsamples of the morphotypes, and then used the criteria to determine the species composition and spatio-temporal distribution of myctophid eggs. Samples were collected monthly in the epipelagic layer off the Pacific coast of northeastern Japan, resulting in 664 net tows that yielded a total of 10,172 myctophid eggs, defined by a smooth and delicate chorion, segmented yolk, and single oil globule. Subsamples of 160 small, 34 intermediate, and 20 large-sized eggs were sequenced, resulting in their classification as Diaphus theta, Diaphus garmani, and Myctophum asperum, respectively. Mean egg diameter was smallest in D. theta (0.75 mm), intermediate in D. garmani (1.06 mm), and largest in M. asperum (1.31 mm). Eggs of D. theta had a markedly narrower perivitelline space (0.17 mm) than D. garmani and M. asperum (0.49 and 0.71 mm, respectively). Based on morphology, the myctophid eggs were dominated by D. theta (86.0%), followed by D. garmani (12.6%) and M. asperum (1.4%). Abundances of early-stage eggs of D. theta and D. garmani were high during the daytime to dusk but low at night and dawn, indicating that spawning would not be active in the epipelagic layer at night. Spawning can be considered to occur in the mesopelagic layer during the early night, and then the eggs rise to reach the epipelagic layer in the daytime. High abundances of D. theta and D. garmani eggs were observed in the slope region from spring to summer, corresponding with the larval distribution reported for the study area.