Partial sympatry of two interspersed Uroteuthis edulis (Loliginidae) clades reveals cryptic stock structure in the southern Taiwan Strait

Abstract

The swordtip squid, Uroteuthis edulis, is an important species for Taiwan's squid fishery industry. Its catches primarily originate from northern regions such as the East China Sea, and considerable contributions come from the Taiwan Strait. Complexities in the species' biology, including the presence of temporally distinct stocks and pronounced phenotypic plasticity, have hindered a comprehensive understanding and effective management, particularly for the understudied southern Taiwan stock. Hence, we investigated the intraspecific relationships of U. edulis, focusing on the genetic connectivity, phylogeny, evolutionary history, seasonal occurrence, and growth profiles in the southern Taiwan Strait. Mitochondrial COI analysis revealed two different clades - Group A (northern) and Group B (southern) (FCT = 81.63%) - each exhibiting distinct demographic expansion histories. These clades further showed partial sympatry and seasonal alternation, with the southern Taiwan Strait serving as their area of overlap. Mitochondrial DNA markers also proved robust in delineating the U. edulis clades and their congeners. The southern group appeared to be a warmer-water cohort, abundant during the warmer months, with a single spawning pulse around April and May. Both males and females showed similar growth trends from paralarvae to adults, diverging only in their adult growth patterns. Our findings demonstrate that temporally staggered occurrence can obscure cryptic lineages, underscoring the importance of genetic techniques for accurate stock delimitation and clearer understanding of evolutionary dynamics shaping U. edulis populations. Due to the substantial variations in distribution range and seasonality across these clades, well-tailored, transboundary resource management initiatives are crucial for their sustainability.