1 School of Marine Sciences, Ningbo University, Ningbo
315211, P. R. China2 Key Laboratory of Marine Ecosystem Dynamics, Second
Institute of Oceanography, Ministry of Natural Resources, Hangzhou
310012, P. R. China3 Observation and Research Station of Yangtze River
Delta Marine Ecosystems, Ministry of Natural Resources, Zhoushan 316022,
P. R. China* Correspondence: lichenghua@nbu.edu.cnAbstract: Sea-level fluctuations in the marginal seas of the
northwestern Pacific Ocean during the Pleistonene have profoundly
influenced the genetic structure of marine organisms. Previous
phylogeographic studies have mainly focused on fish and molluscs; other
taxonomic groups have been poorly studied, which restricts a
comprehensive understanding of the geogenetic patterns of marine species
in this area. To precisely understand how environmental factors and
historical events shape the population structure of the sea penCavernularia obesa in the East China Sea, we determined partial
nucleotide sequences of the mitochondrial cytochrome oxidase subunit I
(COI) gene in 71 individuals from five sites. Results of population
Genetic and demographic analyses revealed a low genetic diversity within
each population. Moreover, the historical population size change showed
that the populations experienced bottlenecks during the Pleistonene. The
sea level of the East China Sea decreased by >100 m when
the glacial advanced during the Pleistonene, which caused the shrinking
of habitats for marine species and eventually resulted in low genetic
diversity within the populations in this area.Keywords: phylogeography; marginal sea; Cavernularia
obesa; sea level fluctuation; Changjiang diluted water
1. Introduction
Phylogeography is the study of the geographic distribution of genetic
lineages within a species and provides a means for inferring historical
and contemporary processes that influence population distribution and
abundance. The phylogeographic distribution of marine species is driven
by historical events and modern environmental factors(Dong et al., 2012;
Lima-Junior et al., 2021). Among these factors, oscillations of climate
and oceanography have caused drastic changes in subtropical and
temperate coastal environments(Avise, 2000).
The marginal seas of the northwestern Pacific Ocean experienced
sea-level fluctuations during late Quaternary glacial cycles(Wang,
1999). When the glacier advanced during the Pleistonene, the East China
Sea (ECS) declined by approximately 130–150 m than present (Waelbroeck
et al., 2002). The ECS shelf was completely exposed as the coastline
migrated approximately 1200 km seaward (Wang, 1999). Consequently, the
ECS was reduced to an elongated enclosed sea ( Okinawa Trough) with an
area <1/3 of its present size (Ni et al., 2014). During
interglacial periods, the ECS was inundated by rising sea levels. These
repeated transgression–regression cycles, together with ocean currents
and recent anthropogenic activities, have greatly impacted the
phylogeographic patterns and population genetic differentiation in this
area. Therefore, the ECS has been proposed as an ideal model for
studying how sea level oscillations are caused by repeated glaciations,
structured distribution ranges, spatial population genetic diversity,
and marine phylogeography (Ni et al., 2014).
The Changjiang River is the largest river in Asia and the third-largest
river in the world. It enters the ECS with about
9×1011 cm3 of freshwater discharge
annually. This huge freshwater flow, named the Changjing diluted water
(CDW), causes significant shifts in various ecological and environmental
parameters in the ECS and acts as a barrier to the genetic connectivity
of marine species. In other marine realms, some well-known biogeographic
boundaries, such as the Basic Isthmus(York et al., 2008), Central
American Isthmus(Coppard and Lessios, 2017), and Cape Hatteras(Mccartney
et al., 2013), have proven to be intraspecific barriers restricting the
gene flow of some wide-ranging taxa. Recently, several studies have
tested the phylogeographic barrier effect of CDW in addition to
biogeographic boundary[1,10-12]. However, the results of these
studies are inconsistent, and controversy has arisen as different
genetic patterns have emerged.
Previous phylogeographic studies on the ECS have mainly focused on
commercially exploited fish and molluscs[12,13]. Although the
genetic structure of commercial species may be affected by invasion
genomics(Jaspers et al., 2021; Rius and Turon, 2020), species from other
diverse taxonomic groups are good candidates for such comparative
studies(Ni et al., 2017). The sea pen C. obesa is a widely
distributed colonial cnidarian found in the northeastern Pacific Ocean
(López-González et al., 2000). Sea pens undergo a two-phase life cycle
of planktonic larvae and sessile adult forms. Mature adults of C.
obesa half-buried in the sediments of the seabed and barely move, and
their dispersion mainly relies on planktonic larvae. The commercial
value of C. obesa is low; therefore, the genetic structure of the
ECS community was unaffected by commercial aquaculture activities. These
characteristics make C. obesa an ideal model species for
phylogeographic studies of the ECS.
In this study, we collected 71 individuals of C. obesa from five
sites in the ECS and used phylogenetic analyses based on mitochondrial
cytochrome oxidase subunit I
(COI) to analyze the range-wide phylogeographic structure and determine
the potential drivers of spatial genetic variation in C. obesa.
2. Materials and Methods
2.1. Sample collection
A total of 71 individuals of C. obesa were collected from five
sites using Agassiz trawl in the ESC from 2016 to 2019. The samples were
frozen at -20 °C. As C. obesa is not an endangered or protected
species, and collection was only carried out from public access areas,
no specific permits were required to collect this species from these
locations. The locations of the sampling sites are shown in Figure 1 and
Table 1.Figure 1. Map of Northwest Pacific showing sampling sites and
haplotypes of C. obesa; YSCC, Yellow Sea Coastal Current; CDW,
Changjiang Diluted Water; and ECSCC, East China Sea Coastal CurrentTable 1. Sampling sites and diversity indices for the five
populations of C. obesa