Plankton Studies Research Articles

Assessing the State of the Pelagic Habitat: A Case Study of Plankton and Its Environment in the Western Irish Sea

Much work had been undertaken on tracking change in the condition of marine pelagic ecosystems and on identifying regime shifts. However, it is also necessary to relate change to states of good ecosystem health or what the European Marine Strategy Framework Directive (MSFD) calls 'Good Environmental Status' (GES). Drawing on existing scientific and legislative principles, including those of OSPAR's 'Strategy to Combat Eutrophication', we propose a framework for assessing the status of what the MSFD calls the 'pelagic habitat' in temperate coastal seas. The framework uses knowledge of local ecohydrodynamic conditions, especially those relating to the stratification and optical environment, to guide expectations of what would be recognised as healthy in terms of ecosystem 'organisation' and 'vigour'. We apply this framework to the seasonally stratified regime of the Western Irish Sea, drawing on published and new work on stratification, nutrient and phytoplankton seasonal cycles, zooplankton, and the implications of plankton community structure and production for higher trophic levels. We conclude that, despite human pressures including nutrient enrichment, and the food-web effects of fisheries, the pelagic ecosystem here is in GES, and hence may be used as a reference for the 'Plankton Index' method of tracking change in state space in seasonally stratified waters.

SyPRID sampler: A large-volume, high-resolution, autonomous, deep-ocean precision plankton sampling system

The current standard for large-volume (thousands of cubic meters) zooplankton sampling in the deep sea is the MOCNESS, a system of multiple opening–closing nets, typically lowered to within 50m of the seabed and towed obliquely to the surface to obtain low-spatial-resolution samples that integrate across 10s of meters of water depth. The SyPRID (Sentry Precision Robotic Impeller Driven) sampler is an innovative, deep-rated (6000m) plankton sampler that partners with the Sentry Autonomous Underwater Vehicle (AUV) to obtain paired, large-volume plankton samples at specified depths and survey lines to within 1.5m of the seabed and with simultaneous collection of sensor data. SyPRID uses a perforated Ultra-High-Molecular-Weight (UHMW) plastic tube to support a fine mesh net within an outer carbon composite tube (tube-within-a-tube design), with an axial flow pump located aft of the capture filter. The pump facilitates flow through the system and reduces or possibly eliminates the bow wave at the mouth opening. The cod end, a hollow truncated cone, is also made of UHMW plastic and includes a collection volume designed to provide an area where zooplankton can collect, out of the high flow region. SyPRID attaches as a saddle-pack to the Sentry vehicle. Sentry itself is configured with a flight control system that enables autonomous survey paths to low altitudes. In its verification deployment at the Blake Ridge Seep (2160m) on the US Atlantic Margin, SyPRID was operated for 6h at an altitude of 5m. It recovered plankton samples, including delicate living larvae, from the near-bottom stratum that is seldom sampled by a typical MOCNESS tow. The prototype SyPRID and its next generations will enable studies of plankton or other particulate distributions associated with localized physico-chemical strata in the water column or above patchy habitats on the seafloor.

Water quality assessments: A case study of plankton and macrobenthic invertebrates of Porto-Novo and parts of Gulf of Guinea

Aspects of physico-chemical characteristics, plankton and macro invertebrates of Porto-Novo and Gulf of Guinea were studied in September 2012 with a view to determine the quality of water using plankton indices. Standard methods were used to determine the physico-chemical parameters. The mean water quality parameters studied were temperature (36.71oC), pH (7.63), transparency (1.97m), total dissolved solids (10.0mg/L), dissolved oxygen (4.73mg/L) and salinity (38.58‰) for Gulf of Guinea. Five divisions; Cyanophyta, Bacillariophyta, Chlorophyta, Euglenophyta and Pyrrhophyta were recorded for both bodies of water with diatoms and dinoflagellates dominating the spectrum. The phytoplankton ofPorto-Novo recorded abundance of Gomphonema spp., Fragilaria crotonensis, Synedrasp. and Nitzschia sp.(diatoms) while that of Gulf of Guinea recorded Ceratium spp. and Gonyaulax spinifera (dinoflagellates). For the zooplankton, Acartia clausii (a copepod) and Penilla avirostris Dana (Cladocera) were the highest in both occurrence and abundance especially at Porto-Novo. The population of benthic fauna was low (20 individuals) dominated by bivalve, Tivela tripla.Keywords: Microalgae, zooplankton, benthos, Porto-Novo, Gulf of Guinea

A simplified, economical, and robust light trap for capturing benthic and pelagic zooplankton

Most of the commonly used light traps for plankton studies are often large, heavy and expensive, and cannot be easily used for pelagic sampling. Although many small, and simple light traps have been designed, most of them are fragile and have weak light sources, allowing only limited collection. In the present study, a new simplified light trap design was described, which is small, lightweight, robust, inexpensive ($70 USD) and easy to construct. The main body of the trap is constructed from a 6L commercial mineral water PET bottle, and the lighting component consists of four 3W white LED bulbs. Pelagic deployment of the new simplified trap and the commonly-used modified Doherty trap was conducted in the same location and compared the composition of the catches including larval fishes and invertebrates. The taxonomic composition of larval fishes collected between the two traps was similar, but it appears that the Doherty trap can collect larger sized larval fishes. Catch composition, wet weights and total number of marine invertebrates collected between the two types of traps are comparable. Ten taxa (copepods, amphipods, crab larvae and juveniles, Cumacea, Euphausiacea, molluscan larvae, isopods, mysids, ostracods and Polychaetes) accounted for over 98% of the total catch of individuals by both trap models. Copepods and crabs larvae were the two most abundant taxa, accounting for more than 55% of the catch. The new and simplified traps therefore represent a robust and comparable design that can be manufactured in large quantities (even in developing countries) and easier in transportation for spatial and temporal studies of plankton dynamics.

Christian Andreas Viktor Hensen and his studies of plankton

This paper gives a brief account of the attempt by the German zoologist Christian Andreas Viktor Hensen to quantify the amount of floating organisms in the sea, which he defined for the first time as plankton. To do this, he invented new collecting and analysing methods, and in 1887 led the first major German oceanographic expedition. Despite arousing criticisms, his methods influenced others to begin quantitative sampling and led to his recognition as one of the founders of ecology.

A long-term study on crustacean plankton of a shallow tropical lake: the role of invertebrate predation

<p>The primary factor that governs the size and species composition of zooplankton is still a controversial issue and temperature is considered the main factor responsible for latitudinal differences. In waters with a narrow temperature range, such as in the tropics, predation may be a more important factor. Nearly three decades of intermittent studies of the crustacean plankton in a shallow tropical lake revealed that the main event that led to their restructuring was the appearance of a second predator, the water mite <em>Krendowskia </em>sp. The new predator and larvae of the dipteran <em>Chaoborus brasiliensis</em> Theobald exerted a combined, although asymmetrical effect on microcrustaceans. The period when the mite was detected was followed by the restructuring of the crustacean plankton community. Predation by these two invertebrates emerged as the factor responsible for community changes, involving an increased contribution of copepods and decreases in the relative abundance of smaller cladoceran species. In the short term, the mite caused a decrease in species richness and the annual mean instantaneous composition of cladocerans, a predominance of large-sized species (<em>Daphnia ambigua </em>Scourfield<em> </em>and <em>Daphnia gessneri</em> Herbst) and the virtual disappearance of small species (e.g., <em>Bosmina tubicen</em> Brehm). The long-term impact resulted in increased species richness and the dominance of large and medium-sized cladocerans, such as <em>D. gessneri</em> and <em>Ceriodaphnia richardi</em> Sars. The larger body size of three cladocerans, the two <em>Daphnia</em> species and <em>B. tubicen</em>, in the long term, may be a response to the dominant predator, <em>Chaoborus</em>. The seasonal variation in the predator abundance, mainly <em>Chaoborus </em>larvae, allowed the prey to recover during the cool season. The copepods <em>Tropocyclops prasinus meridionalis</em> (Fischer) and <em>Thermocyclops decipiens</em> Kiefer were less affected by predation than the cladocerans; their contribution to the crustacean plankton increased 12-28% after the mite appeared. The top-down effect on crustacean plankton did not affect the phytoplankton, which remained dominated by chlorophyceans and the nanoplankton fraction before and after the zooplankton community restructuring.</p>

Rotifer Invasion? On Appearance and Abundance of Tropical Species in Lakes of North-Eastern Poland

ABSTRACT: Trichocerca simoneae De Smet described in 1989 has since been found in tropical and subtropical areas, but was not recorded in Poland before World War II. A bibliography of Polish rotifers by Wiszniewski (1953), although very comprehensive, does not refer to T. simoneae. However, in the studies of plankton of the dystrophic lake, i.e. humic one with water rich in humic acids, acidic pH and brown coloured, carried out in the years 1998–2000, the species dominated. Here, studies in 39 small, inter-forest lakes of north-eastern Poland revealed T. simoneae in 15 of them, often at high densities, up to ca 6000 ind L-1. The possible reasons for this ‘invasion’ are: (1) zooplankton communities in dystrophic lakes are unsaturated with biotic interactions too weak to exclude invaders; CCA and RDA analyses showed that T. simoneae preferred habitats with low number of zooplankton species of low density and dystrophic lakes seem to offer such habitats; (2) long-term deposition of rotifer resting eggs is pro...

Study of Plankton in Qiantang River in Summer and Evaluation of Water Quality

To investigate the plankton and water quality of Qiantang river, plankton of 12 sampling sites of the study area were monitored in summer 2013. Results revealed that 65 genera of phytoplankton including 37 bio-indicators of pollution and 21 genera of zooplankton were observed. Diversity analysis showed that Shannon-Wiener indexes of phytoplankton and zooplankton were at 1.44 to 2.56 and 1.61 to 2.61, plankton biodiversity integrated index were at 0.419 to 0.558, and water quality integrated index were at 0.303 to 1.43. Data analysis suggested that the water quality of Qiantang river could be sorted from α type to β type of moderate pollution, the pollution degree could be sorted from moderate pollution to slight pollution. The evaluation results by using plankton and physicochemical indexes were consistent.

Measuring and calculating geometrical parameters of marine plankton using digital laser holographic imaging

The purpose of this paper is to develop an approach using digital laser holographic imaging for measuring and calculating geometrical parameters of marine plankton. It is very important for study of marine plankton and it facilitates the analysis of marine plankton. Using this approach, we obtained geometrical parameters of marine plankton including perimeter, equivalent diameter, lateral size and so on. It is found that a measurement aberration is less than 8% for copepods sample by using collimating reference wave.

Plankton research in the ROPME Sea Area, Achievements and Gaps

Plankton studies are one of the major items of the marine scientific activities in the ROPME SeaArea, whereas numerous investigations have been conducted on the plankton during the past century. Forphytoplankton, species composition, abundance and biomass, primary production, harmful algal blooms, redtide, toxic species, and eutrophication were considered. Zooplankton studies included species compositionand abundance, certain zooplankton groups and few studies on certain species. The main achievement of these studies was the identification of large number of phytoplankton and zooplankton species in different parts of the RSA and estimation of their abundance.The survey of these studies revealed the occurrence of several gaps in the plankton research in ROPME Sea Area. For example, the majority of these studies were carried out in the inner part, while some of them were done in the middle part. Several studies were based on samples collected once only during certain time of the year or on samples collected from localized areas. The role of the function groups of the both phytoplankton and zooplankton, the harmful algal blooms, toxic phytoplankton species, the role of small phytoplankton groups rather diatoms and dinoflagellates and vertical migration of zooplankton were little concerned. In addition, the continuous record of the Indo-Pacific species entering the inner RSA from the Sea of Oman appeared to be completely missed.The present study attempts to stress the achievements as well as the gaps in the earlier plankton studies in the ROPME Sea Area in order to propose a scientific plan for comprehensive study of plankton community relative to the prevailing environmental conditions in the concerned area.

Extracting DNA from whole organism homogenates and the risk of false positives in PCR based diet studies: A case study using spiny lobster larvae

Better understanding the diet of small marine predators such as the planktonic larvae of spiny lobsters is important for our awareness of interactions within marine assemblages and for species commercialisation. In DNA-based diet studies of small organisms there is a risk that any DNA contaminating the outside of an organism will be detected and falsely assumed to originate from the gut. Experiments with terrestrial predators have overcome the problem of exogenous contamination by treating the exterior of the predator with bleach (sodium hypochlorite). However, the use of bleach is a risky strategy when treating either a rare predator or aquatic predators, which are generally more permeable than terrestrial animals. Many plankton studies have not reported how they dealt with exogenous contamination, or do not use a control during PCR to detect false positives due to exogenous contamination. One approach is to wash the predator with MilliQ filtered water or ethanol and to use the final wash as a PCR template to detect residual DNA. In the present study we report that washing has variable success at removing exogenous contaminants and that using the final wash as a control for exogenous contamination consistently fails. Based on our results we recommend using DNA extracted from a swab of the exterior of the predator as a control for exogenous contamination. We also report on the benefit of using a novel syringe technique to obtain gut content that minimises contact with the predator surface, and therefore the risk of exogenous contamination.

Development and deployment of a point‐source digital inline holographic microscope for the study of plankton and particles to a depth of 6000 m

A point‐source digital inline holographic microscope (DIHM) was designed for the imaging of particles from 50 µm to several millimeters in size. The DIHM operates autonomously without connection to external recording devices or power sources and delivers 4.2 megapixel images at a rate of approximately 7 images s−1, each image representing a snapshot of 1.8 mL seawater. Reliance on largely off‐the‐shelf components, and simplification in its construction makes this camera system adaptable to various particle size ranges and environments, and easy‐to‐operate for nonexpert users. The DIHM produced sharp images of protists with skeletal structures (e.g., acantharians, tintinnids, dinoflagellates), mesoplankton (e.g., copepods, appendicularians, medusae), Trichodesmium colonies and marine snow particles while descending in the water column at 1 m s−1, a typical velocity for deployments of tethered instruments and samplers from oceanographic vessels. To validate the usefulness of the new instrument in an oceanographic context, data are presented of the surface distribution of Trichodesmium spp., and of the vertical frequency distribution of fecal pellets and other particles in the deep sea. The point‐source DIHM has the potential to become a standard instrument on the CTD rosette (i.e., on the basic oceanographic instrument and sampling frame) in the future providing a permanent archival record of the water column that can be mined for specific target particles in the future.

First record of Palaemon macrodactylus Rathbun, 1902 (Decapoda, Palaemonidae) in the western Mediterranean

The Mediterranean Sea is one of the world’s hotspots for marine bio-invasions. Most invasions are first documented based on an initial record of occasional adult captures. However, reports of larval stages could indicate that there is an adult population that is reproducing and therefore well established in the area. The spread of the oriental shrimp, Palaemon macrodactylus, from its native estuarine waters of southeast Asia to new regions worldwide is well documented. We report the first record of this species in the Mediterranean based on the presence of its larval stages in plankton samples. Decapod larvae were collected in five offshore plankton surveys performed off the Balearic Islands (western Mediterranean), and zoeae III and VI of the oriental shrimp werefound among them. Taking into account the duration of the successive developmental stages, and the hydrodynamic characteristics of the study area, these larvae were most probably spawned by adult populations not yet documented. The larvae were found in marine waters despite the fact that adults usually inhabit brackish waters. Our study is a good example of how plankton studies can help to detect larval stages of invasive species before the adult populations are detected.

Preface "Sources and rapid biogeochemical transformation of dissolved organic matter in the Atlantic surface ocean"

One of the most important issues needed to be studied to better understand global fluxes of marine organic matter is to resolve its molecular characteristics and mechanisms which convert fresh, labile biomolecules into semi-labile and refractory dissolved organic compounds (Jiao et al., 2010) which resist degradation for an average time of 5000 years (Bauer et al., 1992; Williams and Druffel, 1987). From a chemical point of view, the persistence of marine dissolved organic matter (DOM) is very unusual. The mostly oxygenand nutrient-rich oceanic water column should be conducive to a rapid microbial degradation of organic matter and subsequent release of CO2. Contrary to this expectation, however, a significant portion of the atmospheric carbon remains in the DOM of the oceans and circulates in global currents on long time scales. Despite the importance of gaining better insights into the molecular mechanisms affecting organic carbon dynamics, very little is known about these processes. Thus, recent reports from the intergovernmental panel on climate change (Denman et al., 2007) barely considered potential changes in quality, quantity and fluxes of marine organic matter. In the oceans, dissolved organic substances constitute an important source of organic carbon and organic nitrogen (Hansell et al., 2009). The pool of dissolved organic carbon (DOC) exceeds the amount of carbon stored in marine animals, plants and bacteria by a factor of about 200. The majority of DOM in the oceans is originally formed from CO2 by plankton and land plants during photosynthesis. The DOM, resulting from this organic material, is directly released into the water by marine plankton and during the degradation of these organisms. Also, DOM can be transported into the oceans via rivers and atmospheric deposits via aerosols. A small fraction of marine DOM, such as persistent organic pollutants or black carbon derivatives, can be derived from anthropogenic activities. DOM can bind to trace metals such as iron and is therefore also responsible for their distribution in the sea. The complex composition of DOM is an enormous analytical challenge, resulting in sparse information on chemical structures. The DOM complexity prevents resolution of single molecules by chromatographic separation or conventional mass spectrometry. However, within the last 10 years analytical instruments, which can deliver molecular signals, improved tremendously. Namely, improvements of nuclear magnetic resonance spectroscopy (NMR) and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) opened a new analytical window for the molecular characterization of natural organic matter (e.g. Stenson et al., 2003; Kujawinski et al., 2004; Koch et al., 2005; Hertkorn et al., 2008; Witt et al., 2009). Meridional transits of polar research vessels offer a great opportunity to perform long-distance surface water transects with continuous water and air sampling, since frequent stops for long station work are usually not possible during such transit cruises. Trace metal (Pohl et al., 2011; Helmers, 1996) and plankton studies (Robinson et al., 2009) and a recent GEOTRACES pilot study in the Atlantic Ocean (Rutgers van der Loeff, 2007) are great examples for the importance of such projects. Previous work along the north–south transect also provided a high resolution comparison of bacterial and primary production (Hoppe et al., 2002). The authors suggested that the observed net heterotrophy required

Synchaeta jollyae (Shiel & Koste) (Rotifera, Synchaetidae): an overlooked species or a newcomer in Neotropical freshwaters?

Populations of Synchaeta jollyae (Shiel & Koste, 1993) (Rotifera), a species recently recorded for the first time in Brazil and South America, were analyzed in reservoirs in Southeast Brazil. Sampling was carried out monthly from August 2006 to July 2007 at Furnas Reservoir in the Rio Grande basin, state of Minas Gerais, and in four cascade reservoirs in the Tietê River basin (Barra Bonita, Bariri, Ibitinga and Nova Avanhandava) state of São Paulo, in June and September 2008 and in January and May 2009. Synchaeta jollyae occurred in most samples and periods. From the results obtained it is evident that S. jollyae occurs in water bodies of varied trophic status but reaches larger populations in eutrophic water bodies and during lower temperature periods. The greatest densities of S. jollyae were found in the eutrophic Bariri Reservoir, on the Tietê River, during the winter. Mann-Whitney test confirmed the significant difference between the population densities in periods of high and low temperatures, with populations reaching higher densities at lower temperatures. It is not yet possible to tell whether S. jollyae is a widely distributed species that has been overlooked in previous plankton studies in South America. Wherever these populations of S. jollyae might have originated, it appears to be a species well established and adapted to a wide range of conditions in the Neotropics.

Potential of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) for the identification of freshwater zooplankton: a pilot study with three Eudiaptomus (Copepoda: Diaptomidae) species

The accurate identification of individuals in zooplankton samples is a crucial step in many plankton studies. Up to now, this has been done primarily by microscopic analysis of morphological characters, and new molecular methodologies are still relatively rarely applied. Another promising technology is matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS), which has had a major impact in applied and systematic microbiology, where it is used for routine high throughput identification of bacteria and fungi. For the present study, we developed a protocol for the rapid acquisition of mass spectra from whole individual copepods. The final protocol enabled us to obtain mass spectra with more than 100 distinct peaks in the mass range of 2000–20 000 Da. A comparison of the mass spectra of three species of Eudiaptomus showed that they could all be clearly discriminated, whereas the mass spectra of different developmental stages and sexes of each particular species were highly similar. Further, a discrimination of con-specific individuals from different habitats was achieved, at least partly, even without extensive optimization of the analytical and statistical procedures. These results indicate the feasibility of identifying copepods by a rapid and simple MALDI-TOF MS analysis, e.g. for population ecology studies.

NONPHOSPHORUS LIPIDS IN PERIPHYTON REFLECT AVAILABLE NUTRIENTS IN THE FLORIDA EVERGLADES, USA(1).

Algal and plant production of nonphosphorus lipids in place of phospholipids is a physiological response to low phosphorus (P) availability. This response has been shown in culture and in marine plankton studies, but examples from freshwater algae remain minimal. Herein, we analyzed the nutrient contents and lipid composition of periphyton communities across the Florida Everglades ecosystem. We hypothesized that in phosphate-poor areas, periphyton in high- and low-sulfate waters would vary the proportion of sulfolipids (SLs) and betaine lipids (BLs), respectively. In phosphate-enriched areas, periphyton would produce more phospholipids (PLs). We observed that at low-P sites, PLs were a minor lipid component. In cyanobacteria-dominated periphyton where sulfate was abundant, BLs were only slightly more abundant than SLs. However, in the low-P, low-sulfate area, periphyton were comprised to a greater degree green algae and diatoms, and BLs represented the majority of the total lipids. Even in a P-rich area, PLs were a small component of periphyton lipid profiles. Despite the phosphorus limitations of the Everglades, periphyton can develop tremendous biomass. Our results suggest a physiological response by periphyton to oligotrophic conditions whereby periphyton increase abundances of nonphosphorus lipids and have reduced proportions of PLs.

Biological Diversity of Phytoplankton of the Lakes of the Republic Bashkortostan, Russia

The results of the study of autotrophic plankton in nine lakes of the Republic of Bashkortostan during the period from 1978 till 2009 were summarized. In total, 587 species and infraspecific taxa of Cyanoprokaryota and algae were revealed. Their α-, β-, and γ-diversity were discussed.

An Unprecedented Aggregation of Whale Sharks, Rhincodon typus, in Mexican Coastal Waters of the Caribbean Sea

Whale sharks, Rhincodon typus, are often perceived as solitary behemoths that live and feed in the open ocean. To the contrary, evidence is accumulating that they are gregarious and form seasonal aggregations in some coastal waters. One such aggregation occurs annually north of Cabo Catoche, off Isla Holbox on the Yucatán Peninsula of Mexico. Here we report a second, much denser aggregation of whale sharks (dubbed “the Afuera”) that occurs east of the tip of the Yucatán Peninsula in the Caribbean Sea. The 2009 Afuera event comprised the largest aggregation of whale sharks ever reported, with up to 420 whale sharks observed in a single aerial survey, all gathered in an elliptical patch of ocean approximately 18 km2. Plankton studies indicated that the sharks were feeding on dense homogenous patches of fish eggs, which DNA barcoding analysis identified as belonging to little tunny, Euthynnus alletteratus. This contrasts with the annual Cabo Catoche aggregation nearby, where prey consists mostly of copepods and sergestid shrimp. Increased sightings at the Afuera coincide with decreased sightings at Cabo Catoche, and both groups have the same sex ratio, implying that the same animals are likely involved in both aggregations; tagging data support this idea. With two whale shark aggregation areas, high coastal productivity and a previously-unknown scombrid spawning ground, the northeastern Yucatán marine region is a critical habitat that deserves more concerted conservation efforts.

Barriers in the pelagic: population structuring of Calanus helgolandicus and C. euxinus in ­European waters

Molecular studies of marine plankton have shown that ecological and/or environmental barriers play an important role in separating populations. Calanoid copepods are central in marine ecosystems, and dramatic biogeographical shifts in copepod assemblages associated with recent climate warming have been reported. We examined spatial population structuring in European waters of the Atlantic Ocean and Mediterranean Sea of Calanus helgolandicus and its sister species, C. euxi nus, from the Black Sea based on genetic and morphometric characters. The aims were to identify barriers to dispersal, relate these to hydrographic characteristics and infer historical patterns of distribution and demography. We analysed a 408 bp fragment of the mitochondrial 16S gene (316 individuals), prosome to urosome length relationships (212 individuals) and sea surface temp - eratures obtained from 19 European sites. Estimates of genetic differentiation between samples and hierarchical analyses of molecular variance indicated strong spatial population structuring between, as well as within, basins. We identified 7 phylogeographic groups: Fjords, Oceanic inflow, NE Atlantic/ Tyrrhenian, Adriatic, Mljet Island, Aegean, and Black Sea, which explained 39.7% of the total genetic variation. Based on genetic data, C. euxinus is considered to be a differentiated popula- tion within the C. helgolandicus distribution range because the most important genetic barrier sepa- rates western and eastern Mediterranean populations. Morphometric barriers largely reflect sea sur- face temperature barriers and are not congruent with the main genetic barriers. Contrary to recent findings for C. finmarchicus, we conclude that C. helgolandicus/C. euxinus populations are not con- nected by high levels of dispersal and have been vulnerable to past climatic changes.