Pelagic Sargassum community change over a 40-year period: temporal and spatial variability

Pelagic forms of the brown algae (Phaeophyceae) Sargassum spp. and their conspicuous rafts are defining characteristics of the Sargasso Sea in the western North Atlantic. Given rising temperatures and acidity in the surface ocean, we hypothesized that macrofauna associated with Sargassum in the Sargasso Sea have changed with respect to species composition, diversity, evenness, and sessile epibiota coverage since studies were conducted 40 years ago. Sargassum communities were sampled along a transect through the Sargasso Sea in 2011 and 2012 and compared to samples collected in the Sargasso Sea, Gulf Stream, and south of the subtropical convergence zone from 1966 to 1975. Mobile macrofauna communities exhibited changes in community structure and declines in diversity and evenness within a 6-month time period (August 2011–February 2012). Equivalent declines in diversity and evenness were recorded in the same region (Sargasso Sea, 25°–29°N) in 1972–1973. Recent community structures were unlike any documented historically, whether compared to sites of the same latitude range within the Sargasso Sea, or the broader historical dataset of sites ranging across the Sargasso Sea, Gulf Stream, and south of the subtropical convergence zone. Recent samples also recorded low coverage by sessile epibionts, both calcifying forms and hydroids. The diversity and species composition of macrofauna communities associated with Sargassum might be inherently unstable. While several biological and oceanographic factors might have contributed to these observations, including a decline in pH, increase in summer temperatures, and changes in the abundance and distribution of Sargassum seaweed in the area, it is not currently possible to attribute direct causal links.     

The phytal animals on the thalli of Sargassum serratifolium in the Sargassum region,with reference to their seasonal fluctuations

In the Sargassum region on the coast of Mukaishima, in the Inland Sea of Japan, the phytal animals living on Sargassum serratifolium, and the alga itself as a habitat for phytal animals, were studied from August, 1966 to August, 1968. The standing crop of Sargassum/m² bottom was 4.93 kg/m² (March, 1967) and 3.53 kg/m² (February, 1968) in the most luxuriant season, and 0.4 to 0.5 kg/m² (July, 1967 and 1968) in the off-season. The individual number of phytal animals per mean plant, without sessile fauna, reached a maximum in late winter and early spring (about 130,000 in 1967, 266,000 in 1968) when Sargassum was luxuriant, and a minimum in summer (about 15,000 in both years) when Sargassum was declining. Among the phytal animals, benthonic copepods (Harpacticoida) were very abundant in most seasons. Their dominance decreased in spring, whilst nematode dominance increased. It appears that such seasonal changes are closely related to the standing crop of S. serratifolium. Fluctuations in numbers of most groups of the meiofauna of phytal animals, such as foraminiferans, ostracods, isopods, amphipods, copepods, tanaids and nematodes, are connected with changes in the standing Sargassum crop. Seasonal fluctuations of some groups, such as echinoderms, actinians, mysids and decapod crustaceans (macrofauna), were, however, independent of Sargassum crop variations. The individual number of phytal animals/m² bottom in the Sargassum region were compared with findings of previous studies in other habitats, such as naked sandy and mud bottoms.Contribution No. 100 from the Mukaishima Marine Biological Station.     

Spatial heterogeneity of phytoplankton populations in estuarine surface microlayers

Phytoplankton populations are concentrated in surface microlayer waters of the York River and Mobjack Bay, Virginia, USA. The degree of vertical heterogeneity was variable and seemed most dependent on weather conditions (vertical mixing) and total phytoplankton cell densities. Vertical stratification also exhibited some taxocoenotic dependency. Structurally, the phytoplankton communities of the surface microlayer were less diverse than subsurface communities. In general, values of informational diversity (H'), evenness (J), and richness (S and r) are lowest in the surface microlayer and increase with depth (to 3.0 m). Mechanisms which structure and maintain vertical heterogeneity are discussed.This work was supported in part by funds from the National Science Foundation Research Applied to National Needs Program grants to the Virginia Institute of Marine Science and the Chesapeake Research Consortium.Contribution No. 791 of the Virginia Institute of Marine Science, and Contribution No. 62 of the South Carolina Marine Resources Center.     

Fungi from the Sargasso Sea

Fungi inhabiting Sargassum spp. from the Sargasso Sea were examined. Three ascomycetes were found, two of which are new records for this environment. Haloguignardia oceanica (Ferdinandsen et Winge) Kohlm. comb. nov. was collected only once before, over 50 years ago. Haloguignardia tumefaciens (Cribb et Herbert) Cribb et Cribb was known only from Australia and New Zealand. Imperfect states of these ascomycetes and fruiting bodies (acervuli) of a hyperparasitic deuteromycete (cf. Sphaceloma sp.) were found for the first time in the galls of the two species. Lindra thalassiae Orpurt et al., previously known only from the sea grass Thalassia testudinum Köngig, develops in air bladders of Sargassum sp., causing a shedding of infested vesicles. Possible reasons for the rarity of fungal species and diseased plants in the Sargasso Sea are diseussed.Dedicated respectfully to my teacher, Professor Dr. H Drawert (Universität Famburg), on the occasion of his 60th birthday.     

Symbiosis between bacteria and oceanic clonal sea star larvae in the western North Atlantic Ocean

Bacteria were present in the majority of clonal sea star bipinnariae (Luidia sp.) collected between June 1987 and August 1990 from oceanic populations in the subtropical western North Atlantic Ocean. Light and electron microscopy revealed dense aggregations of bacteria within lobes of the epidermal cuticle in the gastric region of larvae. Gram-negative rods and less-common elongate spiral bacteria were observed, always in close association with branching epidermal microvilli. Intact and partially digested bacteria occurred in vesicles and phagosomes within epidermal cells, a probable indication of phagocytotic activity by the host. The association of larvae of Luidia sp. and bacteria ranged widely on both geographical and temporal scales. During July and August 1988, larvae were abundant (1.6 to 11.1 m³) in the mixed surface layer (<10 to 100 m) of stations in the Gulf Stream, the Sargasso Sea and the North Equatorial Drift, and 96% (mean of 5 stations, n=8 to 10 larvae in each station) of the individuals surveyed harbored bacteria beneath their cuticle. In a single station on the Florida Current sampled during winter, spring, and summer months between 1989 and 1991, 79 to 90% of the larvae harbored bacteria. The incidence of symbiosis was higher in actively cloning larvae than in non-cloning larvae. This is the first documentation of a symbiosis in field populations of echinoderm larvae.     

Primary productivity and related data from tropical and subtropical marine sediments

Measurements in situ were made of oxygen exchange and ¹⁴C fixation in calcareous sediments off the coast of Florida, USA, and at various sites in the Caribbean Sea at depths to 60 m. Sediment samples were analyzed for total organic carbon, nitrogen and photosynthetic pigments. In some cases, measurements were made of the pH of sediment interstitial water and its concentration of total CO₂. In the Caribbean Sea, routine hydrological sampiing was undertaken and in situ plankton productivity determined.Contribution No. 1522 from the University of Miami, Rosenstiel School of Marine and Atmospheric Science, 10 Rickenbacker Causeway, Miami, Florida 33149, USA.     

Lethal limits and the zoogeography of the faunal assemblages of Coastal Carolina waters

The offshore waters of Cape Hatteras, North Carolina, USA, is an area of dynamic encounter between the northerly flowing, warm waters of the Gulf Stream and the southerly flowing, colder water of the Virginia coastal current. The faunal assemblages north of the Cape and those associated with a reeflike formation southe of the Cape in the Gulf Stream are distinct, each having predominantly cold- or warm-water affinities. To assess the zoogeographical significance of temperature, high and low thermal limits of both adult and larvae of species from these 2 regions were determined. Lethal salinity limits of the first stage zoeae of a few Crustacea were determined. In general, southern affinity species survived higher temperatures than northerly displanced species. In some cases, species with overlapping distributional limits had different thermal tolerances, which could be correlated with differential microhabitat selection. Many of the southern affinity species could not survive low temperatures which were characteristic of either the water mass northe of Cape Hatteras (4 °C) or the waters between the Gulf Stream and the shore (10 °C). A few species were able to seasonally acclimate to temperature extremes. Zoeae of reef species are the least resistant to both low temperature and low salinity.Supported by Grant NSF GB 7435 from the National Science Foundation. The authors thank Duke University Marine Laboratory and the Cooperative Program of Biological Oceanography for the use of the R. V. Eastward. The Cooperative Program is supported through the National Science Foundation Grant GB-8189.     

Effects of chronic oil pollution on a salt-marsh grass community

A small, naturally defined saltmarsh in Chesapeake Bay (USA) was repeatedly dosed with a No. 2 fuel oil. The production, decay rates, physical characteristics and hydrocarbon content of the dominant grass species present in the marsh were routinely monitored. Spartina alterniflora was the only grass species displaying effects of the oil dosing; this is attributed to the grass's location in the marsh. A substantial portion of the S. alterniflora in the marsh was killed by the oil dosing; the remaining portion evidenced sublethal effects including delayed development in the spring, increased density, and reduced mean weight per stem. The second annual cohort of shoots, usually produced in late summer and early fall, was suppressed almost entirely. Decay rates of S. alterniflora in the oiled marsh were higher and peaked later than decay rates in a control marsh. Oil which entered the roots and rhizomes of dead S. alterniflora was retained in a relatively undergraded state for at least 7 months after dosing stopped.Virginia Institute of Marine Science Contribution No. 945     

Salinity and substratum effects on condition index of the bivalve Rangia cuneata

The condition index (CI) of the clam Rangia cuneata was measured monthly at 8 stations in 2 substrate types and 3 salinity regimes in the James River estuary, Virginia, USA. CI was higher in clams from sand bottoms than from mud, and decreased in both substrata in fresher water. Values were lowest in early spring and at a maximum in early autumn, with a pronounced autumn peak in the sand substratum. This pattern coincided with the annual salinity cycle. A field experiment showed that some factor associated with the water overlying a sand or mud substratum, rather than the nature of the substratum itself, was important in determining CI. The hypothesis is presented that this factor may be the higher suspended solids concentrations measured immediately over mud bottoms.Virginia Institute of Marine Science Contribution No. 782.     

Mitochondrial DNA and electronic tracking reveal population structure of Atlantic bluefin tuna (Thunnus thynnus)

Population subdivision was examined in Atlantic bluefin tuna (Thunnus thynnus) through sequencing of the control region of the mitochondrial genome. A total of 178 samples from the spawning grounds in the Gulf of Mexico, Bahamas and Mediterranean Sea were analyzed. Among the samples from these locations were 36 electronically tagged bluefin tuna that were tagged in the North Atlantic and subsequently traveled to one of these known spawning grounds during the spawning season. Bluefin tuna populations from the Gulf of Mexico and the Mediterranean Sea were found to be genetically distinct based on Φ_st, and sequence nearest neighbor analyses, showing that these two major spawning areas support independent stocks. Sequence nearest neighbor analysis indicated significant population subdivision among the Gulf of Mexico, western Mediterranean and eastern Mediterranean Sea. However, it was not possible to find significant pairwise differences between any sampling areas when using all samples. If only samples that had a high likelihood of assignment to a specific spawning site were used (young of the year, spawning adults), the differentiation increased among all sampling areas and the Western Mediterranean Sea was distinct from the Eastern Mediterranean Sea and the Gulf of Mexico. It was not possible to distinguish samples from the Bahamas from those collected at any of the other sampling sites. These data support tagging results that suggested distinctness of the Gulf of Mexico, Eastern and Western Mediterranean Sea spawning areas. This level of stock differentiation is only possible if Atlantic bluefin tuna show strong natal homing to individual spawning grounds.     

Horizontal movement of ocean sunfish, <Emphasis Type="Italic">Mola mola</Emphasis>, in the northwest Atlantic

Data were retrieved from 25 ocean sunfish (Mola mola) that were tagged with pop-up satellite archival tags in the southern Gulf of Maine (n = 6), off Nantucket Island (n = 17), and off the coast of Georgia (n = 2) between September 2005 and March 2008. Tags remained attached from 7 to 242 days, with a mean attachment period of (X ± SD) 107.2 ± 80.6 days. Ocean sunfish tagged in the Gulf of Maine and southern New England left those areas in the late summer and early autumn and moved south along the continental shelf break. Fish traveled as far south as the Bahamas and the Gulf of Mexico. By moving south, sunfish experienced similar mean sea surface temperatures throughout the tagging period. The maximum straight-line distance traveled by a tagged Mola mola was 2,520 km in 130 days. Two tagged ocean sunfish entered the Gulf of Mexico, one in the December and one in July. Movements were associated with frontal features created by the Gulf Stream and fish moved farther offshore in 2007 when the Gulf Stream was deflected from the shelf break.     

Movements of bluefin tuna (<Emphasis Type="Italic">Thunnus thynnus</Emphasis>) in the northwestern Atlantic Ocean recorded by pop-up satellite archival tags

Pop-up satellite archival tags were implanted into 68 Atlantic bluefin tuna (Thunnus thynnus Linnaeus), ranging in size from 91 to 295 kg, in the southern Gulf of Maine (n=67) and off the coast of North Carolina (n=1) between July 2002 and January 2003. Individuals tagged in the Gulf of Maine left that area in late fall and overwintered in northern shelf waters, off the coasts of Virginia and North Carolina, or in offshore waters of the northwestern Atlantic Ocean. In spring, the fish moved either northwards towards the Gulf of Maine or offshore. None of the fish crossed the 45°W management line (separating eastern and western management units) and none traveled towards the Gulf of Mexico or the Straits of Florida (known western Atlantic spawning grounds). The greatest depth recorded was 672 m and the fish experienced temperatures ranging from 3.4 to 28.7°C. Swimming depth was significantly correlated with location, season, size class, time of day, and moon phase. There was also evidence of synchronous vertical behavior and changes in depth distribution in relation to oceanographic features.Communicated by J.P. Grassle, New Brunswick     

Trophic implications of cross-shelf copepod distributions in the Southeastern Bering Sea

Spring distributions of some numerically dominant copepods reflect associations with two distinct water masses separated along the 80- to 100-m isobaths. Seaward of this middle shelf front, the oceanic Bering Sea hosts populations of Calanus cristatus, C. plumchrus, and Eucalanus bungii bungii; Metridia pacifica, Oithona similis, and Pseudocalanus spp. are also present. The large oceanic species are much less abundant in waters shallower than 80 m where the community is seasonally dominated by smaller copepods, O. similis, Acartia longiremis, and Pseudocalanus spp. Experimental and field-derived estimates of carbon ingestion indicate that the oceanic/outer shelf copepods can occasionally graze the equivalent of the daily plant production and probably routinely remove 20–30% of the primary productivity. Conversely, stocks of middle shelf copepods rarely ingest more than 5% of the plant carbon productivity. During 45 d between mid April to late May, 1979, approximately three times more organic matter was ingested m^-2 by the outer shelf/oceanic copepod community than by middle shelf species. This imbalance in cross-shelf grazing permits middle shelf phytoplankton stocks to grow rapidly to bloom proportions, and to sink ungrazed to the seabed. Over the outer shelf and particularly along the shelf break, a much closer coupling to phytoplankton supports a large biomass of oceanic grazers. Here, copepod stocks approaching 45 g dry wt m^-2 occur in late spring as a narrow band at the shelf break.Supported by National Science Foundation Grant DPP 76-23340Contribution no. 485, Institute of Marine Science, University of Alaska, Fairbanks     

Incidence,reproduction and feeding of Stylochus zebra,a polyclad turbellarian symbiont of hermit crabs

The hermit crab Pagurus pollicaris is the common host for the polyclad turbellarian Stylochus zebra in the Atlantic coastal waters from Massachusetts to North Carolina, USA. S. zebra is reported for the first time from two other hermit crabs, Pagurus impressus and Petrochirus diogenes. It was not found with Clibanarius vittatus or Pagurus longicarpus, although the latter serves as a host along the Gulf coast of Florida. The incidence of the polyclad with P. pollicaris in Massachusetts is more than twice that in populations to the south. Multiple infestations (up to 7 worms/crab) occur in over 50% of the infested crabs from Massachusetts, but they are rare elsewhere. Worms living in clear plastic shells with crabs orient in the body whorl dorsal to the host. Egg masses of worms are cemented in the same position within gastropod shells harboring the crabs. In the laboratory, the worm feeds on the gastropod Crepidula plana, a common inhabitant of shells occupied by P. pollicaris. Its feeding behavior is described. The relationship between S. zebra and P. pollicaris appears to be generally commensalistic, but it could be more complicated if the predation of crab embryos observed in the laboratory is confirmed under natural conditions.Contribution No. 755 from the Virginia Institute of Marine Science, Gloucester Point, Virginia, USA.     

Fatty acid composition of flora and fauna associated with Sargassum mats in the Gulf of Mexico

Feeding ecology of organisms associated with floating Sargassum in the northwestern Gulf of Mexico was assessed using fatty acids. Nineteen groups were collected from the Sargassum community including four autotrophs, eight invertebrates, five juvenile fishes, and two adult fishes. Spatial and temporal variability in polyunsaturated fatty acid (PUFA) signatures of selected taxa (Sargassum fluitans [autotroph], Leander tenuicornis [primary heterotroph], Balistes capriscus [secondary heterotroph]) was examined to quantify natural variation within these dietary tracers. Although PUFA signatures varied seasonally for all three taxa, no differences were detected between samples collected in year 2000 and 2001 or from different sample locations in the northwest Gulf. PUFA signatures made up 16.3–62.3% of the total fatty acid composition of main autotrophs present in the pelagic environment [particulate organic matter (POM), epiphytic algae, S. fluitans, S. natans], and PUFA profiles of selected primary producers were distinct. Specifically, levels of 20:5n−3, 22:5n−3, and 22:6n−3 were significantly higher in POM than Sargassum spp. or epiphytic algae (Cladophora sp.). Dominant PUFA in the tissue of invertebrate and vertebrate consumers were 18:2n−6, 20:4n−6, 20:5n−3, 22:5n−3, 22:6n−3 and multivariate analyses indicated that PUFA signatures of all consumers were highly similar to POM. As a result, heterotrophs utilizing the Sargassum complex may rely heavily on phytoplankton production rather than production by Sargassum or associated epiphytic algae.     

Heterotrophic activity associated with particulate size fractions in a Spartina alterniflora salt-marsh estuary,Sapelo Island,Georgia, USA,and the continental shelf waters

An investigation was undertaken to determine the distribution and activity of the heterotrophic microplankton associated with particles from a Georgia salt marshestuarine ecosystem to the western edge of the Sargasso Sea. Heterotrophic activity was determined by the uptake of ¹⁴C-glucose. More than 80% of the activity was associated with detritus greater than 3 m in creek, river and coastal (within 4 km of shore) waters. In the Gulf Stream, approximately 80% of the heterotrophic activity was in the fraction less than 3 m. In the estuary, total heterotrophic activity fluctuated with the tides; the greatest activity occurred near low ebb tide at all locations. The lowest activity was measured at slack low and high tides. In creek water most of the heterotrophic activity was associated with particles between 14 and 180 m, whereas in coastal waters (less than 4 km from shore) most of the activity was in the fraction greater than 180 m.Contribution No. 331 from the University of Georgia Marine Institute, Sapelo Island, Georgia 31327, USA.     

Species-specific grazing rates of heterotrophic dinoflagellates in oceanic waters,measured with a dual-label radioisotope technique

A dual-isotope method was developed to measure grazing rates and food preferences of individual species of heterotrophic dinoflagellates from natural populations, collected from the Slope, Gulf Stream, and Sargasso Sea and from a transect from Iceland to New England, in 1983. The isotope method measures the grazing rates of microzooplankton which cannot be separated in natural populations on the basis of size. Tritiated-thymidine and ¹⁴C-bicarbonate were used to label natural heterotrophic and autotrophic food, respectively. Nine oceanic dinoflagellate species in the genera Protoperidinium, Podolampas, and Diplopsalis fed on both heterotrophic and autotrophic food particles with clearance rates of 0.4 to 8.0 l cell^-1 h^-1, based on ³H incorporation, and 0.0 to 28.3 l cell^-1 h^-1, based on ¹⁴C incorporation. Two dinoflagellate species, Protoperidinium ovatum and Podolampas palmipes, fed only on ³H-labelled food particles. Several species of dinoflagellates fed on bacteria (<1 m) which had been prelabelled with ³H-thymidine. The clearance rates of heterotrophic dinoflagellates and ciliates were similar and within the range of tintinnid ciliate clearance rates reported in the literature. As heterotrophic dinoflagellates and ciliates can have comparable abundances in oceanic waters, we conclude that heterotrophic dinoflagellates may have an equally important impact as microheterotrophic grazers of phytoplankton and bacteria in oceanic waters.Partially supported by a grant from the National Science Foundation, OCE-81-17744     

Natural diets of vertically migrating zooplankton in the Sargasso Sea

The feeding preferences of three common diel vertically migrating zooplankton were investigated from December 1999 to October 2000 at the U.S. JGOFS Bermuda Atlantic Time-Series Study (BATS) station in the Sargasso Sea. Gut content analysis of the copepods Pleuromamma xiphias (Giesbrecht) and Euchirella messinensis (Claus) and of the euphausiid Thysanopoda aequalis (Hansen) indicated that all three species fed on a wide variety of phytoplankton, zooplankton, and detrital material. Diet changes generally reflected seasonal trends in phytoplankton community structure. However, species-specific feeding preferences and differences in feeding selectivity among the three species were evident, and in general agreement with feeding habits predicted from the analysis of mouthpart morphology. The euphausiid T. aequalis fed equally on more different food types compared to both copepod species. The copepod P. xiphias consumed a diverse assemblage of phytoplankton from late winter through the summer (78-93% of gut items, by number, were phytoplankton) and based its diet more strongly on carnivorous feeding in autumn and early winter (31% and 61% of gut items were phytoplankton, respectively). E. messinensis showed the greatest feeding specialization, with a strong preference for pennate diatoms in winter and spring and for coccolithophorids during late summer and fall (constituting 67-93% of gut items by number). All three species consumed diatoms more than other phytoplankton taxa, even though diatoms form only a small fraction of the phytoplankton biomass in the Sargasso Sea. Although the majority of gut items identified were phytoplankton cells, the relative biomass contribution of these small cells may be lower than that of zooplankton and detritus. Zooplankton on which the three species primarily preyed were protozoans and crustaceans, but also included other metazoans such as chaetognaths and cnidarians. Marine snow was also an important component of the diet in all three species, with typically >50% and rarely <20% of the gut content being olive-green debris. Marine snow from larvacean houses was found in the guts of all three species, while E. messinenis appeared to selectively consume marine snow aggregates enriched with bicapitate Nitzschia spp. Large cyanobacteria (>4 µm in diameter) found in guts were also likely consumed with marine snow. The species-specific differences in the diets of these three migrating species suggest that an individual species approach is important in determining how feeding habits affect the structure of pelagic food webs and carbon cycling in the sea. Electronic supplementary material to this paper can be obtained by using the Springer LINK server located at http://dx.doi.org/10.1007/s00227-002-0815-8.     

Feeding ecology of the loggerhead sea turtle Caretta caretta in the Northwestern Gulf of Mexico

Digestive tract contents collected from carcasses of 82 loggerhead sea turles (Caretta caretta) found on the south Texas coast from 1986 through 1988 were examined. Benthic invertebrates were the predominant prey. Sea pens (Virgularia presbytes), crabs, and mollusks accounted for 94% of the dry weight of the digestive tract samples. Temporal changes in the percent occurrence and percent dry weight of sea pens, crabs, and mollusks in the digestive tract samples were significant. Loggerheads fed primarily on sea pens during spring, then primarily on crabs during summer and fall. The increase of crabs in the loggerhead diet paralleled the annual increase in the abundance of crabs in the Gulf of Mexico. Sea pens were located nearshore at depths of 6 to 12 m and had a disjunct distribution. Present address: Texas A&M University, Department of Biology, College Station, Texas 77843-3258, USA     

Cryptic isolation of Gulf of California shovelnose guitarfish evidenced by mitochondrial DNA

The shovelnose guitarfish Rhinobatos productus is an evolutionarily, ecologically, and economically important ray, with a continuous distribution from San Francisco, California (USA), to Mazatlan, Sinaloa, and in the Gulf of California (Mexico). Regional studies have revealed morphometric differences between shovelnose from the Gulf of California and the Pacific coast of Baja California, which may result from phenotypic plasticity in the presence of high levels of gene flow or from a degree of genetic differentiation in the presence of cryptic isolation within a continuous distribution. We used PCR-RFLP of the mitochondrial control region to assess the degree of genetic differentiation between Gulf of California and Pacific shovelnose guitarfish. We found very high levels of molecular diversity (averages: h=0.77, =1.19%), which may be associated with historically large and stable populations, as well as very significant levels of genetic differentiation between gulf and Pacific samples (²=64, P<0.0001; _ST=0.63, P<0.0001, mean nucleotide divergence d=2.47%). We found a deep phylogeographic break between haplotypes from the gulf and the Pacific, which may suggest the existence of cryptic species but clearly indicates more than one evolutionarily significant unit of R. productus. Our results show a pattern of genetic structure and levels of differentiation consistent with the geological history of the region. Furthermore, these findings have wide-ranging implications for the management and conservation of cartilaginous fish in Mexico, as they reveal the existence of biological diversity that will go unnoticed without the genetic scrutiny of intraspecific variation and that is highly relevant for much needed management and conservation efforts.Communicated by P.W. Sammarco, Chauvin