We conducted monthly fishery-independent sampling using scientific gillnets and longlines at two seagrass shoals and surrounding soft-bottom habitat in the northeast Gulf of Mexico. Using non-metric ...multidimensional scaling, we explored the effects of abiotic variables on temporal patterns of fish community structure, analyzed temporal patterns using permutational multivariate analysis of variance, and tested for cyclicity in community structure. Additionally, we used generalized additive models to explore the effects of environmental variables on immigration and emigration phases of dominant taxa. We found strong seasonality and cyclicity in assemblages captured by both gear types, with depauperate winter communities and diverse assemblages in warmer month. Our results suggest that temperature may determine the timing of immigration and duration of the residency period of dominant taxa, but photoperiod may cue immigration and emigration in adults when the purpose of those migrations includes predicable reproductive functions (e.g., parturition). We found evidence of partial migration in juvenile life stages of some coastal sharks and hypothesize that rising temperatures due to climate change may have variable effects on residency patterns over ontogeny.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
South Florida has been identified as a genetic break for multiple mobile marine taxa but the mechanisms that impede gene flow largely remain unknown. To understand how South Florida functions as a ...barrier for blacknose shark, a highly migratory species that has genetically diverged Atlantic and Gulf populations, patterns of genetic variation were assessed in 212 individuals sampled from the Atlantic, eastern Gulf, and Florida Keys at 2213 nuclear-encoded SNP-containing loci. Results support divergence between the Gulf and Atlantic (
F
ST
~ 0.002,
P
< 0.05), and 51 individuals caught in the Keys were assigned to the Gulf, as compared to only two individuals that assigned to the Atlantic, indicating that Florida Keys is largely composed of Gulf individuals. Further, two to three migrants were identified, all of which were Gulf individuals captured in the Atlantic. The results indicate that South Florida does not prevent individual movement between the Gulf and Atlantic and that the Keys may be a seasonal mixing zone. However, the Gulf and Atlantic remain genetically independent, suggesting that region-specific reproductive behavior/compatibility, or aspects of movement ecology, such as swimming energetics or temperature-driven interannual variability in migratory range, may maintain divergence rather than a physical barrier in South Florida.
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DOBA, EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, IZUM, KILJ, KISLJ, MFDPS, NLZOH, NUK, OBVAL, OILJ, PILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, SIK, UILJ, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
Coastal areas serve as vital habitat for many marine fishes and often host broad gradients of environmental conditions that result in diverse species assemblages. Understanding the influence of ...environmental variables on the distribution and abundance of fishes is especially important for devising appropriate management strategies. We documented large fish community structure in the Apalachicola Bay system from 2018 to 2021 using fishery-independent longline and gillnet surveys. Spatiotemporal variation in fish assemblages was evaluated with permutational multivariate ANOVA, while community- and species-level habitat relationships were described across gradients of environmental variables with non-metric multidimensional scaling and generalized additive models. We captured 5085 fishes from 17 elasmobranch and 33 bony fish species, with gafftopsail catfish
Bagre marinus
, blacktip shark
Carcharhinus limbatus
, Atlantic sharpnose shark
Rhizoprionodon terraenovae
, and hardhead catfish
Ariopsis felis
comprising 82.1% of the total catch. Fish communities varied spatially throughout the system, and changes in these communities were most strongly correlated with gradients in salinity, clarity, and depth. We documented higher species richness and density of young elasmobranchs in our surveys compared to adjacent systems, supporting past suggestions that this area serves as important juvenile shark habitat. We suggest that flow from the Apalachicola River, transporting nutrients to the system and supporting numerous environmental niches across spatiotemporal scales, plays an influential role in maintaining the elevated diversity and density of elasmobranchs observed in this study.
Migration is common in marine animals,1–5 and use of the map-like information of Earth’s magnetic field appears to play an important role.2,6–9 While sharks are iconic migrants10–12 and well known ...for their sensitivity to electromagnetic fields,13–20 whether this ability is used for navigation is unresolved.14,17,21,22 We conducted magnetic displacement experiments on wild-caught bonnetheads (Sphyrna tiburo) and show that magnetic map cues can elicit homeward orientation. We further show that use of a magnetic map to derive positional information may help explain aspects of the genetic structure of bonnethead populations in the northwest Atlantic.23–26 These results offer a compelling explanation for the puzzle of how migratory routes and population structure are maintained in marine environments, where few physical barriers limit movements of vagile species.
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•Sharks are known to undergo precise, long-distance migrations•The navigational mechanism used to facilitate these movements is unresolved•We show that sharks use the Earth’s magnetic field for homeward orientation•This ability is useful for navigation and possibly maintaining population structure
Keller et al. provide evidence that sharks use the map-like information from the geomagnetic field as a navigational aid. This ability is useful for navigating during the long-distance migrations that these species are known for and possibly maintaining population structure in marine environments, where few physical barriers limit movement.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
When identifying potential trophic cascades, it is important to clearly establish the trophic linkages between predators and prey with respect to temporal abundance, demographics, distribution, and ...diet. In the northwest Atlantic Ocean, the depletion of large coastal sharks was thought to trigger a trophic cascade whereby predation release resulted in increased cownose ray abundance, which then caused increased predation on and subsequent collapse of commercial bivalve stocks. These claims were used to justify the development of a predator-control fishery for cownose rays, the "Save the Bay, Eat a Ray" fishery, to reduce predation on commercial bivalves. A reexamination of data suggests declines in large coastal sharks did not coincide with purported rapid increases in cownose ray abundance. Likewise, the increase in cownose ray abundance did not coincide with declines in commercial bivalves. The lack of temporal correlations coupled with published diet data suggest the purported trophic cascade is lacking the empirical linkages required of a trophic cascade. Furthermore, the life history parameters of cownose rays suggest they have low reproductive potential and their populations are incapable of rapid increases. Hypothesized trophic cascades should be closely scrutinized as spurious conclusions may negatively influence conservation and management decisions.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
The great hammerhead (Sphyrna mokarran) is a highly mobile, large‐bodied shark primarily found in coastal‐pelagic and semi‐oceanic waters across a circumtropical range. It is a target or by‐catch ...species in multiple fisheries, and as a result, rapid population declines have occurred in many regions. These declines have contributed to the species being assessed as globally critically endangered on the IUCN Red List. Although conservation and management measures have yielded promising results in some regions, such as the United States, high levels of at‐vessel and post‐release mortality remain a major concern to the species population recovery. This examined the vertical space use and thermal range of pop‐off archival satellite–tagged S. mokarran in the western North Atlantic Ocean, expanding the understanding of the ecological niche of this species and providing insight into by‐catch mitigation strategies for fisheries managers. The results showed that S. mokarran predominantly used shallow depths (75% of records <30 m) and had a narrow temperature range (89% of records between 23 and 28°C). Individual differences in depth use were apparent, and a strong diel cycle was observed, with sharks occupying significantly deeper depths during the daytime. Furthermore, two individuals were confirmed pregnant with one migrating from the Bahamas to South Carolina, U.S.A., providing further evidence of regional connectivity and parturition off the U.S. East Coast. The findings suggest that S. mokarran may be vulnerable to incidental capture in the western North Atlantic commercial longline fisheries due to substantial vertical overlap between the species and the gear. The results can be incorporated into conservation and management efforts to develop and/or refine mitigation measures focused on reducing the by‐catch and associated mortality of this species, which can ultimately aide S. mokarran population recovery in areas with poor conservation status.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Background
The hammerhead sharks (family Sphyrnidae) are an immediately recognizable group of sharks due to their unique head shape. Though there has long been an interest in hammerhead development, ...there are currently no explicit staging tables published for any members of the group. The bonnethead Sphyrna tiburo is the smallest member of Sphyrnidae and is abundant in estuarine and nearshore waters in the Gulf of Mexico and Western North Atlantic Ocean. Due to their relative abundance, close proximity to shore, and brief gestation period, it has been possible to collect and document multiple embryonic specimens at progressive stages of development.
Results
We present the first comprehensive embryonic staging series for the Bonnethead, a viviparous hammerhead shark. Our stage series covers a period of development from stages that match the vertebrate phylotypic period, from Stage 23, through stages of morphological divergence to complete development at birth—Stage 35). Notably, we use a variety of techniques to document crucial stages that lead to their extreme craniofacial diversity, resulting in the formation of one of the most distinctive characters of any shark species, the cephalofoil or hammer‐like head.
Conclusion
Documenting the development of hard‐to‐access vertebrates, like this viviparous shark species, offers important information about how new and diverse morphologies arise that otherwise may remain poorly studied. This work will serve as a platform for future comparative developmental research both within sharks and across the phylogeny of vertebrates, underpinning the extreme potential of craniofacial development and morphological diversity in vertebrate animals.
Key Findings
A staging series for one of the smallest hammerhead species.
Observing the developmental timing and growth of fins and cephalofoil in hammerheads.
This work will serve as a platform for future comparative developmental research both with hammerhead sharks and across chondrichthyan fishes.
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
The relationship between phoretic diskfishes and their hosts is a classic example of marine symbiosis, yet surprisingly few studies have quantified this trophic relationship. We investigated the ...hypothesis that by consuming host parasites and prey scraps phoretic diskfishes (Echeneidae) feed at a higher relative trophic position than free-living individuals through expanded foraging opportunities. We used carbon and nitrogen stable isotope analysis of muscle tissue from both free-living and commensal sharksuckers
Echeneis naucrates
and their hosts, supplemented with gut-content analysis, to investigate this hypothesis. Our analysis revealed that commensal sharksuckers likely occupy higher relative trophic positions than free-living sharksuckers. The importance of scavenging host prey decreased ontogenetically as sharksuckers shifted from symbiotic phoresis to free-swimming behavior, leading to an ontogenetic change in which obligately commensal juveniles occupy higher trophic positions than free-living adults which are only facultatively commensal. Relative differences in δ
13
C and δ
15
N among individual host-commensal pairs varied among host species, suggesting potential differences in foraging opportunities among host taxa.
Modes of reproduction and embryonic development vary greatly among the elasmobranchs, and prior studies have suggested that the energetic toll of embryogenesis in lecithotrophic species depletes ...embryonic organic matter by 20% or more. Matrotrophic species experience a lesser reduction or an increase in organic matter during embryogenesis. To investigate the maternal–embryonic nutritional relationship, we measured changes in organic matter from fertilization to near-parturition in embryos of Centrophorus granulosus and Etmopterus princeps. Embryos of C. granulosus experienced a reduction of 19.5% in organic matter, while E. princeps embryos experienced a reduction of 7.7% in organic matter over the course of embryonic development, suggesting some level of matrotrophy occurs, particularly for the latter species. Uterine villi were present in both species and developed concurrently with the embryos, increasing in length and thickness while becoming progressively vascularized. Embryos of C. granulosus were dissected to track the partitioning of water, organic matter, and inorganic matter to the liver, external yolk sac, internal yolk sac, digestive tract, and evicerated body throughout development. Mating was aseasonal for both species and spatially-mediated segregation by sex and maturity stage was observed. Ovarian cycles were concurrent for C. granulosus and consecutive for E. princeps. Size at maturity for C. granulosus was determined to be 111cm TL for males and 143cm TL for females, with an average fecundity of 5.3 embryos (range=4–7). Size at maturity for E. princeps was determined to be 56.5cm TL for males and 61cm TL for females north of the Azores and 54cm TL for males and 69cm TL for females near the Charlie Gibbs Fracture Zone. Average fecundity was 11.2 embryos (range=7–18) for this species. This is the first reporting of reproductive parameters for these two species, and the information provided will be valuable for informing stock assessment models in areas where these species are fished.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Globally, sawfishes are threatened with extinction because they occur in coastal habitats, which are disproportionally affected by human activities. In particular, sawfishes are susceptible to myriad ...fisheries that operate in the same regions. Unfortunately, these well-documented threats can be only partly mitigated because of ongoing human resource needs. However, the outlook for sawfishes is optimistic if we build on 5 successful research and conservation approaches developed over the past 2 decades: (1) local ecological knowledge, (2) identification of nurseries, (3) state-of-the-art techniques, (4) tagging studies, and (5) threat mitigation. If appreciation of healthy ecosystems that contain top predators like sawfishes can be fostered by healthy human populations where sawfishes occur (e.g. ecosystem management, promotion of sustainable livelihoods), and if local knowledge can be used to influence scientific studies that inform customized management decisions, this group of iconic species will have a chance to recover. In addition, promoting sample collection, sample archiving, and collaboration, especially in instances of sawfish mortality, will maximize the knowledge gained, despite often limited funding and limited samples. The first 2 decades of the 21st century have seen the first steps taken toward sawfish recovery, but there is more to do. Ultimately, translation of research results into conservation actions that include enforcement and fisher behavior change will be the key to sustained recovery. This Overview provides a context for the papers in the Theme Section 'Biology and ecology of sawfishes', and we incorporate them as part of a mini-review of research on this threatened group to provide a future outlook.
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