The shark-like rays (Rhinopristiformes) are among the most threatened species of cartilaginous fishes. The guitarfishes (Rhinobatidae) are one of 5 families in the order, with 62% of species assessed ...as Vulnerable or higher by the International Union for the Conservation of Nature (IUCN). Species-specific fisheries and conservation efforts have been limited, however, due to unresolved taxonomic issues and poor species descriptions. Presently, there are 3 described species of
Rhinobatos
from the southwestern Indian Ocean (SWIO):
R. austini
,
R. holcorhynchus
, and
R. nudidorsalis
. These 3 species have been mistaken for one another and are assessed as Data Deficient by the IUCN. Since the descriptions of
R. austini
and
R. nudidorsalis
, additional specimens have become available and a rediagnosis of these 3 species is required to clarify their taxonomic status. In the present study, morphometrics from 4 additional congener Indian Ocean species of
Rhinobatos
assessed by the IUCN were analyzed and serve as comparative material. In addition to a traditional morphological analysis, morphometrics from all 7 species were analyzed using a principal component analysis (PCA) and linear discriminant analysis (LDA). Results show distinct clusters for the SWIO
Rhinobatos
and indicate the nasal region is effective in differentiating species.
R. austini
,
R. holcorhynchus
, and
R. nudidorsalis
are confirmed as distinct species and are rediagnosed based on new material. These rediagnoses provide taxonomic clarity for SWIO
Rhinobatos
and will aid in species-specific identification, leading to improvements in conservation and fisheries monitoring and management.
Wedgefishes (Rhinidae) are threatened by unsustainable fishing globally, and especially in the Southwest Indian Ocean (SWIO), due to their high-value fins in the shark trade. The whitespotted ...wedgefish
Rhynchobatus djiddensis
and the bottlenose wedgefish
R. australiae
are both classified as Critically Endangered on the IUCN Red List of Threatened Species, yet a lack of species-specific knowledge and taxonomic uncertainty still exists within this genus. Genetic approaches aid in taxonomic classification and identifying distinct populations for targeted conservation. Morphological specimen identification of samples (n = 189) collected across the SWIO was confirmed based on the cytochrome oxidase c subunit I (
COI
) and/or nicotinamide adenine dehydrogenase subunit 2 (
ND2
) gene regions. The genetic diversity and population structure within and between species and sampling locations were investigated using a dual marker approach: (1) 2 concatenated mitochondrial gene regions, namely
COI
and the control region (n = 117), and (2) 9 nuclear microsatellite markers (n = 146). The overall genetic diversity was moderate, with an indication that different evolutionary forces are at play on a mitochondrial versus nuclear level. The 2 species were delineated based on both marker types, and for
R. djiddensis
, the sampling locations of South Africa and Mozambique were genetically homogeneous. For
R. australiae
, significant differentiation was found between sampling locations, with Madagascar and Tanzania being genetically the most similar. This information provides critical insights into the distribution range and population structure of the whitespotted wedgefish species complex that can support the sustainable management of wedgefishes.
Skates by virtue of their abundance and widespread occurrence appear to play an influential role in the food webs of demersal marine communities. However, few quantitative dietary studies have been ...conducted on this elasmobranch group. Therefore, to better understand the ecological role of skates, standardized diet compositions and trophic level (TL) values were calculated from quantitative studies, and compared within and among skate and shark taxa. Prey items were grouped into 11 general categories to facilitate standardized diet composition and TL calculations. Trophic level values were calculated for 60 skate species with TL estimates ranging from 3.48 to 4.22 (mean TL = 3.80 ± 0.02 SE). Standardized diet composition results revealed that decapods and fishes were the main prey taxa of most skate species followed by amphipods and polychaetes. Correspondingly, cluster analysis of diet composition data revealed four major trophic guilds, each dominated by one of these prey groups. Fish and decapod guilds were dominant comprising 39 of 48 species analyzed. Analysis of skate families revealed that the Arhynchobatidae and Rajidae had similar TL values of 3.86 and 3.79 (t-test, P = 0.27), respectively. The Anacanthobatidae were represented by a single species, Cruriraja parcomaculata, with a TL of 3.53. Statistical comparison of TL values calculated for five genera (Bathyraja, Leucoraja, Raja, Rajella, Rhinoraja) revealed a significant difference between Bathyraja and Rajella (t-test, P = 0.03). A positive correlation was observed between TL and total length (L T) with larger skates (e.g. >100 cm L T) tending to have a higher calculated TL value (>3.9). Skates were found to occupy TLs similar to those of several co-occurring demersal shark families including the Scyliorhinidae, Squatinidae, and Triakidae. Results from this study support recent assertions that skates utilize similar resources to those of other upper trophic-level marine predators, e.g. seabirds, marine mammals, and sharks. These preliminary findings will hopefully encourage future research into the trophic relationships and ecological impact of these interesting and important demersal predators.
Eight Bathyraja skate species, Bathyraja aleutica, Bathyraja interrupta, Bathyraja lindbergi, Bathyraja maculata, Bathyraja minispinosa, Bathyraja parmifera, Bathyraja taranetzi and Bathyraja ...trachura, collected along the eastern Bering Sea continental slope were examined for reproductive features. The sex ratio for six of eight Bathyraja skate species studied was c. 1 : 1 with males dominating for B. interrupta and B. parmifera. Five of eight species examined, B. aleutica, B. lindbergi, B. parmifera, B. taranetzi and B. trachura approximately equalled or exceeded the maximum reported total length, LT. Maximum LT for three species (B. interrupta, B. minispinosa and B. parmifera) revealed that the largest individuals were males and that in three other species (B. lindbergi, B. maculata and B. trachura) both females and males grew to approximately the same size. Both females and males matured at approximately the same LT for seven of eight species. Only female B. aleutica matured at a much larger size than males. All species matured within a narrow size range following a protracted juvenile stage and once mature these skates appeared to grow very little. First maturity for all species occurred at ≥80% of their maximum LT. The number of oocytes did not significantly differ between the left and right ovary for any species. The total number of mature oocytes increased with LT for six of eight species examined. The number of oocytes for one species, B. aleutica, increased in number until 145 cm LT, then declined in those individuals greater in length. The egg cases of all species are described, of which four (B. lindbergi, B. parmifera, B. taranetzi and B. trachura) have never been previously reported. Evidence is presented that suggest skates may be limited to as little as one and as many as 13 spawning seasons, depending on the species, and that senescence may occur in older individuals.
Southern Africa has one of the richest and most diverse chondrichthyan faunas in the world, comprising all 13 orders, 49 families, 111 genera and approximately 204 species. This represents nearly 20% ...of all known chondrichthyans, and includes 117 shark, 79 batoid and 8 chimaera species. A greater diversity of chondrichthyan species is seen on the east coast (n = 175) compared to the west coast (n = 96), especially among the families Carcharhinidae and Dasyatidae. The cumulative number of species added to the fauna has increased over the past 112 years from 28 to 204. A golden age of southern African chondrichthyan research occurred between 1967 and 1976 when 46 species were added to the fauna, 16 of which were newly described. A total of 5 families, 23 genera and 130 species have been described from southern Africa. Three families, 17 genera and 75 species are still considered valid. In all, 59 researchers have authored new species descriptions of southern African chondrichthyans, with 95 of 130 species being described by just 14 authors. The most new species described were by JLB Smith with 18, followed by J Müller and FGJ Henle, who collaborated to author 12 new species descriptions. A review of the conservation status of 189 chondrichthyan species indicates that 29% have been assessed as Vulnerable or higher (more threatened), a much higher percentage than the 17.4% of chondrichthyans globally. Of particular concern is that 13% are species endemic to southern Africa. Despite increased chondrichthyan awareness and popularity, most research and conservation efforts centre on a few charismatic species, while the future of little-known species remains ‘lost’ from the public and scientific consciousness.
The large size, high trophic level and wide distribution of Hexanchiformes (cow and frilled sharks) should position this order as important apex predators in coastal and deep‐water ecosystems. This ...review synthesizes available information on Hexanchiformes, including information not yet published, with the purpose of evaluating their conservation status and assessing their ecological roles in the dynamics of marine ecosystems. Comprising six species, this group has a wide global distribution, with members occurring from shallow coastal areas to depths of c. 2500 m. The limited information available on their reproductive biology suggests that they could be vulnerable to overexploitation (e.g. small litter sizes for most species and suspected long gestation periods). Most of the fishing pressure exerted on Hexanchiformes is in the form of commercial by‐catch or recreational fishing. Comprehensive stock and impact assessments are unavailable for most species in most regions due to limited information on life history and catch and abundance time series. When hexanchiform species have been commercially harvested, however, they have been unable to sustain targeted fisheries for long periods. The potentially high vulnerability to intense fishing pressure warrants a conservative exploitation of this order until thorough quantitative assessments are conducted. At least some species have been shown to be significant apex predators in the systems they inhabit. Should Hexanchiformes be removed from coastal and deep‐water systems, the lack of sympatric shark species that share the same resources suggests no other species would be capable of fulfilling their apex predator role in the short term. This has potential ecosystem consequences such as meso‐predator release or trophic cascades. This review proposes some hypotheses on the ecology of Hexanchiformes and their role in ecosystem dynamics, highlighting the areas where critical information is required to stimulate research directions.
In this study, fishes and invertebrates collected from the continental slope (1000 m) of the eastern North Pacific Ocean were analysed using stable‐isotope analysis (SIA). Resulting trophic positions ...(TP) were compared to known diets and habitats from the literature. Dual isotope plots indicated that most species groups (invertebrates and fishes) sorted as expected along the carbon and nitrogen axes, with less intraspecific variability than interspecific variability. Results also indicated an isotopically distinct benthic and pelagic food web, as the benthic food web was more enriched in both nitrogen and carbon isotopes. Trophic positions from SIA supported this finding, resulting in the assignment of fishes to different trophic positions from those expected based on published dietary information. These differences can be explained largely by the habitat of the prey and the percentage of the diet that was scavenged. A mixing model estimated dietary contributions of prey similar to those of the known diet of Bathyraja trachura from stomach‐content analysis (SCA). Linear regressions indicated that trophic positions calculated from SIA and SCA, when plotted against B. trachura total length for 32 individuals, exhibited similar variation and patterns. Only the TP from SCA yielded significant results (stomach content: P < 0·05, stable isotope: P > 0·05).
The big (Raja binoculata), California (R. inornata), longnose (R. rhina), and sandpaper (Bathyraja kincaidii), skates are commonly found on soft-bottom regions of the central California continental ...shelf and upper slope. The feeding ecology of this assemblage was compared to evaluate the degree of trophic separation among species, based on the results of previous species-specific diet studies. Specimens were collected from fishery independent trawl surveys conducted during September 2002-March 2003 at depths of 9-536 m. Using single and compound measures, diet composition of small (<=60 cm TL) and large (>60 cm TL) individuals were compared within continental shelf (<=200 m) and slope (>200 m) regions using traditional, multivariate, and novel techniques. Diet compositions within size classes were similar in both regions. Diet compositions between size classes generally differed, however, with fishes more important and crustaceans (especially shrimps and euphausiids) less important in the diets of large individuals. Crabs contributed substantially to skate diet compositions on the shelf, but were uncommon prey items at deeper depths, probably because of their relative scarcity in slope waters. Conversely, cephalopods were common prey items at slope depths, but were rarely ingested at shelf depths. The studied skate assemblage appears to consist primarily of generalist crustacean and fish predators that exhibit high dietary overlap at similar sizes. It is possible that resource competition among skates and groundfish species has been reduced because of considerable recent declines in the biomass of upper trophic level groundfishes. Skates may therefore play important roles in contemporary benthic food web dynamics off central California.
Skates (Rajiformes: Rajoidei) are a highly diverse fish group, comprising more valid species than any other group of cartilaginous fishes. The high degree of endemism exhibited by the skates is ...somewhat enigmatic given their relatively conserved body morphology and apparent restrictive habitat, e.g. soft bottom substrates. Skates are primarily marine benthic dwellers found from the intertidal down to depths in excess of 3,000 m. They are most diverse at higher latitudes and in deepwater, but are replaced in shallower, warm temperate to tropical waters by stingrays (Myliobatodei). The number of valid skate species has increased exponentially, with more species having been described since 1950 (n = 126) than had been described in the previous 200 years (n = 119). Much of the renaissance in skate systematics has largely been through the efforts of a few individuals who through author-coauthor collaboration have accounted for 78 of the 131 species described since 1948 and for nine of 13 genera named since 1950. Furthermore, detailed regional surveys and accounts of skate biodiversity have also contributed to a better understanding of the diversity of the skates. A checklist of the living valid skate species is presented.
Marine protected areas (MPAs) can potentially alter food web dynamics by increasing the density of fishes within their borders. Such increases in the density of potential competitors can cause ...generalist predators to contract the scope of their diets. This study investigated the effects of increased conspecific fish density on the diets of gopher rockfish Sebastes carnatus at a 35 yr old MPA in Point Lobos, California, and at 4 newly established MPAs in Año Nuevo, Point Lobos, Piedras Blancas, and Point Buchon in central California. Analyses were conducted for 707 stomachs collected from 2007 to 2009. Diets did not differ inside versus outside the old Point Lobos MPA in terms of prey richness, evenness, composition, or gopher rockfish trophic level. However, fish outside the MPA had greater levels of individual specialization. No consistent differences in these metrics were observed inside versus outside the 4 new MPAs, although prey composition and evenness did differ significantly among geographic locations. Diets at Año Nuevo, the most northern and shallow location, consisted predominantly of Cancer spp. and porcelain crabs (Porcellanidae), while diets from southern, deeper locations were dominated by brittle stars (Ophiuroidea). The case study of the old Point Lobos MPA indicates that fish feeding ecology may not change in an MPA after several decades. Differences in prey observed among geographic locations suggest variation in the community composition among central California’s new MPAs, which may influence the effect of each MPA on food web dynamics over time.
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