Diet and reproduction in the Australian butterfly ray Gymnura australis from northern and north-eastern Australia

Phylogenetic analysis of viviparity, matrotrophy, and other reproductive patterns in chondrichthyan fishes

The reproductive diversity of extant cartilaginous fishes (class Chondrichthyes) is extraordinarily broad, reflecting more than 400 million years of evolutionary history. Among their many notable reproductive specialisations are viviparity (live-bearing reproduction) and matrotrophy (maternal provision of nutrients during gestation). However, attempts to understand the evolution of these traits have yielded highly discrepant conclusions. Here, we compile and analyse the current knowledge on the evolution of reproductive diversity in Chondrichthyes with particular foci on the frequency, phylogenetic distribution, and directionality of evolutionary changes in their modes of reproduction. To characterise the evolutionary transformations, we amassed the largest empirical data set of reproductive parameters to date covering nearly 800 extant species and analysed it via a comprehensive molecular-based phylogeny. Our phylogenetic reconstructions indicated that the ancestral pattern for Chondrichthyes is 'short single oviparity' (as found in extant holocephalans) in which females lay successive clutches (broods) of one or two eggs. Viviparity has originated at least 12 times, with 10 origins among sharks, one in batoids, and (based on published evidence) another potential origin in a fossil holocephalan. Substantial matrotrophy has evolved at least six times, including one origin of placentotrophy, three separate origins of oophagy (egg ingestion), and two origins of histotrophy (uptake of uterine secretions). In two clades, placentation was replaced by histotrophy. Unlike past reconstructions, our analysis reveals no evidence that viviparity has ever reverted to oviparity in this group. Both viviparity and matrotrophy have arisen by a variety of evolutionary sequences. In addition, the ancestral pattern of oviparity has given rise to three distinct egg-laying patterns that increased clutch (brood) size and/or involved deposition of eggs at advanced stages of development. Geologically, the ancestral oviparous pattern arose in the Paleozoic. Most origins of viviparity and matrotrophy date to the Mesozoic, while a few that are represented at low taxonomic levels are of Cenozoic origin. Coupled with other recent work, this review points the way towards an emerging consensus on reproductive evolution in chondrichthyans while offering a basis for future functional and evolutionary analyses. This review also contributes to conservation efforts by highlighting taxa whose reproductive specialisations reflect distinctive evolutionary trajectories and that deserve special protection and further investigation.

Reproductive biology, length-weight relationship and diet of co-occurring butterfly rays, Gymnura poecilura and Gymnura zonura, in Malaysian waters

Recent IUCN assessments had resulted in up listing of the status of butterfly rays due to concerns of overfishing, but inadequate biological understanding of these rays prevents meaningful conservation and management measures. Therefore, this study was undertaken to address knowledge gaps in the reproductive biology and diet of longtail butterfly ray (Gymnura poecilura) and zone tail butterfly ray (Gymnura zonura) in Malaysian waters. From surveys of landing sites and fish markets from years 2017 to 2022, size (disc width, DW), weight and maturity were recorded, and stomachs were collected from 94 G. poecilura (N = 39 females and 55 males) and 20 G. zonura (N = 10 females and 10 males) specimens. The length-weight relationships were significantly different between sexes for G. poecilura. The size at maturity (DW50) was estimated to be 476.0 mm (females), 385.0 mm (males) for G. poecilura and 442.0 mm (combined) for G. zonura. The number of embryos ranged from 1 to 6, and the embryo size was between 73.90 to 130.44 mm DW. Dietary analysis of stomach contents revealed that fish prey was dominant in both G. poecilura [94.4% Index of Relative Importance (IRI)] and G. zonura (100% IRI). Ontogenetic shift was seen in G. poecilura that fed on more variety of prey items, including shrimps, squids and crabs with an increase in body size. Both species co-occur all along coastal Malaysia although G. zonura is rarely encountered from fisheries surveys along the Strait of Malacca. Given similar habitat associations and dietary habits, G. poecilura may be able to outcompete G. zonura across their shared habitat range. The validity of G. japonica and G. micrura records in Malaysia remains questionable and requires future investigation.

Discerning the dietary habits of the smooth butterfly ray Gymnura lessae using two distinct methods, otolith identification and metagenetics

Two different methods, metagenetics and free-otolith identification, were used to identify prey in the stomach contents of 531 Gymnura lessae captured by trawling in Mobile Bay, Alabama 2016-2018. Both methods were found to produce analogous results and were therefore combined into a single complete dataset. All prey were teleosts; the families Sciaenidae and Engraulidae were the most important prey (prey specie index of relative importance 89.3% I_PSRI ). Multivariate analyses indicated that the diet of G. lessae varied with sex and seasonality. Specifically, variability was probably due to morphologically larger females consuming larger teleost prey species compared with males, whereas seasonal variability was probably due to changes in the available prey community composition. The findings indicate that both metagenetics and free otolith identification, used independently or complementarily, offer robust means of characterising dietary habits for teleost-specialised species such as G. lessae, which may play an important role in the structure and maintenance of coastal food webs such as those in Mobile Bay.

Reproductive characteristics and reproductive tract anatomy of the California butterfly ray Gymnura marmorata (Myliobatiformes: Gymnuridae)

The California butterfly ray Gymnura marmorata is frequently caught by artisanal fisheries in north-western Mexico (Baja California Sur; BCS). The aim of this study was to estimate reproductive characteristics and describe the micro and macroscopic reproductive structures of female and male G. marmorata. Sampling was carried out at two fisheries landing sites located on the west coast BCS, from 2008 to 2012. The largest disc width (W_D ) ever recorded for this species was 131.0 cm and females were larger than males. The male sample was composed exclusively of adult specimens, with a median size at 50% maturity (W_D50 ) estimated at 44.9 cm W_D . For females, size at onset of maturity was estimated at 63.0 cm W_D ; W_D50 , at 64.7 cm W_D ; and size at pregnancy at 80.1 cm W_D . Gonadal asymmetry was observed for the first time in this species. Indeed, in males only the left testis was functional (testicular asymmetry), while in females, only the left ovary was functional (ovarian asymmetry).

Trace metals and persistent organic pollutants contamination in batoids (Chondrichthyes: Batoidea): A systematic review

Batoids (Chondrichthyes: Batoidea; e.g. stingrays, skates, and guitarfish) comprise more than 55% of elasmobranch taxa and represent ecologically important predators in benthic and pelagic habitats. Although overexploitation and habitat degradation are the two biggest threats to batoid populations, coastal and oceanic pollution is also a pervasive potential threat. In this systematic review, we compile published scientific literature on trace metals and persistent organic pollutants (POPs) contamination in elasmobranch species of the Batoidea superorder and present contamination patterns, exposure effects, and potential human exposure risks to most reported contaminants. We found batoids to accumulate a wide range of trace metals, including mercury (Hg), arsenic (As), lead (Pb), copper (Cu), cadmium (Cd) and zinc (Zn). Accumulation of POPs is reported for chlordanes, dichlorodiphenyltrichloroethane (DDT), polychlorinated biphenyl (PCB), dieldrin, Heptachlor epoxide, hexachlorobenzene and perfluoroalkyl substances (PFAS). Hg levels in muscle tissue were significantly different among oceanic basins and habitats, consistent with previous global assessments of Hg oceanic background levels. Some batoid species presented Hg levels higher than large pelagic teleost fishes and comparable to sharks. Ecological traits such as, bottom feeding, upper trophic position and elasmobranch-specific physiology and metabolism are discussed as potential factors associated with Hg uptake and accumulation in batoids. Some species exceeded USEPA's maximum contamination safety limits in edible tissues for Hg, As and ΣPCBs. For most trace metals and POPs, there is a lack of studies focusing on contamination levels in batoids. We recommend future research increasing reporting on POPs and trace metals besides Hg in batoids to further investigate the role of Elasmobranch as a bioindicator for marine pollution.

The evolution of underwater flight: The redistribution of pectoral fin rays, in manta rays and their relatives (Myliobatidae)

Batoids are a diverse clade of flat cartilaginous fishes that occur primarily in benthic marine habitats. The skates and rays typically use their flexible pectoral fins for feeding and propulsion via undulatory swimming. However, two groups of rays have adopted a pelagic or bentho-pelagic lifestyle and utilize oscillatory swimming-the Myliobatidae and Gymnuridae. The myliobatids have evolved cephalic lobes, anteriorly extended appendages that are optimized for feeding, while their pectoral fins exhibit several modifications that likely arose in association with functional optimization of pelagic cruising via oscillatory flight. Here, we examine variation in fin ray distribution and ontogenetic timing of fin ray development in batoid pectoral fins in an evolutionary context using the following methods: radiography, computed tomography, dissections, and cleared and stained specimens. We propose an index for characterizing variation in the distribution of pectoral fin rays. While undulatory swimmers exhibit symmetry or slight anterior bias, we found a posterior shift in the distribution of fin rays that arose in two distinct lineages in association with oscillatory swimming. Undulatory and oscillatory swimmers occupy nonoverlapping morphospace with respect to fin ray distribution illustrating significant remodeling of pectoral fins in oscillatory swimmers. Further, we describe a derived skeletal feature in anterior pectoral fins of the Myliobatidae that is likely associated with optimization of oscillatory swimming. By examining the distribution of fin rays with clearly defined articulation points, we were able to infer evolutionary trends and body plan remodeling associated with invasion of the pelagic environment. Finally, we found that the number and distribution of fin rays is set early in development in the little skate, round stingray, and cownose ray, suggesting that fin ray counts from specimens after birth or hatching are representative of adults and therefore comparable among species.

Comparative anatomy of the extraocular muscles in four Myliobatoidei rays (Batoidea, Myliobatiformes)

Extraocular muscles are classically grouped as four rectus and two oblique muscles. However, their description and potential associations with species behavior are limited. The objective was to characterize extraocular muscles in four Myliobatoidei rays from diverse habitats with divergent behaviors. Heads (10 per species) of Dasyatis hypostigma, Gymnura altavela, Mobula thurstoni and Pteroplatytrygon violacea were decalcified and dissected to characterize and describe extraocular muscles. Principal component analysis (PCA) was used to evaluate relationships between muscle length and species; for P. violacea, D. hypostigma and G. altavela, these were qualitatively and quantitatively consistent with the general pattern of extraocular muscles in vertebrates. In contrast, for M. thurstoni, the two oblique muscles were completely fused and there was a seventh extraocular muscle, named m. lateral rectus β (both were apparently novel findings in this species). There were also significant differences in eye disposition in the chondrocranium. The PCA axis 1 (rectus muscles) and PCA axis 2 (oblique muscles) accounted for 98.47% of data variability. Extraocular muscles had significant differences in length and important anatomical differences among sampled species that facilitated grouping species according to their life history. In conclusion, extraocular muscles are not uniform in all vertebrate species, thereby providing another basis for comparative studies.

Microanatomy of the male and female reproductive tracts in the long-tailed butterfly ray Gymnura poecilura, an elasmobranch with unusual characteristics

The anatomy of the male and female reproductive systems was investigated in the long-tailed butterfly ray Gymnura poecilura using gross observation and light microscopy. The testes are highly asymmetrical, to the extent that only the left testis is functional and the right testis is completely absent. Both of the male genital ducts are present and symmetrical, although spermatozoa only occur in the left duct. The genital ducts are straight and unconvoluted, with regular incomplete internal partitions throughout. Females do not possess a right ovary, nor do the oviducal glands exhibit distinct club and papillary zones, and the baffle zone lacks baffle plates. In all sections of the gland, the tubules display different secretory activities depending on the proximity to the gland lumen. The gland produces a thin egg membrane that encases each egg individually, while the endometrium is formed into trophonemata.

A comparative analysis of feeding and trophic level ecology in stingrays (Rajiformes; Myliobatoidei) and electric rays (Rajiformes: Torpedinoidei)

Standardised diets and trophic level (T_L) estimates were calculated for 75 ray species from the suborders Myliobatoidei (67 spp.) and Torpedinoidei (8 spp.). Decapod crustaceans (31.71±3.92%) and teleost fishes (16.45±3.43%) made the largest contribution to the standardised diet of the Myliobatoidei. Teleost fishes (37.40±16.09%) and polychaete worms (31.96±14.22%) were the most prominent prey categories in the standardised diet of the suborder Torpedinoidei. Cluster analysis identified nine major trophic guilds the largest of which were decapod crustaceans (24 species), teleost fishes (11 species) and molluscs (11 species). Trophic level estimates for rays ranged from 3.10 for Potamotrygon falkneri to 4.24 for Gymnura australis, Torpedo marmorata and T. nobiliana. Secondary consumers with a T_L <4.00 represented 84% of the species examined, with the remaining 12 species (16%) classified as tertiary consumers (T_L ≥4.00). Tertiary consumers included electric rays (Torpedo, 3 spp. and Hypnos, 1 sp.), butterfly rays (Gymnura, 4 spp.), stingrays (2 spp.) and Potamotrygonid stingrays (2 spp.). Feeding strategies were identified as the primary factor of influence with respect to Myliobatoidei and Torpedinoidei T_L estimates with inter-family comparisons providing the greatest insight into Myliobatoidei and Torpedinoidei relationships.

Feeding ecology and dietary comparisons among three sympatric Neotrygon (Myliobatoidei: Dasyatidae) species

The diets of Neotrygon picta, Neotrygon kuhlii and Neotrygon annotata are described using trophic level analysis, index of relative importance (I(RI) ) and non-metric multidimensional scaling ordination (nMDS). They were secondary consumers with trophic levels of between 3·55 and 3·58. Carids were the most important prey category in the diets of N. picta (82·0%I(RI) ) and N. annotata (69·4%I(RI) ); Polychaeta (72·3%I(RI) ) was the most important prey category for N. kuhlii. A significant difference was detected between the volumetric dietary data of all three species (ANOSIM, global R-statistic = 0·742, P < 0·01) and between sequential N. picta and N. kuhlii size classes (ANOSIM, global R-statistics = 0·367-0·403, P < 0·01). Changes in polychaete and carid prey proportions were the primary sources of diet variability. Dietary differences may be linked to variations in disc width, mouth morphology and feeding strategies. This study provides insights into the level of variability within and between the diets and feeding behaviours of Neotrygon species, and the potential for resource partitioning to occur when these species coexist.

Life history of the blackspotted whipray Himantura astra

Features of the life history of Himantura astra from north-east Australia were examined including its age and growth, reproduction and diet. Centrum edge and marginal increment ratio analyses were used to validate annual band formations with the Gompertz growth function providing the best fit to male (W(D∞) = 722·7 mm, k = 0·104) and female (W(D∞) = 821·8 mm, k = 0·073) disc width (W(D))-at-age data. At 29 years, the maximum age of females was higher than males (18 years). Sizes at 50% sexual maturity (W(D50)) for males and females were 469·3 and 462·3 mm, respectively. Ages at sexual maturity (A(M50)) were reported at 7·32 (males) and 8·67 (females) years. An index of relative importance (I(RI)) revealed carid shrimps (77·9%), brachyurans (12·1%) and stomatopods (4·9%) as the most important prey groups, with prey diversity increasing with W(D) from 0·92 to 1·63 (Shannon-Weiner index). This study provides significant insights into the biology of H. astra and contributes to the ongoing development of fisheries-based risk assessments for this species.