Expanding records of occurrence of the whale shark (Rhincodon typus) in The Bahamas and a call for sustainable tourism practices.

This report updates our understanding of whale shark occurrences in The Bahamas by drawing upon a variety of data sources. Our findings reveal previously unreported sighting locations, often associated with tourism activities, underscoring the pivotal role played by nontraditional data sources in addressing knowledge gaps. These revelations emphasize the ongoing necessity for monitoring efforts. Additionally, we have found cases that raise concerns related to unregulated human-shark interactions in the region, highlighting the pressing need for sustainable tourism practices within Bahamian waters.

Persistent Organic Pollutants in Whale Shark (Rhincodon typus) Skin Biopsies from Bahía de Los Ángeles, Mexico.

The whale shark (Rhincodon typus) is a filter-feeding organism that can be considered a sentinel species, and Bahía de los Ángeles (BLA) in the Gulf of California is an important sighting site for these elasmobranchs. This filter-feeding organism can be considered a pollutant sampler from the marine environment. Persistent organic pollutants are toxic compounds with high mobility and environmental persistence, bioaccumulation and trophic transfer. Among these are polycyclic aromatic hydrocarbons (PAHs) and organochlorine pesticides (OCPs). The present work aimed to determine concentrations of PAHs and OCPs in whale shark skin biopsies, collected in 2021 at BLA. Mean detected levels of PAHs and OCPs were 279.4 ng/g dw (dry weight) and 1478.1 ng/g dw, respectively. Analysis of similarities between the ordered sizes (4.2-7.6 m) and the concentrations of PAHs and OCPs indicated no significant differences. Individual PAHs detected indicate pyrogenic and petrogenic sources; the presence of pesticides at levels higher than those of hydrocarbons may be related to agricultural activity in the areas surrounding the Baja California peninsula. This study is the first report of PAH levels in R. typus for the Gulf of California and Mexico.

Microplastic occurrence and phthalate ester levels in neuston samples and skin biopsies of filter-feeding megafauna from La Paz Bay (Mexico).

The impacts of microplastics on filter feeders megafauna have recently received increased attention. These organisms are potentially exposed to plastic ingestion and the release of added/sorbed contaminants during feeding activities. An assessment of microplastic abundance and the chemical impact of Phthalates esters (PAEs) were performed in neustonic samples and skin biopsies of Balaenoptera physalus and Rhincodon typus inhabiting the Gulf of California (Mexico). Sixty-eight percent of the net tows contained plastics with a maximum of 0.24 items/m3 mainly composed of polyethylene fragments. PAE levels were detected both in environmental and skin biopsy samples, with the highest values in the fin whale specimens (5291 ng/g d.w). Plasticizer fingerprint showed a similar distribution pattern between neustonic samples and filter-feeding species, with DEHP and MBP having the highest concentrations. The detection of PAE levels confirmed their potential role as plastic tracers and give preliminary information about the toxicological status of these species feeding in La Paz Bay.

Trends in the evolution of the elasmobranch melanocortin-2 receptor: Insights from structure/function studies on the activation of whale shark Mc2r.

To understand the mechanism for activation of the melanocortin-2 receptor (Mc2r) of the elasmobranch, Rhincodon typus (whale shark; ws), wsmc2r was co-expressed with wsmrap1 in CHO cells, and the transfected cells were stimulated with alanine-substituted analogs of ACTH(1-24) at the "message" motif (H6F7R8W9) and the "address" motif (K15K16R17R18P19). Complete alanine substitution of the H6F7R8W9 motif blocked activation, whereas single alanine substitution at this motif indicated the following hierarchy of position importance for activation: W9 > R8, and substitution at F7 and H6 had no effect on activation. The same analysis was done on a representative bony vertebrate Mc2r ortholog (Amia calva; bowfin; bf) and the order of position importance for activation was W9 > R8 = F7, (alanine substitution at H6 was negligible). Complete alanine substitution at the K15K16R17R18P19 motif resulted in distinct outcomes for wsMc2r and bfMc2r. For bfMc2r, this analog blocked activation-an outcome typical for bony vertebrate Mc2r orthologs. For wsMc2r, this analog resulted in a shift in sensitivity to stimulation of the analog as compared to ACTH(1-24) by two orders of magnitude, but the dose response curve did reach saturation. To evaluate whether the EC2 domain of wsMc2r plays a role in activation, a chimeric wsMc2r was made in which the EC2 domain was replaced with the EC2 domain from a melanocortin receptor that does not interact with Mrap1 (i.e., Xenopus tropicalis Mc1r). This substitution did not negatively impact the activation of the chimeric receptor. In addition, alanine substitution at a putative activation motif in the N-terminal of wsMrap1 did not affect the sensitivity of wsMc2r to stimulation by ACTH(1-24). Collectively, these observations suggest that wsMc2r may only have a HFRW binding site for melanocortin-related ligand which would explain how wsMc2r could be activated by either ACTH or MSH-sized ligands.

Whale shark rhodopsin adapted to deep-sea lifestyle by a substitution associated with human disease.

Spectral tuning of visual pigments often facilitates adaptation to new environments, and it is intriguing to study the visual ecology of pelagic sharks with secondarily expanded habitats. The whale shark, which dives into the deep sea of nearly 2,000 meters besides near-surface filter feeding, was previously shown to possess the 'blue-shifted' rhodopsin (RHO), which is a signature of deep-sea adaptation. In this study, our spectroscopy of recombinant whale shark RHO mutants revealed that this blue shift is caused dominantly by an unprecedented spectral tuning site 94. In humans, the mutation at the site causes congenital stationary night blindness (CSNB) by reducing the thermal stability of RHO. Similarly, the RHO of deep-diving whale shark has reduced thermal stability, which was experimentally shown to be achieved by site 178 and 94. RHOs having the natural substitution at site 94 are also found in some Antarctic fishes, suggesting that the blue shift by the substitution at the CSNB site associated with the reduction in thermal stability might be allowed in cold-water deep-sea habitats.

Narrowing, twisting, and undulating: Complicated movement in shark spiral intestine inferred using ultrasound.

Shark intestine presents a complicated three-dimensional morphology, characterized by the development of a coiled internal septum. A basic question regarding the intestine is its movement. This lack of knowledge has prevented the testing of the hypothesis on its functional morphology. The present study, to our knowledge, for the first time, visualized the intestinal movement of three captive sharks using an "underwater ultrasound" system. The results indicated that the movement of the shark intestine involved strong twisting. We suspect that this motion is the mechanism that tightens the coiling of the internal septum, enhancing compression of the intestinal lumen. Our data also revealed the presence of active undulatory movement of the internal septum, of which the undulatory wave propagated in the opposite (anal-to-oral) direction. We hypothesize that this motion decreases the flow rate of the digesta and increases absorptive time. These observations indicate that the kinematics of the shark spiral intestine are more complicated than expected based on morphology, and the fluid flow in the intestine is likely highly regulated by intestinal muscular activity.

Microbiome structure in large pelagic sharks with distinct feeding ecologies.

BACKGROUND: Sharks play essential roles in ocean food webs and human culture, but also face population declines worldwide due to human activity. The relationship between sharks and the microbes on and in the shark body is unclear, despite research on other animals showing the microbiome as intertwined with host physiology, immunity, and ecology. Research on shark-microbe interactions faces the significant challenge of sampling the largest and most elusive shark species. We leveraged a unique sampling infrastructure to compare the microbiomes of two apex predators, the white (Carcharodon carcharias) and tiger shark (Galeocerdo cuvier), to those of the filter-feeding whale shark (Rhincodon typus), allowing us to explore the effects of feeding mode on intestinal microbiome diversity and metabolic function, and environmental exposure on the diversity of microbes external to the body (on the skin, gill). RESULTS: The fecal microbiomes of white and whale sharks were highly similar in taxonomic and gene category composition despite differences in host feeding mode and diet. Fecal microbiomes from these species were also taxon-poor compared to those of many other vertebrates and were more similar to those of predatory teleost fishes and toothed whales than to those of filter-feeding baleen whales. In contrast, microbiomes of external body niches were taxon-rich and significantly influenced by diversity in the water column microbiome. CONCLUSIONS: These results suggest complex roles for host identity, diet, and environmental exposure in structuring the shark microbiome and identify a small, but conserved, number of intestinal microbial taxa as potential contributors to shark physiology.

Pharmacological properties of whale shark (Rhincodon typus) melanocortin-2 receptor and melancortin-5 receptor: Interaction with MRAP1 and MRAP2.

In the current study, the whale shark (ws; Rhincodon typus) melanocortin-2 receptor (MC2R) co-expressed with wsMRAP1 in Chinese Hamster Ovary (CHO) Cells could be stimulated in a dose dependent manner by ACTH(1-24) with an EC50 of 2.6 × 10-10 M ± 9.7 × 10-11. When the receptor was expressed alone, stimulation was only observed at [10-6 M]. A comparable increase in sensitivity to stimulation by srDes-Ac-αMSH was also observed when the receptor was co-expressed with wsMRAP1. Furthermore, co-expression with wsMRAP1 significantly increased the trafficking of wsMC2R to the plasma membrane of CHO cells. Surprisingly, co-expression with wsMRAP2 also increased sensitivity to stimulation by ACTH(1-24) and srDes-Ac-αMSH, and increased trafficking of the receptor to the plasma membrane. These observations are in sharp contrast to the response of MC2R orthologs of bony vertebrates which have an obligate requirement for co-expression with MRAP1 for both trafficking to the plasma membrane and activation, whereas, co-expression with MRAP2 increases trafficking, but has minimal effects on activation. In addition, when comparing the activation features of wsMC2R with those of the elephant shark MC2R and red stingray MC2R orthologs, both similarities and differences are observed. The spectrum of features for cartilaginous fish MC2R orthologs will be discussed. A second objective of this study was to determine whether wsMC5R has features in common with wsMC2R in terms of ligand selectivity and interaction with wsMRAP paralogs. While wsMC5R can be activated by either srACTH(1-24) or srDes-Ac-αMSH, and co-expression with wsMRAP1 enhances this activation, wsMRAP1 had no effect on the trafficking of wsMC5R. In addition, co-expression with wsMRAP2 had no positive or negative effect on either ligand sensitivity or trafficking of wsMC5R.

Microplastics in fecal samples of whale sharks (Rhincodon typus) and from surface water in the Philippines.

Marine plastic abundance has increased over the past 60 years and microplastics (< 5 mm) constitute a primary component of such litter. Filter-feeding megafauna, such as the whale shark, might be particularly affected by microplastic pollution as their feeding mode requires filtration of up to thousands of cubic meters of water. In addition, the habitat range of whale sharks intersects with several recognized microplastic pollution hotspots, among which is the Coral Triangle. Direct evidence for microplastic ingestion in whale sharks however, has not yet been presented. Here we show that whale shark scat collected in the Philippines from 2012 to 2019 contained a mean of 2.8 microplastics g- 1. Contrary to our expectations, the microplastic concentration in the scat remained consistent from 2012 to 2019. Water samples from the study site in 2019 indicated that the local microplastic pollution (5.83 particles m- 3) was higher than in surface waters in other whale shark habitats, but well below other pollution hot-spots found in Southeast Asia and China (range: 100-4100 particles m- 3). With the predicted growth in plastic use, leading to increased plastic marine pollution, whale sharks are expected to become more exposed to this form of pollution. To what extent microplastic ingestion impacts the overall health status of this endangered species remains an open question. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s43591-021-00017-9.

Behavioural response of a whale shark during the passage of a typhoon.

During a behavioural survey of a tagged whale shark (Rhincodon typus) conducted in 2019 in the waters off Kagoshima, Japan, a typhoon passed close to the area under surveillance. As the typhoon approached, monitoring of the shark's movements indicated that it dived to depths of up to 90 m, and during this period, the authors recorded the effects of the typhoon-induced waves. They also detected changes in the vertical thermal structure of the waters, possibly due to the disturbance caused by the typhoon.

The whale shark genome reveals patterns of vertebrate gene family evolution.

Chondrichthyes (cartilaginous fishes) are fundamental for understanding vertebrate evolution, yet their genomes are understudied. We report long-read sequencing of the whale shark genome to generate the best gapless chondrichthyan genome assembly yet with higher contig contiguity than all other cartilaginous fish genomes, and studied vertebrate genomic evolution of ancestral gene families, immunity, and gigantism. We found a major increase in gene families at the origin of gnathostomes (jawed vertebrates) independent of their genome duplication. We studied vertebrate pathogen recognition receptors (PRRs), which are key in initiating innate immune defense, and found diverse patterns of gene family evolution, demonstrating that adaptive immunity in gnathostomes did not fully displace germline-encoded PRR innovation. We also discovered a new toll-like receptor (TLR29) and three NOD1 copies in the whale shark. We found chondrichthyan and giant vertebrate genomes had decreased substitution rates compared to other vertebrates, but gene family expansion rates varied among vertebrate giants, suggesting substitution and expansion rates of gene families are decoupled in vertebrate genomes. Finally, we found gene families that shifted in expansion rate in vertebrate giants were enriched for human cancer-related genes, consistent with gigantism requiring adaptations to suppress cancer.

Using species connectivity to achieve coordinated large-scale marine conservation efforts in the Red Sea.

In the face of increasing anthropogenic threats, coastal nations need to reach common ground for effective marine conservation. Understanding species' connectivity can reveal how nations share resources, demonstrating the need for cooperative protection efforts. Unfortunately, connectivity information is rarely integrated into the design of marine protected areas (MPAs). This is exemplified in the Red Sea where biodiversity is only nominally protected by a non-cohesive network of small-sized MPAs, most of which are barely implemented. Here, we showcase the potential of using connectivity patterns of flagship species to consolidate conservation efforts in the Red Sea. We argue that a large-scale MPA (LSMPA) would more effectively preserve Red Sea species' multinational migration routes. A connectivity-informed LSMPA approach provides thus one avenue to unite coastal nations toward acting for the common good of conservation and reverse the global decline in marine biodiversity.

Wound-healing capabilities of whale sharks (Rhincodon typus) and implications for conservation management.

Wound healing is important for marine taxa such as elasmobranchs, which can incur a range of natural and anthropogenic wounds throughout their life history. There is evidence that this group shows a high capacity for external wound healing. However, anthropogenic wounds may become more frequent due to increasing commercial and recreational marine activities. Whale sharks are particularly at risk of attaining injuries given their use of surface waters and wildlife tourism interest. There is limited understanding as to how whale sharks recover from injuries, and often insights are confined to singular opportunistic observations. The present study makes use of a unique and valuable photographic data source from two whale shark aggregation sites in the Indian Ocean. Successional injury-healing progression cases were reviewed to investigate the characteristics of injuries and quantify a coarse healing timeframe. Wounds were measured over time using an image standardization method. This work shows that by Day 25 major injury surface area decreased by an average of 56% and the most rapid healing case showed a surface area reduction of 50% in 4 days. All wounds reached a point of 90% surface area closure by Day 35. There were differences in healing rate based on wound type, with lacerations and abrasions taking 50 and 22 days to reach 90% healing, respectively. This study provides baseline information for wound healing in whale sharks and the methods proposed could act as a foundation for future research. Use of a detailed classification system, as presented here, may also assist in ocean scale injury comparisons between research groups and aid reliable descriptive data. Such findings can contribute to discussions regarding appropriate management in aggregation areas with an aim to reduce the likelihood of injuries, such as those resulting from vessel collisions, in these regions or during movements between coastal waters.

Donsol: an important reproductive habitat for the world's largest fish Rhincodon typus?

Little is known about the reproductive ecology of the whale shark Rhincodon typus. On 15 March 2020, a free-swimming neonate whale shark was found in the shallow, coastal waters of Donsol, Philippines. The total length of the male shark was 60 cm, falling within the species known size-at-birth. This is the third occurrence of neonatal whale sharks reported in the area, and coupled with the occurrence of juveniles and adults, and anecdotal mating and precopulatory behaviour reported herein, the importance of the site for this endangered species is highlighted.

The whale shark genome reveals how genomic and physiological properties scale with body size.

The endangered whale shark (Rhincodon typus) is the largest fish on Earth and a long-lived member of the ancient Elasmobranchii clade. To characterize the relationship between genome features and biological traits, we sequenced and assembled the genome of the whale shark and compared its genomic and physiological features to those of 83 animals and yeast. We examined the scaling relationships between body size, temperature, metabolic rates, and genomic features and found both general correlations across the animal kingdom and features specific to the whale shark genome. Among animals, increased lifespan is positively correlated to body size and metabolic rate. Several genomic traits also significantly correlated with body size, including intron and gene length. Our large-scale comparative genomic analysis uncovered general features of metazoan genome architecture: Guanine and cytosine (GC) content and codon adaptation index are negatively correlated, and neural connectivity genes are longer than average genes in most genomes. Focusing on the whale shark genome, we identified multiple features that significantly correlate with lifespan. Among these were very long gene length, due to introns being highly enriched in repetitive elements such as CR1-like long interspersed nuclear elements, and considerably longer neural genes of several types, including connectivity, activity, and neurodegeneration genes. The whale shark genome also has the second slowest evolutionary rate observed in vertebrates to date. Our comparative genomics approach uncovered multiple genetic features associated with body size, metabolic rate, and lifespan and showed that the whale shark is a promising model for studies of neural architecture and lifespan.

Body temperature stability in the whale shark, the world's largest fish.

It is generally assumed that the body temperature of large animals is less likely to change because of their large body size, resulting in a high thermal inertia and a smaller surface area to volume ratio. The goal of this study was to investigate the stability of body temperature in large fish using data from field experiments. We measured the muscle temperatures of free-ranging whale sharks (Rhincodon typus), the largest extant fish globally, and investigated their ectothermic physiology and the stability of their body temperature. The muscle temperature changed substantially more slowly than the water temperature fluctuations associated with vertical movements, and the whole-body heat-transfer coefficients (HTCs) of whale sharks estimated using heat-budget models were lower than those of any other fish species measured to date. The heat-budget models also showed that internal heat production does not contribute to changes in muscle temperature. A comparative analysis showed that the HTC at cooling in various fish species including both ectothermic and endothermic species ranging from 10-4 to 103 kg was proportional to body mass-0.63 This allometry was present regardless of whether the fish were ectothermic or endothermic, and was an extension of the relationship observed in previous studies on small fish. Thus, large fish have the advantage of body temperature stability while moving in environments with large temperature variations. Our results suggest that the large body size of whale sharks aids in preventing a decrease in body temperature during deep excursions to more than 1000 m depths without high metabolic costs of producing heat.

Pharmacological properties of whale shark (Rhincodon typus) melanocortin-2 receptor and melancortin-5 receptor: Interaction with MRAP1 and MRAP2.

In the current study, the whale shark (ws; Rhincodon typus) melanocortin-2 receptor (MC2R) co-expressed with wsMRAP1 in Chinese Hamster Ovary (CHO) Cells could be stimulated in a dose dependent manner by ACTH(1-24) with an EC50 of 2.6 × 10-10 M ± 9.7 × 10-11. When the receptor was expressed alone, stimulation was only observed at [10-6 M]. A comparable increase in sensitivity to stimulation by srDes-Ac-αMSH was also observed when the receptor was co-expressed with wsMRAP1. In addition, co-expression with wsMRAP1 significantly increased the trafficking of wsMC2R to the plasma membrane of CHO cells. Surprisingly, co-expression with wsMRAP2 also increased sensitivity to stimulation by ACTH(1-24) and srDes-Ac-αMSH, and increased trafficking of the receptor to the plasma membrane. These observations are in sharp contrast to the response of MC2R orthologs of bony vertebrates which have an obligate requirement for co-expression with MRAP1 for both trafficking to the plasma membrane and activation, and while co-expression with MRAP2 increases trafficking, it has minimal effects on activation. In addition, when comparing the activation features of wsMC2R with those of the elephant shark MC2R and red stingray MC2R orthologs, both similarities and differences are observed. The spectrum of features for cartilaginous fish MC2R orthologs will be discussed. A second objective of this study was to determine whether wsMC5R has features in common with wsMC2R in terms of ligand selectivity and interaction with wsMRAP paralogs. While wsMC5R can be activated by either srACTH(1-24) or srDes-Ac-αMSH, and co-expression with wsMRAP1 enhances this activation, wsMRAP1 had no effect on the trafficking of wsMC5R. Co-expression with wsMRAP2 had no positive or negative effect on either ligand sensitivity or trafficking of wsMC5R.

Body temperature stability observed in the whale sharks, the world's largest fish.

It is generally assumed that the body temperature of large animals is less likely to change due to their large body size, resulting in a high thermal inertia and a smaller surface area to volume ratio. The goal of this study was to investigate the stability of body temperature in large fish using data from field experiments. We measured the muscle temperatures of free-ranging whale sharks (Rhincodon typus), the largest extant fish globally, and investigated their ectothermic physiology and the stability of their body temperatures. The measured muscle temperature of the whale sharks changed substantially more slowly than the water temperature fluctuations associated with vertical movements, and the whole-body heat-transfer coefficients (HTC) of whale sharks estimated using heat-budget models were lower than those of any other fish species measured to date. The heat-budget models also showed that internal heat production does not contribute to changes in muscle temperature. A comparative analysis showed that the HTC at cooling in various fish species including both ectothermic and endothermic species ranging from 10-4 to 103 kg was proportional to body mass-0.63. This allometry was present regardless of whether the fish were ectothermic or endothermic, and was an extension of the relationship observed in previous studies on small fish. Thus, large fish have the advantage of body temperature stability while moving in environments with large temperature variations. Our results suggest that the large body size of whale sharks aids in preventing a decrease in body temperature during deep excursions to more than 1000 m depths without high metabolic costs of producing heat.

The retinal pigments of the whale shark (Rhincodon typus) and their role in visual foraging ecology.

The spectral tuning properties of the whale shark (Rhincodon typus) rod (rhodopsin or Rh1) and long-wavelength-sensitive (LWS) cone visual pigments were examined to determine whether these retinal pigments have adapted to the broadband light spectrum available for surface foraging or to the narrowband blue-shifted light spectrum available at depth. Recently published whale shark genomes have identified orthologous genes for both the whale shark Rh1 and LWS cone opsins suggesting a duplex retina. Here, the whale shark Rh1 and LWS cone opsin sequences were examined to identify amino acid residues critical for spectral tuning. Surprisingly, the predicted absorbance maximum (λmax) for both the whale shark Rh1 and LWS visual pigments is near 500 nm. Although Rh1 λmax values near 500 nm are typical of terrestrial vertebrates, as well as surface foraging fish, it is uncommon for a vertebrate LWS cone pigment to be so greatly blue-shifted. We propose that the spectral tuning properties of both the whale shark Rh1 and LWS cone pigments are most likely adaptations to the broadband light spectrum available at the surface. Whale shark melanopsin (Opn4) deactivation kinetics was examined to better understand the underlying molecular mechanisms of the pupillary light reflex. Results show that the deactivation rate of whale shark Opn4 is similar to the Opn4 deactivation rate from vertebrates possessing duplex retinae and is significantly faster than the Opn4 deactivation rate from an aquatic rod monochromat lacking functional cone photoreceptors. The rapid deactivation rate of whale shark Opn4 is consistent with a functional cone class and would provide the animal with an exponential increase in the number of photons required for photoreceptor signaling when transitioning from photopic to scotopic light conditions, as is the case when diving.

Trace Elements in Tissues of Whale Sharks (Rhincodon typus) Stranded in the Gulf of California, Mexico.

Concentration of essential (Se, Zn and Cu) and non-essential (As, Cd, Hg and Pb) trace elements were measured in selected tissues of two dead whale sharks (Rhincodon typus) stranded in the Gulf of California (GC) in 2017 and 2018. Concentrations of Cd and Pb in the skeletal muscle of the whale shark from La Paz Bay, GC were higher compared to a previous study on whale shark from China. The shark from La Paz Bay also presented higher concentration of Pb in the epidermis, compared to the same tissue of the other whale shark stranded in Punta Bufeo, GC. The Hg in all analysed tissues was lower than those documented in carnivorous sharks. Molar ratio Se:Hg shows an excess of Se over Hg in all the tissues sampled in both sharks.