PDF Freshwater Fish Physiology
PhD Elasmobranch Physiology
BSc Marine Biology - Honours, Co-op
My Research
Elasmobranch intestine: how does the acquisition
of dietary nitrogen affect osmoregulation?
Our current understanding of marine elasmobranch (sharks, skates, and rays) osmoregulation is focused on the gills (site of nitrogen excretion), kidney (site of nitrogen retention), and rectal gland (site of sodium and chloride concentration); however, the intestine is the main site of nitrogen acquisition from ingested prey. My current research looks at how nitrogen (ammonia and urea) moves across intestinal tissues and what internal conditions influence their transport.
It's a shark-eat-shark world
Sharks retain urea as part of their hydration-strategy to counter-balance the osmotic pressure and dehydrating properties of living in a salt water environment. Generally, sharks consume prey containing little-to-no urea (fish, crabs, whales, etc). However, some shark species are known to consume other elasmobranchs (sharks, skates, or rays) which contain high concentrations of urea in their tissues. During a necropsy I personally observed 6 whole dogfish that had been ingested by a 12 foot blue shark during one feeding event, as evidenced by consistent decomposition of all the ingested dogfish.
This made me ask the question: what happens to all of the urea when one shark eats another shark?
Turns out, not much.
Abstract: Marine elasmobranchs are nitrogen-limited owing to the requirement of nitrogen for both somatic growth and urea-based osmoregulation, and due to the loss of urea across the gills and kidney as nitrogenous waste. In this study we used in vitro stomach and intestinal gut sacs to investigate the effects of consuming a urea-rich meal (700 mM within a 2% body-mass ration of food-slurry) on nitrogen movement across the gastrointestinal (GI) tract of North Pacific spiny dogfish (Squalus acanthias suckleyi). Plasma urea concentrations did not differ between fasted (359 ±19 mM), urea-poor fed (340 ±16 mM), and urea-rich fed (332 ± 24 mM) dogfish. Interestingly, in vitro gut sacs of urea-rich fed dogfish showed no net urea absorption from the lumen over 3 h incubation, which contrasts previously published data on urea-poor fed dogfish that absorb urea from the lumen. In addition, ammonium (NH4+) concentration within the gut sac intestinal lumen significantly increased from 0.62 to 4.35 mM over 3 h. This is likely due to a combination of tissue production and microbial urease activity in the intestine. The overall results highlight the ability of S. a. suckleyi to regulate and maintain internal nitrogen concentrations despite the addition of excess dietary urea.
Hoogenboom, J. L., Weinrauch, A. M., Wood, C. M., and Anderson, W. G. 2020.
The effects of digesting a urea-rich meal on North Pacific spiny dogfish (Squalus acanthias suckleyi)
Comparative Biochemistry and Physiology Part A. 249:110775.
https://doi.org/10.1016/j.cbpa.2020.110775
Understanding dietary nitrogen usage in marine elasmobranchs
After feeding, where does dietary nitrogen go? Is it all shuffled into the ornithine-urea cycle to produce urea? Is it transported around the body as ammonia? How quickly does is it used to produce proteins and amino acids?
Abstract: Marine elasmobranchs are ureosmotic, retaining large concentrations of urea to balance their internal osmotic pressure with that of the external marine environment. The synthesis of urea requires exogenous nitrogen acquired primarily through the gastrointestinal tract. This is the first study to describe the incorporation of dietary nitrogen within a marine elasmobranch. North Pacific spiny dogfish (Squalus acanthias suckleyi) were fed 7 mM 15NH4Cl in a 2% ration by-body-mass of herring-slurry via gavage. Dietary labelled nitrogen was tracked from ingestion to tissue-incorporation and synthesis of nitrogenous compounds (urea, glutamine, bulk amino acids, protein) in the intestinal spiral valve, plasma, liver, and skeletal muscle. Within 20 h post-feeding, we found labelled 15N was incorporated into all nitrogenous compounds of interest within most of the tissues examined. The highest 𝛿15N values were seen in the anterior region of the spiral valve at 20 h post-feeding, suggesting this region was particularly important in assimilating the dietary nitrogen. Enrichment of the 15N-compounds in all tissues examined was sustained throughout the 168 h digestion period, highlighting the ability of these animals to retain and use dietary nitrogen for both somatic and osmoregulatory processes.
Hoogenboom, J. L. and Anderson W.G. 2023. Using 15N to determine the metabolic fate of dietary nitrogen in North Pacific spiny dogfish (Squalus acanthias suckleyi). Journal of Experimental Biology. 226 (13): jeb-244921.
Nitrogen transport along the intestinal spiral valve of cloudy catsharks (Scyliorhinus torazame)
A collaboration with Dr. Susumo Hyodo's Physiology Lab
at the Atmosphere and Ocean Research Institute - University of Tokyo
The mRNA expression of known nitrogen transporters (Rhp2, Rhbg, UT) were investigated within the intestinal spiral valve of cloudy catsharks.
Abstract: As part of their osmoregulatory strategy, marine elasmobranchs retain large quantities of urea to balance the osmotic pressure of the marine environment. The main source of nitrogen used to synthesize urea comes from the digestion and absorption of food across the gastrointestinal tract. In this study we investigated possible mechanisms of nitrogen movement across the spiral valve of the cloudy catshark (Scyliorhinus torazame) through the molecular identification of two Rhesus glycoprotein ammonia transporters (Rhp2 and Rhbg) and a urea transporter (UT). We used immunohistochemistry to determine the cellular localizations of Rhp2 and UT. Within the spiral valve, Rhp2 was expressed along the apical brush-border membrane, and UT was expressed along the basolateral membrane and the blood vessels. The mRNA abundance of Rhp2 was significantly higher in all regions of the spiral valve of fasted catsharks compared to fed catsharks. The mRNA abundance of UT was significantly higher in the anterior spiral valve of fasted catsharks compared to fed. The mRNA transcript of four ornithine urea cycle (OUC) enzymes were detected along the length of the spiral valve and in the renal tissue, indicating the synthesis of urea via the OUC occurs in these tissues. The presence of Rhp2, Rhbg, and UT along the length of the spiral valve highlights the importance of ammonia and urea movement across the intestinal tissues, and increases our understanding of the mechanisms involved in maintaining whole-body nitrogen homeostasis in the cloudy catshark.
Hoogenboom, J.L., Wong, M.K.S., Hyodo, S. and Anderson, W.G., 2023. Nitrogen transporters along the intestinal spiral valve of cloudy catshark (Scyliorhinus torazame): Rhp2, Rhbg, UT. Comparative Biochemistry and Physiology Part A: Molecular & Integrative Physiology, 280:111418.
https://doi.org/10.1016/j.cbpa.2023.111418
Investigating nitrogen movement in North Pacific spiny dogfish (Squalus acanthias suckleyi), with focus on UT, Rhp2, and Rhbg mRNA abundance
Abstract: For ureosmotic marine elasmobranchs, the acquisition and retention of nitrogen is critical for the synthesis of urea. To better understand whole-body nitrogen homeostasis, we investigated mechanisms of nitrogen trafficking in North Pacific spiny dogfish (Squalus acanthias suckleyi). We hypothesized the presence of nitrogen within the spiral valve lumen would affect both the transport of nitrogen and the mRNA abundance of a urea transporter (UT) and two ammonia transport proteins (Rhp2, Rhbg) within the intestinal epithelium. The in vitro preincubation of intestinal tissues in NH4Cl, intended to simulate dietary nitrogen availability, showed that increased ammonia concentrations did not significantly stimulate the net uptake of total urea or total methylamine. We also examined the mRNA abundance of UT, Rhp2, and Rhbg in the gills, kidney, liver, and spiral valve of fasted, fed, excess urea fed, and antibiotic-treated dogfish. After fasting, hepatic UT mRNA was significantly lower, and Rhp2 mRNA in the gills was significantly higher than the other treatments. Feeding significantly increased Rhp2 mRNA levels in the kidney and mid spiral valve region. Both excess urea and antibiotics significantly reduced Rhbg mRNA levels along all three spiral valve regions. The antibiotic treatment also significantly diminished UT mRNA abundance levels in the anterior and mid spiral valve, and Rhbg mRNA levels in the kidney. In our study, no single treatment had significantly greater influence on the overall transcript abundance of the three transport proteins compared to another treatment, demonstrating the dynamic nature of nitrogen balance in these ancient fish.
Hoogenboom, J.L. and Anderson, W.G., 2023. Investigating nitrogen movement in North Pacific spiny dogfish (Squalus acanthias suckleyi), with focus on UT, Rhp2, and Rhbg mRNA abundance. Journal of Comparative Physiology B, 193:439–451.
Nitrogen handling in the elasmobranch gut: a role for microbial urease
Abstract: Ureotelic elasmobranchs require nitrogen for both protein growth and urea-based osmoregulation, and therefore are probably nitrogen-limited in nature. Mechanisms exist for retaining and/or scavenging nitrogen in the gills, kidney, rectal gland and gut, but as yet, the latter are not well characterized. Intestinal sac preparations of the Pacific spiny dogfish shark (Squalus acanthias suckleyi) incubated in vitro strongly reabsorbed urea from the lumen after feeding, but mucosal fluid ammonia concentrations increased with incubation time. Phloretin (0.25 mmol l−1, which blocked urea reabsorption) greatly increased the rate of ammonia accumulation in the lumen. A sensitive [14C] urea-based assay was developed to examine the potential role of microbial urease in this ammonia production. Urease activity was detected in chyme/intestinal fluid and intestinal epithelial tissue of both fed and fasted sharks. Urease was not present in gall-bladder bile. Urease activities were highly variable among animals, but generally greater in chyme than in epithelia, and greater in fed than in fasted sharks. Comparable urease activities were found in chyme and epithelia of the Pacific spotted ratfish (Hydrolagus colliei), a ureotelic holocephalan, but were much lower in ammonotelic teleosts. Urease activity in dogfish chyme was inhibited by acetohydroxamic acid (1 mmol l−1) and by boiling. Treatment of dogfish gut sac preparations with acetohydroxamic acid blocked ammonia production, changing net ammonia accumulation into net ammonia absorption.We propose that microbial urease plays an important role in nitrogen handling in the elasmobranch intestine, allowing some urea-N to be converted to ammonia, which is then reabsorbed for amino acid synthesis or reconversion to urea
Wood, C.M., Liew, H.J., De Boeck, G., Hoogenboom, J.L. and Anderson, W.G., 2019.
Nitrogen handling in the elasmobranch gut: a role for microbial urease. Journal of Experimental Biology, 222(3).
How do basking sharks use the Bay of Fundy, Canada?
Little is currently known about the population dynamics of basking sharks (Cetorhinus maximus) at regional or local scales. Using a long-term sighting database (1994–2012) and photo-identification of individuals, we studied the seasonal and inter-annual patterns in basking shark occurrence and site fidelity in the Bay of Fundy, Canada.
Basking sharks can be identified by markings on their dorsal fins
From unique dorsal fin markings, 98 individual sharks were identified from photographs taken between 1997 and 2012. Four of individual basking sharks were re-sighted in subsequent years, with the longest interval between re-sightings being 9.1 years. These re-sightings suggest some site fidelity by individuals and demonstrate the longevity of some mark-types on the first dorsal fin. This study highlights the role of long-term sightings and photographic records as population assessment tools for regional scale-monitoring of a globally vulnerable species.
Environmental predictors and temporal patterns of basking shark (Cetorhinus maximus) occurrence in the
lower Bay of Fundy, Canada
Abstract: Little is currently known about the population dynamics of basking sharks (Cetorhinus maximus) at regional or local scales. Using a long-term sighting database (1994–2012) and photo-identification of individuals, we studied the seasonal and inter-annual patterns in basking shark occurrence and site fidelity in the Bay of Fundy, Canada. Zero-inflated negative binomial models quantified spatial, temporal and environmental predictors of shark sighting rates. The probability of sighting a basking shark increased in August, and in deep water offshore; this may reflect the distribution and availability of calanoid copepod prey. Sea-surface temperature (SST) had no effects on shark sightings, but there was a negative correlation between the North Atlantic Oscillation (NAO) index and shark sightings lagged at two and four years; the former possibly due to the position of the Gulf Stream and the latter likely a result of the lagged influence of the NAO on copepod abundance. The model also showed a significant decline in the occurrence of basking sharks within the Bay of Fundy over the study period. From unique markings on dorsal fins, 98 individual sharks were identified from photographs taken between 1997 and 2012. Four of these individuals were re-sighted in subsequent years, and the longest interval between re-sightings was 9.1 years. These re-sightings suggest some site fidelity by individuals and demonstrate the longevity of some mark-types on the first dorsal fin. This study highlights the role of long-term sightings and photographic records as population assessment tools for regional scale-monitoring of a globally vulnerable species
Hoogenboom, J.L., Wong, S.N., Ronconi, R.A., Koopman, H.N., Murison, L.D. and Westgate, A.J., 2015. Environmental predictors and temporal patterns of basking shark (Cetorhinus maximus) occurrence in the lower Bay of Fundy, Canada. Journal of Experimental Marine Biology and Ecology, 465:24-32
Sperm whales fluke markings differ between
the Atlantic and Pacific oceans
My undergraduate Honours thesis focused on historical predation pressures that sperm whales face in the Atlantic and Pacific oceans. Throughout their life times, individuals accumulate markings and scars from predation, as well as intraspecific interactions with other sperm whales. These markings can be used to identify individuals over many years.
Photo-identification of marine mammals
Photo-identification is a non-invasive tool researchers can use to identify wildlife. Many characteristics, such as marks and scars are long-lasting and can be used to track individuals for days, months, or years. This long-term tracking can give insight into life cycle, lifespan, migration, and social patterns. My research with the
Whitehead Cetacean Research Lab at
Dalhousie University focused on the photo-identification of marine mammals such as long-finned pilot whales, sperm whales, and Northern bottlenosed whales. I used these photo-id techniques to create the first catalogue of basking sharks that frequent the Bay of Fundy.
