Feeding habits of the blue shark, Prionace glauca, and salmon shark, Lamna ditropis, in the transition region of the Western North Pacific

We describe the feeding habits of 70 blue sharks (Prionace glauca) and 39 salmon sharks (Lamna ditropis) caught at 0–7 m depth at night by research drift gillnets in the transition region of the western North Pacific during April–May of 1999 and 2000. Blue sharks of 50–175 cm total length fed on a large variety of prey species, consisting of 24 species of cephalopods and 16 species of fishes. Salmon sharks of 69–157 cm total length fed on a few prey species, consisting of 10 species of cephalopods and one species of fish. Important prey for the blue sharks were large, non-active, gelatinous, meso- to bathypelagic cephalopods (e.g., Chiroteuthis calyx, Haliphron atlanticus, Histioteuthis dofleini and Belonella borealis) and small myctophid fishes. Important prey for the salmon sharks were mid-sized, active, muscular, epi- to mesopelagic squids (e.g. Gonatopsis borealis, Onychoteuthis borealijaponica and Berryteuthis anonychus). Our results suggest that blue sharks feed on cephalopods mainly during the daytime when they descend to deep water. Salmon sharks may feed opportunistically with no apparent diurnal feeding period. Blue sharks and salmon sharks have sympatric distribution in the transition region in spring; they have different feeding habits and strategies that reduce competition for food resources.     

Archival Tagging Of School Shark, Galeorhinus Galeus, in Australia: Initial Results

Archival tags were used to study the movement and depth behaviour of school sharks, Galeorhinus galeus, in southern australia. Thirty fish were tagged in late 1997, and to date there have been nine recaptures (30% recapture rate). Periods at liberty varied from 8 days to 18 months. The sharks spent about 80% of their time on the continental shelf, and appeared to swim close to the bottom during the day. At night they often ascended for periods of up to several hours, except at times around the full moon. When in deep water, the sharks typically descended at dawn to depths of up to 600m, before ascending at dusk. It was not possible to use the light data from the tags to estimate position when the sharks were in deep water, because they were often at depths beyond the sensitivity of the tag. In shallower water, longitude was estimated from the light data but latitude was estimated from the maximum daily depth, assuming the fish were on the bottom. The timing of the dives in deepwater appeared sufficiently regular to offer the prospect of using it to estimate longitude. We propose future research using archival tags on this species should address questions about female reproductive migrations, pelagic behaviour and vertical movements.     

Habitat selection by juvenile lemon sharks,Negaprion brevirostris

Synopsis We surgically implanted ultrasonic transmitters in 38 lemon sharks,Negaprion brevirostris, and manually tracked the sharks for 1–153 days. This yielded 2281 positional fixes recorded at 15-min intervals. We used these positional data with availability data of four environmental variables (water depth, temperature, salinity, and bottom type), sampled at 213 stations along 15 transects, to examine usage of habitat. All sharks used contours of water depth, water temperature, and bottom type disproportionately to the availability of these variables in the study site. Specifically, juvenile lemon sharks selected shallower, warmer water with an underlying rocky or sandy substrate, perhaps for predator avoidance. This is the first report on habitat selection by any elasmobranch.     

Space Utilization and Swimming Depth of White Sharks, Carcharodon carcharias, at the South Farallon Islands, Central California

This paper presents information on the movements of white sharks, Carcharodon carcharias, at the South Farallon Islands (SFI), central California. Acoustic telemetry techniques provided preliminary data on the diurnal space utilization, movement patterns and swimming depths of four white sharks, ranging from approximately 3.7 to 4.9m in length. Sharks swam within about 10m of the bottom to depths of approximately 30m, but in deeper water they tended to stray more from the bottom. Activity spaces for time periods tracked ranged from 1.84 to 9.15km². Indications are that an inverse relationship exists between length and activity space. During the time tracked, larger individuals swam within particular areas around the islands whereas smaller individuals did not restrict their movements in the same manner. Values of a site attachment index were inversely related to length for all sharks tracked. The site attachment indices, apparent inverse relationship between total length and activity space and observations on telemetered and other known individuals support a hypothesis that larger sharks possess site fidelity in their search for prey at SFI, within and between years. With the high frequency of predation by white sharks on juvenile northern elephant seals at SFI in the fall, the majority of the sharks' movements are probably related to their search for these pinniped prey. These data provide preliminary evidence that white sharks at SFI may search for prey by swimming in a particular area over a number of days or weeks, traversing the area in a manner which maximizes coverage, and swimming close to the bottom or at a distance far enough from the surface to remain cryptic from prey.     

Short‐term survival and movements of Atlantic sharpnose sharks captured by hook‐and‐line in the north‐east Gulf of Mexico

Ultrasonic telemetry was used to compare post‐release survival and movements of Atlantic sharpnose sharks Rhizoprionodon terraenovae in a coastal area of the north‐east Gulf of Mexico. Ten fish were caught with standardized hook‐and‐line gear during June to October 1999. Atlantic sharpnose sharks were continuously tracked after release for periods of 0·75 to 5·90 h and their positions recorded at a median interval of 9 min. Individual rate of movement was the mean of all distance and time measurements for each fish. Mean ± s.e . individual rate of movement was 0·45 ± 0·06 total lengths per second (L_T s^?1) and ranged from 0·28 to 0·92 L_T s^?1 over all fish. Movement patterns did not differ between jaw and internally hooked Atlantic sharpnose sharks. Individual rate of movement was inversely correlated with bottom water temperature at capture (r² = 0·52, P ≤ 0·05). No consistent direction in movement was detected for Atlantic sharpnose sharks after release, except that they avoided movement towards shallower areas. Capture‐release survival was high (90%), with only one fish not surviving, i.e. this particular fish stopped movement for a period of 10 min. Total rate of movement was total distance over total time (m min^?1) for each Atlantic sharpnose shark. Mean total rate of movement was significantly higher immediately after release at 21·5 m min^?1 over the first 1·5 h of tracking, then decreased to 11·2 m min^?1 over 1·5–6 h, and 7·7 m min^?1 over 3–6 h (P ≤ 0·002), which suggested initial post‐release stress but quick recovery from capture. Thus, high survival (90%) and quick recovery indicate that the practice of catch‐and‐release would be a viable method to reduce capture mortality for R. terraenovae.     

Vertical distribution in and isolation of bacteria from Lake Vanda: an Antarctic lake

Vertical distribution of bacteria in Lake Vanda, an Antarctic meromictic lake, was examined by the acridine orange epifluorescence direct count method. Total bacteria were 10⁴–10⁵ cells · ml^–1 in the water at 55 m depth and above, and increased drastically to 10⁷ cells · ml^–1 in the bottom water. Filamentous or long rodshaped bacteria occurred at a high frequency in the upper layers, but in the bottom layers most bacteria were coccoidal or short rods. Mean bacterial cell volume in water of between 10 m and 60 m deep was fairly large compared with common bacterial populations in seawater and lake water. Aerobic heterotrophic bacteria were recovered from the water of a depth of 30 m and above, and were assumed to belong to Caulobacter. Viable heterotrophic bacteria were not recovered from the high salinity deep water by media prepared with the same deep water. Phototrophic purple non-sulphur bacteria were isolated by enrichment cultures from water at 55 m depth.     

Homeothermy in adult salmon sharks, <Emphasis Type="Italic">Lamna ditropis</Emphasis>

Salmon sharks, Lamna ditropis, belong to a small group of sharks that possess vascular counter-current heat exchangers (retia mirabilia) allowing retention of metabolically generated heat, resulting in elevated body temperatures. The capacity of free-swimming lamnid sharks to regulate rates of heat gain and loss has not been demonstrated. Using acoustic telemetry, we recorded swimming depth and stomach temperature from four free-swimming salmon sharks in Prince William Sound, Alaska. Temperature data were obtained over time periods ranging from 3.8 to 20.7 h. Temperature profiles of the water column were obtained concurrently for use as estimates of ambient temperature. Mean stomach temperature among four individuals tracked ranged from 25.0 to 25.7°C. These sharks defended specific elevated temperatures regardless of changes in ambient temperature, which ranged from about 5–16°C. The maximum observed elevation of stomach temperature over ambient was 21.2°C. Because stomach temperatures were so strictly maintained relative to changes in ambient temperature, a thermal rate coefficient, k, (°C min^–1 °C thermal gradient^–1) for cooling of 0.053 min^–1 was obtained via a `control' experiment with a dead salmon shark. We show that free-swimming adult salmon sharks maintain a specific stomach temperature independent of changes in ambient temperature through a combination of physical and physiological means, and essentially function as homeotherms. This unique ability is probably the underlying factor in the evolutionary niche expansion of salmon sharks into boreal waters and in their ability to actively pursue and capture highly active prey such as salmon.     

Seasonal foraging movements and migratory patterns of female Lamna ditropis tagged in Prince William Sound, Alaska

Conventional and electronic tags were used to investigate social segregation, distribution, movements and migrations of salmon sharks Lamna ditropis in Prince William Sound, Alaska. Sixteen salmon sharks were tagged with satellite transmitters and 246 with conventional tags following capture, and were then released in Prince William Sound during summer 1999 to 2001. Most salmon sharks sexed during the study were female (95%), suggesting a high degree of sexual segregation in the region. Salmon sharks congregated at adult Pacific salmon Oncorhynchus spp. migration routes and in bays near Pacific salmon spawning grounds in Prince William Sound during July and August. Adult Pacific salmon were the principal prey in 51 salmon shark stomachs collected during summer months in Prince William Sound, but the fish appeared to be opportunistic predators and consumed sablefish Anoplopoma fimbria, gadids, Pacific herring Clupea pallasi, rockfish Sebastes spp. and squid (Teuthoidea) even when adult Pacific salmon were locally abundant. As Pacific salmon migrations declined in late summer, the salmon sharks dispersed; some continued to forage in Prince William Sound and the Gulf of Alaska into autumn and winter months, while others rapidly moved south‐east thousands of kilometres toward the west coasts of Canada and the U.S. Three movement modes are proposed to explain the movement patterns observed in the Gulf of Alaska and eastern North Pacific Ocean: ‘focal foraging’ movements, ‘foraging dispersals’ and ‘direct migrations’. Patterns of salmon shark movement are possibly explained by spatio‐temporal changes in prey quality and density, an energetic trade‐off between prey availability and water temperature, intra‐specific competition for food and reproductive success. Transmissions from the electronic tags also provided data on depth and water temperatures experienced by the salmon sharks. The fish ranged from the surface to a depth of 668 m, encountered water temperatures from 4·0 to 16·8° C and generally spent the most time above 40 m depth and between 6 and 14° C (60 and 73%, respectively).     

Survivorship of Cyclops abyssorum tatricus (Cyclopoida, Copepoda) and Boeckella gracilipes (Calanoida, Copepoda) under ambient levels of solar UVB radiation in two high-mountain lakes

We performed in situ experiments during the summer of 1995 and 1996 to assess the potential effect of solar ultraviolet B (UVB) radiation (290-320 nm) on the survival of Cyclops abyssorum tatricus Kozminski and Boeckella gracilipes Daday. These species are numerically dominant within the crustacean zooplankton living in two high-mountain lakes, one located in the Austrian Alps [Gossenkollesee (GKS), 2417 m above sea level, maximum depth 9.9 m] and another in the Chilean Andes (Laguna Negra, 2700 m above sea level, maximum depth 320 m). The copepods were incubated in quartz tubes (1 1) or in quartz tubes wrapped with Mylar D® to exclude most of the UVB radiation. The organisms were exposed at 0.5 m depth for 10-72 h on cloudless days. Both lakes were very transparent to UVB and 10% of the surface radiation at the nominal wavelength of 305 nm was still present at 9.6 m in GKS and at 12.8 in Laguna Negra. These species migrate vertically and have a maximum daytime distribution close to the bottom (C.abyssorum tatricus) or below 15 m depth (B.gracilipes). Both species were red, but the carotenoid concentration was higher in C.abyssorum tatricus than in B.gracilipes (6.5 and 2.3 g mg^-1 dry weight, respectively). UV-absorbing compounds with a maximum absorption at 334 nm were also detected. Cyclops abyssorum tatricus was highly resistant to UVB radiation and no significant lethal effect was observed. Boeckella gracilipes had a mortality 5 times higher in the treatment receiving full sunlight than in the Mylar treatment (3.2%) only when exposed for 70 h. The resistance of B.gracilipes was higher than that reported in the literature for the same species, suggesting the existence of intraspecific differences in UV sensitivity.      

Age and growth of the sandbar shark, Carcharhinus plumbeus, in Hawaiian waters through vertebral analysis

Age and growth estimates were determined for the sandbar shark, Carcharhinus plumbeus, from Oahu, Hawaii in the central Pacific Ocean. Age estimates were obtained through vertebral centra analysis of 187 sharks. We verified our age estimates through marginal increment analysis of centra and oxytetracycline marking methods of at liberty sandbar sharks. Sizes of sampled sharks ranged from 46 to 147 cm pre-caudal length. Four growth models were fitted to length-at-age data; two forms of the von Bertalanffy growth model, the Gompertz growth model, and a logistic growth model. Males and females exhibited statistically significant differences in growth, indicating that females grow slower and attain larger sizes than males. Growth parameter estimates revealed slower growth rates than previously estimated (based on captive specimens) for Hawaiian sandbar sharks. The von Bertalanffy growth model using empirical length-at-birth provided the best biological and statistical fit to the data. This model gave parameter estimates of L _∞ = 138.5 cm PCL and k = 0.12 year⁻¹ for males and L _∞ = 152.8 cm PCL, k = 0.10 year⁻¹ for females. Male and female sandbar sharks mature at approximately 8 and 10 years of age, respectively.     

Distribution and food habits of the slender smoothhound,Gollum attenuatus,from the waters around New Zealand

Three hundred and seventy-three females and 385 malesof Gollum attenuatus were caught with bottom longlines from the Challenger Plateau, Wanganella Bank and Three Kings Ridge around New Zealand. The sharks ranged in size from 440 to 1092 mm TL for females and 442 to 1067 mm TL for males. Catch rates (number of fish per 100 hooks) were 0.41 for the Challenger Plateau, 0.14 for the Wanganella Bank, and 4.52 for the Three Kings Ridge. The sex ratio was 1:1. In terms of 100-m depth intervals,G. attenuatus was most abundant in 500 to 599 m depths. There was a marked decrease in catch rates with increased distance of the hooks from the bottom. Stomachs contained such varied items as sharks, teleost fishes, gastropods, squids, octopi, decapods, isopods, brittle stars, and human garbage. The data in this study suggest thatG. attenuatus consumed, in order of importance, fishes and crustaceans.     

Vertical Movement Patterns and Ontogenetic Niche Expansion in the Tiger Shark,Galeocerdo cuvier

Sharks are top predators in many marine ecosystems and can impact community dynamics, yet many shark populations are undergoing severe declines primarily due to overfishing. Obtaining species-specific knowledge on shark spatial ecology is important to implement adequate management strategies for the effective conservation of these taxa. This is particularly relevant concerning highly-mobile species that use wide home ranges comprising coastal and oceanic habitats, such as tiger sharks, Galeocerdo cuvier. We deployed satellite tags in 20 juvenile tiger sharks off northeastern Brazil to assess the effect of intrinsic and extrinsic factors on depth and temperature usage. Sharks were tracked for a total of 1184 d and used waters up to 1112 m in depth. The minimum temperature recorded equaled 4°C. All sharks had a clear preference for surface (< 5 m) waters but variability in depth usage was observed as some sharks used mostly shallow (< 60 m) waters whereas others made frequent incursions into greater depths. A diel behavioral shift was detected, with sharks spending considerably more time in surface (< 10 m) waters during the night. Moreover, a clear ontogenetic expansion in the vertical range of tiger shark habitat was observed, with generalized linear models estimating a ~4-fold increase in maximum diving depth from 150- to 300-cm size-classes. The time spent in the upper 5 m of the water column did not vary ontogenetically but shark size was the most important factor explaining the utilization of deeper water layers. Young-of-the-year tiger sharks seem to associate with shallow, neritic habitats but they progressively move into deeper oceanic habitats as they grow larger. Such an early plasticity in habitat use could endow tiger sharks with access to previously unavailable prey, thus contributing to a wider ecological niche.     

Biology of the Galapagos shark,Carcharhinus galapagensis, in Hawai'i

Synopsis Catch records from the Hawai'i Cooperative Shark Research and Control Program, which operated in Hawai'i from 1967–1969, were examined and data on the Galapagos shark,Carcharhinus galapagensis were analyzed. A total of 304 Galapagos sharks was caught, predominantly with longlines. More female sharks were caught than males, and the catch was skewed geographically. On the island of O'ahu the highest catch rates occurred along the north and south coasts. High catch rates also occurred near points of land, where longshore currents converge. Average depth of capture was greater for juveniles (45.1 m) and mature males (60.2 m), than for subadults (38.8 m) and mature female sharks (34.2 m). Males appear to reach maturity between 205 and 239 cm total length, and females between 215 and 245 cm. Litter size ranged from 4 to 16 pups, with an average of 8.7. In Hawaiian waters Galapagos sharks are born at just over 80 cm total length. Mating and parturition apparently occur early in the year, and gestation is estimated to be about 12 months. Stomach contents consisted mainly of teleosts and benthic prey, and ontogenetic changes in diet occurred as sharks increased in size. Sharks consumed a smaller proportion of teleosts and more elasmobranchs with increasing size. Dietary diversity also increased with increasing size of shark.     

Size frequency,dispersal distances and variable growth rates of young sharks in a multi-species aggregation

During a mark–recapture survey from November 2014 until April 2017, 333 neonatal and juvenile blacktip reef sharks Carcharhinus melanopterus and 302 neonatal and juvenile sicklefin lemon sharks Negaprion acutidens were tagged and measured at the uninhabited and isolated St. Joseph Atoll (Republic of Seychelles). Both species demonstrated seasonal reproductive synchronicity and relatively large sizes at birth. Despite the extended times at liberty > 2.5 years, the majority of recaptures were found in close proximity to the initial tagging location (< 500 m). Annual growth rates of C. melanopterus (n = 24) and N. acutidens (n = 62) ranged from 6.6 to 31.7 cm year⁻¹ (mean ± SE; 16.2 ± 1.2 cm year⁻¹) and 0.2 to 32.2 cm year⁻¹ (11.8 ± 1 cm year⁻¹), respectively and are to date the most variable ever recorded in wild juvenile sharks. High abundances of both species coupled with long-term and repeated recaptures are indicative of a habitat where juveniles can reside for their first years of life. However, large variability in annual growth rates in both species may suggest high intra and interspecific competition induced by a possibly resource limited, isolated habitat.     

Vertical distribution of organic constituents in an Antarctic lake: Lake Fryxell

Vertical distribution of organic constituents, i.e. total organic carbon (TOC), hydrocarbons, fatty acids and hydroxy acids in water and sediment samples from Lake Fryxell (77° 35 S, 163° 15 E) of southern Victoria Land, Antarctica were studied to elucidate their features in relation to stratification of the lake waters and likely distribution of microorganisms. The TOC content of the surface water (5.0 m; just below the ice cover of 4.50 m thickness) was 1.4 mg l^–1. It increased markedly with depth and attained a maximum value of 21.7 mg C l^–1 at a depth of 17.5 m, but decreased to the bottom (13.3 mg C l^–1). The high TOC content of the anoxic bottom layers (> 15 m) is attributable to the concentration of refractory organic substances over long periods following the degradation of labile organic constituents. Hydrocarbons were not found in the water column, but the major constituent of the bottom sediment was n-C_{29 : 2} alkene. Total concentrations of fatty acids in the oxic layers ( 10 m) were highest at 10.0 m and much higher than those in the anoxic layers (> 10 m), probably reflecting the phytoplankton population. The content of branched (iso and anteiso) fatty acids and 3-hydroxy acids in the anoxic layers were much greater than those in the oxic layers which would seem to reflect the distribution of bacterial abundance. The differences of organic composition between the water column and sediments imply that sinking dead organisms were quickly degraded in the lake bottom. Also, the composition of microorganisms in the water column must be very different from that in the sediments.     

Tidal Influence on Spatial Dynamics of Leopard Sharks, Triakis semifasciata, in Tomales Bay, California

We used ultrasonic telemetry to determine the movement directions and movement rates of leopard sharks, Triakis semifasciata, in Tomales Bay, California. To analyze tide and time of day effects, we surgically implanted transmitters in the peritoneal cavities of one male and five female leopard sharks, which we located during summer for three to five sampling sessions lasting 12 to 24h each. All leopard sharks showed strong movement direction patterns with tide. During incoming tides, sharks moved significantly (p<0.0001) towards the inner bay, apparently to exploit the extensive inner bay muddy littoral zones' food resources. On outgoing tides, sharks showed significant (p<0.0001) movements towards the outer bay. During high tide, there was no discernible pattern to their movements (p=0.092). Shark movement rates were significantly (p<0.0001) greater during dark periods (mean±SE: 10.5±1.0m min^–1), compared with fully lighted ones (6.7±0.5m min^–1). Movement rates of longer sharks tended to be greater than those of shorter ones (range means±SE: 5.8±0.6m min^–1 for the 91cm shark, to 12.8±1.6m min^–1 for the 119cm shark), but the leopard sharks' overall mean movement rate (8.1±0.5m min^–1) was slower than other (more pelagic) sharks.     

Emergence of Chironomidae from Findley Lake and two ponds in the Cascade Mountains,U.S.A.

From Findley Lake and two ponds in the Cascade Mountains (U.S.A.) Chironomidae started to emerge as soon as the ice thawed in 1972 and 1973. The 1051 Chironomidae that emerged per m² from Findley Lake included 307 individuals ofTanytarsus and 358 ofProcladius. Tanytarsus outnumberedChironomus at all depths from 0 to 19.3 m. The 1506 Chironomidae that emerged per m² from the 4.5 m deep pond included 997 individuals ofTanytarsus. The 1490 Chironomidae that emerged per m² from the 2 m deep pond included 1363 Tanytarsus. The emerging biomass was 216 and 186 mg dry weight per m² in Findley Lake and the 4.5 m deep pond, respectively, and only 78 mg in the 2 m deep pond. Most species had their maximum emergence where there was organic detritus from the surrounding forest.Orthocladius emerged from sand bottom at 2.7 to 5.2 m depth.Stictochironomus emerged from mud and sand at 8.4 m depth.Chironomus emerged from soft mud at 15 to 27.5 m depth.     

Demography and movement patterns of leopard sharks (Triakis semifasciata) aggregating near the head of a submarine canyon along the open coast of southern California, USA

The demography, spatial distribution, and movement patterns of leopard sharks (Triakis semifasciata) aggregating near the head of a submarine canyon in La Jolla, California, USA, were investigated to resolve the causal explanations for this and similar shark aggregations. All sharks sampled from the aggregation site (n?=?140) were sexually mature and 97.1 % were female. Aerial photographs taken during tethered balloon surveys revealed high densities of milling sharks of up to 5470 sharks ha^?1. Eight sharks were each tagged with a continuous acoustic transmitter and manually tracked without interruption for up to 48 h. Sharks exhibited strong site-fidelity and were generally confined to a divergence (shadow) zone of low wave energy, which results from wave refraction over the steep bathymetric contours of the submarine canyon. Within this divergence zone, the movements of sharks were strongly localized over the seismically active Rose Canyon Fault. Tracked sharks spent most of their time in shallow water (≤2 m for 71.0 % and ≤10 m for 95.9 % of time), with some dispersing to deeper (max: 53.9 m) and cooler (min: 12.7 °C) water after sunset, subsequently returning by sunrise. These findings suggest multiple functions of this aggregation and that the mechanism controlling its formation, maintenance, and dissolution is complex and rooted in the sharks’ variable response to numerous confounding environmental factors.     

Predation and depth effects on abundance and size distribution of an invasive bivalve, the golden mussel Limnoperna fortunei, in a dam reservoir

We investigated the predation and depth effects on abundance and size distribution of an invasive freshwater bivalve, Limnoperna fortunei, post-larvae using screened (5-, 25-, and 50-mm mesh openings) and unscreened cages at three depths (6, 12, and 1 m above the bottom) in a dam reservoir, Lake Ohshio of Japan. The densities of L. fortunei on the unscreened cages were much lower than those of the 5-mm mesh screened cages at any depth. The predation rates estimated by dividing the density of the unscreened cage by that of the 5-mm mesh screened cage were 97.3% at 6 m depth, 96.6% at 12 m depth, and 95.8% at the bottom. Shell lengths of L. fortunei post-larvae were small on bottom cages, and their size distribution was significantly different from those at other depths. These results suggest that predation and depth affected the abundance and size distribution of L. fortunei in the lentic environment. The impact of L. fortunei invasion on the food web and the possibility of regulating their population by predation pressures are also discussed.     

Unique osmoregulatory morphology in primitive sharks: an intermediate state between holocephalan and derived shark secretory morphology

Discovery of an unusual rectal gland in the Atlantic sixgill shark Hexanchus vitulus led us to examine the rectal glands of 31 species of sharks to study diversity in rectal-gland morphology. Twenty-four of 31 species of sharks had digitiform glands (mean width–length ratio ± SD = 0.17 ± 0.04) previously assumed to be characteristic of all elasmobranchs regardless of habitat depth or phylogenetic age. Rectal glands from the family Somniosidae were kidney bean-shaped (mean width: length ± SD = 0.46 ± 0.05); whereas those from families Echinorhinidae and Hexanchidae were lobulate (mean width: length ± SD = 0.55 ± 0.06). Rectal gland width: length were different among species with digitiform morphology and lobulate morphology (ANOVA; R² = 0.9; df = 15, 386; 401, F = 219.24; P < 0.001). Histological and morphological characteristics of the digitiform morphology from deep-sea sharks were similar to those from shallow-water sharks. Histology of lobulate rectal glands from hexanchids were characterised by tubule bundles separated by smooth muscle around a central lumen. Additionally, we examined plasma chemistry of four species of sharks with digitiform rectal glands and two species with lobulate rectal-gland morphology to see if there were differences between morphologies. Plasma chemistry analysis showed that urea and trimethylamine N-oxide (TMAO) followed the piezolyte hypothesis, with TMAO being highest and urea being lowest in deep-sea sharks. Among electrolytes, Na⁺ was highest in species with lobulate rectal glands. Hexanchids and echinorhinids both have lobulate rectal glands similar to those of holocephalans, despite the more than 400 million years separating these two groups. The morphological similarities between the lobulate rectal-gland anatomy of primitive sharks and the secretory morphology of holocephalans may represent an intermediate state between Holocephali and derived shark species.