sharks of the open ocean || pelagic elasmobranch diversity

Chapter 2

Pelagic Elasmobranch Diversity

Leonard J. V. Compagno

Abstract

Pelagic sharks include oceanic and semi-oceanic species of sharks and rays, active free-swimming species that live in the world’s oceanic basins and over the continental and insular slopes and rises. The group is relatively small, comprising about 6% of the world’s total number of cartilaginous fi sh species. Pelagic sharks show few derived body plans or ecomorphotypes compared to coastal shark species, but have two ecomorphotypes special-ized for life in the open ocean (macroceanic and microceanic). They are best represented among the orders Squaliformes, Carcharhiniformes, Lamniformes, and Rajiformes, with a few members of the Hexanchiformes and Orectolobiformes and none from the other shark orders. This chapter presents an annotated checklist and discussion of pelagic sharks, and also considers the impact of human activities on these species.

Key words: biodiversity, taxonomy, pelagic sharks, ecomorphotypes, Squaliformes, Carcharhiniformes, Lamniformes, Rajiformes, Hexanchiformes, Orectolobiformes.

Introduction

There have been few reviews of the biodiversity of pelagic sharks (Casey et al., 1992). The term “shark” as used here includes the batoids (skates and rays) as well as the true sharks. “Pelagic shark” refers to highly mobile species that are not closely associated with the sea bottom, as opposed to benthic or bottom-dwelling sharks. “Oceanic” species include those pelagic sharks that live their lives in whole or in part in the ocean basins away from continental landmasses, although some of these species come to the edges of the continental and insular shelves (in water about 200 m deep) and may come close inshore over the shelves to feed, breed, or partake in other activities, including social inter-actions. These oceanic species may be epipelagic, confi ned to the upper, sunlit level or epipelagic zone of the oceans (from the surface to about 200 m deep); mesopelagic, living in the twilight zone below the epipelagic zone where little light penetrates (from 200 to 1,000 m); or bathypelagic, inhabiting the sunless zone below 1,000 m and extending to the deep slopes, rises, ocean fl oor, and trenches (down to 6,000 m or more). “Semipelagic” sharks penetrate oceanic waters but concentrate close to continental landmasses over the continental slopes and rises.

Sharks of the Open Ocean: Biology, Fisheries and Conservation. Edited by M. D. Camhi, E. K. Pikitch and E. A. Babcock

© 2008 Blackwell Publishing Ltd. ISBN: 978-0632-05995-9

Pelagic Elasmobranch Diversity 15

Pelagic shark diversity

The diversity of pelagic sharks is very low compared to that of shelf- and slope-dwelling sharks; Compagno (1990) estimated that about 2% of cartilaginous fi shes were oceanic. Using the present taxonomical estimate of 1,164 species of cartilaginous fi shes, including undescribed and dubious species, a revised estimate of the numbers of oceanic and semipe-lagic sharks suggests that 2.7% of all living cartilaginous fi shes are oceanic, while an addi-tional 2.8% are semipelagic (Table 2.1). The orders Squaliformes and Lamniformes dominate the oceanic group in species diversity, while members of the order Carcharhiniformes prob-ably dominate in biomass. The orders Carcharhiniformes, Rajiformes, and Squaliformes are the most diverse semipelagic sharks. No chimaeras (Chimaeriformes) are oceanic or semi-pelagic, with most species living on the continental and insular slopes.

Compagno (1990) analyzed sharks in terms of ecomorphotypes, which can include diverse taxa that may or may not be phyletically related but are grouped together by simi-larities in morphology, habitat, and behavior. Oceanic sharks exhibit several of these eco-morphotypes: the high-speed or tachypelagic, tuna-shaped morphotype for members of Lamnidae, such as the porbeagle, salmon shark, and shortfi n mako; the archipelagic or modifi ed tachypelagic superpredator morphotype for the white shark (also Lamnidae); the macroceanic morphotype of large oceanic sharks with long pectoral fi ns, such as the blue shark, oceanic whitetip shark, silky shark, threshers, longfi n mako, and megamouth shark; the microceanic morphotype of small- to moderate-sized oceanic sharks with long, fusi-form bodies and small pectoral fi ns, including dwarf members of the Squaliformes and certain members of the Lamniformes such as the crocodile shark; the modifi ed rajoben-thic or skatelike rhomboidal form of the pelagic stingray; and the aquilopelagic or eagle-ray morphotype of oceanic devilrays (Mobulidae). Semipelagic sharks share morphotypes found in slope- and shelf-dwelling cartilaginous fi shes, including several types not seen in specialized oceanic sharks.

The following survey describes the oceanic and semipelagic sharks, arranged by order and family. General distributional information on these groups is derived in part from

Table 2.1 Number of oceanic and semipelagic shark species among six orders, and as a percentage of all species in the respective orders.*

Order Number of % of Number of % of oceanic species semipelagic species species in order species in order

Orectolobiformes 1 2.9 – –Lamniformes 11 73.3 3 20.0Carcharhiniformes 3 1.1 15 5.3Hexanchiformes – – 2 33.3Squaliformes 9–13 6.0–8.7 3–5 2.0–3.3Rajiformes 2–3 0.3–0.5 8 1.2Total number of species (and % of all 26–31 31–33 cartilaginous species) (2.2–2.6%) (2.6–2.8%)

*Percentages were calculated by dividing the maximum number of oceanic or semipelagic species into the maximum number of species in each order.

16 Sharks of the Open Ocean

Bigelow and Schroeder (1948, 1953, 1957), Compagno (1984, 1988, 2001, 2002, 2007a, b), Compagno and Niem (1998), Compagno et al. (1995, 1999, 2005), Ebert (1990), Fowler (1941), Garman (1913), Last and Stevens (1994), McEachran and de Carvalho (2002), McEachran and Notarbartolo di Sciara (1995), and Notarbartolo di Sciara (1987), as well as from unpublished database and fi eld report fi les on chondrichthyan distribution devel-oped by the writer over the past two decades for FAO publications and other work.

Order Orectolobiformes: carpet sharksFamily Rhincodontidae: whale sharksRhincodon typus (Smith, 1828). Whale shark. Oceanic, semipelagic, and littoral.

Carpet sharks are mostly benthic and inshore species, though some are littoral and more active swimmers (families Stegostomatidae, Ginglymostomatidae). A notable exception among orectoloboids is the whale shark, a pelagic species often found near oceanic islands that feeds on plankton in both coastal and oceanic waters. Its oceanic movements have been studied by conventional tag recoveries and by satellite tracking in the eastern Pacifi c and Indian Ocean.

Order Lamniformes: mackerel sharksFamily Mitsukurinidae: goblin sharksMitsukurina owstoni (Jordan, 1898). Goblin shark. Semipelagic, benthic on continental slopes.

Family Odontaspididae: sand tiger sharksOdontaspis ferox (Risso, 1810). Smalltooth sand tiger. Littoral, benthic, and semipelagic, on ridge systems and seamounts far from land.Odontaspis noronhai (Maul, 1955). Bigeye sand tiger. Oceanic and rarely littoral.

Family Pseudocarchariidae: crocodile sharksPseudocarcharias kamoharai (Matsubara, 1936). Crocodile shark. Oceanic and semipelagic.

Family Megachasmidae: megamouth sharksMegachasma pelagios (Taylor, Compagno and Struhsaker, 1983). Megamouth shark. Oceanic and semipelagic.

Family Alopiidae: thresher sharksAlopias pelagicus (Nakamura, 1935). Pelagic thresher. Oceanic and occasionally littoral.Alopias superciliosus (Lowe, 1839). Bigeye thresher. Oceanic, semipelagic, and occasion-ally littoral and benthic on the continental slopes.Alopias vulpinus (Bonnaterre, 1788). Thresher shark. Oceanic, semipelagic, and littoral.

Family Cetorhinidae: basking sharksCetorhinus maximus (Gunnerus, 1765). Basking shark. Littoral, semipelagic, and occa-sionally oceanic.


Family Lamnidae: mackerel sharksCarcharodon carcharias (Linnaeus, 1758). Great white shark. Oceanic, semipelagic, lit-toral, and occasionally benthic on the continental slopes.Isurus oxyrinchus (Rafi nesque, 1810). Shortfi n mako. Oceanic, semipelagic, and littoral where continental shelves are narrow.Isurus paucus (Guitart Manday, 1966). Longfi n mako. Oceanic, rarely littoral where con-tinental shelves are narrow.Lamna ditropis (Hubbs and Follett, 1947). Salmon shark. Oceanic, semipelagic, and lit-toral where continental shelves are narrow.Lamna nasus (Bonnaterre, 1788). Porbeagle shark. Oceanic, semipelagic, and littoral.

Although presently a small group compared to their previous diversity in Cretaceous and Cenozoic times, the order Lamniformes has a disproportionate number of species that are oceanic, semipelagic, or wide-ranging in oceanic, shelf, and slope waters. The sand tiger shark family (Odontaspididae) has an apparently oceanic, epipelagic, and possibly meso-pelagic large species, Odontaspis noronhai, with a microceanic morphotype. This species also occurs on continental and insular slopes, as does O. ferox, another sand tiger species that may be semipelagic (but that needs confi rmation). The goblin shark (Mitsukurinidae) is a slope-dweller that may be semipelagic. The crocodile shark (Pseudocarcharias kamo-harai) and the megamouth shark (Megachasma pelagios) are, respectively, microceanic and macroceanic epipelagic species that occasionally are littoral on the continental shelves. As a group the thresher sharks (Alopiidae) are littoral and oceanic (with a macroceanic morphotype), but the bigeye thresher (Alopias superciliosus) is a slope-dweller. The bask-ing shark (Cetorhinus maximus) is usually considered a littoral, continental species, but rare records from oceanic islands (e.g., the Hawaiian Islands) and sight records over the continental slopes or even the ocean basins in the western North Atlantic suggest that this is also a semipelagic and at least occasionally oceanic species. Most of the mackerel sharks (Lamnidae) are both littoral and oceanic, except for the longfi n mako (Isurus paucus), which seldom penetrates continental waters. The white shark (Carcharodon carcharias) is usually considered a littoral, continental species like the basking shark, but catches of white sharks in mid-ocean and near oceanic islands and ongoing satellite-tagging and genetic studies demonstrate oceanic movements. The white shark apparently descends to the upper slopes and may have the widest habitat and geographic range of any chondrichthyan.

Order Carcharhiniformes: ground sharksFamily Triakidae: houndsharksGaleorhinus galeus (Linnaeus, 1758). Tope shark. Littoral and semipelagic.

Family Carcharhinidae: requiem sharksCarcharhinus albimarginatus (Rüppell, 1837). Silvertip shark. Littoral and semipelagic.Carcharhinus altimus (Springer, 1950). Bignose shark. Semipelagic and benthic on the upper continental slopes.Carcharhinus brachyurus (Günther, 1870). Bronze whaler. Littoral and semipelagic.Carcharhinus brevipinna (Müller and Henle, 1839). Spinner shark. Littoral and semipelagic.Carcharhinus falciformis (Bibron, 1839). Silky shark. Oceanic and semipelagic.


Carcharhinus galapagensis (Snodgrass and Heller, 1905). Galapagos shark. Littoral and semipelagic.Carcharhinus leucas (Valenciennes, 1839). Bull shark. Littoral and freshwater, occasion-ally semipelagic.Carcharhinus limbatus (Valenciennes, 1839). Blacktip shark. Littoral and semipelagic.Carcharhinus longimanus (Poey, 1861). Oceanic whitetip shark. Oceanic and semipelagic.Carcharhinus obscurus (Lesueur, 1818). Dusky shark. Littoral and semipelagic.Carcharhinus plumbeus (Nardo, 1827). Sandbar shark. Littoral and semipelagic.Carcharhinus signatus (Poey, 1868). Night shark. Semipelagic.Galeocerdo cuvier (Peron and Lesueur, 1822). Tiger shark. Littoral and semipelagic.Prionace glauca (Linnaeus, 1758). Blue shark. Oceanic and semipelagic, littoral where continental shelves are narrow.

Family Sphyrnidae: hammerhead sharksSphyrna lewini (Griffi th and Smith, 1834). Scalloped hammerhead. Littoral and semipelagic.Sphyrna mokarran (Rüppell, 1837). Great hammerhead. Littoral and semipelagic.Sphyrna zygaena (Linnaeus, 1758). Smooth hammerhead. Littoral and semipelagic.

Most carcharhiniform sharks are shelf- or slope-dwellers. Three members of the requiem shark family Carcharhinidae, the blue shark (Prionace glauca), silky shark (Carcharhinus falciformis), and oceanic whitetip shark (C. longimanus), are oceanic species and are perhaps the most important oceanic sharks in terms of biomass. Several species of large carcharhinids, the larger hammerheads (Sphyrnidae), and one species of houndshark – the soupfi n or tope shark (Galeorhinus galeus) – are semipelagic as well as littoral, while one of the carcharhinids, the night shark (C. signatus), is more strictly offshore and semi-pelagic and occurs at or beyond the edge of the continental shelves of the tropical and warm-temperate Atlantic.

Order Hexanchiformes: cow and frilled sharksFamily Chlamydoselachidae: frilled sharksChlamydoselachus anguineus (Garman, 1884). Frilled shark. Benthic and semipelagic.

Family Hexanchidae: sixgill and sevengill sharksHexanchus griseus (Bonnaterre, 1788). Bluntnose sixgill shark. Benthic and semipelagic.

No known hexanchiform sharks are truly oceanic. The frilled shark (Chlamydoselachus anguineus) is a slope-dweller that can occur well off the bottom and may be semipe-lagic in some places. The bluntnose sixgill shark (Hexanchus griseus) has an enormous geographic and bathymetric range and occurs off oceanic islands and on ridges and seamounts. It is primarily a shelf- and slope-dweller, but may also be semipelagic.

Order Squaliformes: dogfi sh sharksFamily Etmopteridae: lanternsharksEtmopterus gracilispinis (Krefft, 1968). Broadband lanternshark. Benthic and semipelagic.


Etmopterus pusillus (Lowe, 1839). Smooth lanternshark. Benthic and semipelagic.Miroscyllium sheikoi (Dolganov, 1986). Rasptooth dogfi sh. Benthic and possibly semipelagic.Trigonognathus kabeyai (Mochizuki and Ohe, 1990). Viper dogfi sh. Benthic and possibly semipelagic.

Family Somniosidae: sleeper sharksScymnodalatias albicauda (Taniuchi and Garrick, 1986). Whitetail dogfi sh. Oceanic.Scymnodalatias garricki (Kukuyev and Konovalenko, 1988). Azores dogfi sh. Possibly oceanic.Scymnodalatias oligodon (Kukuyev and Konovalenko, 1988). Sparsetooth dogfi sh. Possibly oceanic.Scymnodalatias sherwoodi (Archey, 1921). Sherwood dogfi sh. Possibly oceanic.Zameus squamulosus (Günther, 1877). Velvet dogfi sh. Benthic and semipelagic.

Family Dalatiidae: kitefi n sharksEuprotomicroides zantedeschia (Hulley and Penrith, 1966). Taillight shark. Oceanic and benthic.Euprotomicrus bispinatus (Quoy and Gaimard, 1824). Pygmy shark. Oceanic and benthic.Heteroscymnoides marleyi (Fowler, 1934). Longnose pygmy shark. Oceanic.Isistius brasiliensis (Quoy and Gaimard, 1824). Cookiecutter or cigar shark. Oceanic.Isistius labialis (Meng, Chu and Li, 1985). South China cookiecutter shark. Oceanic.Isistius plutodus (Garrick and Springer, 1964). Largetooth cookiecutter shark. Oceanic and semipelagic.Mollisquama parini (Dolganov, 1984). Pocket shark. Possibly semipelagic.Squaliolus aliae (Teng, 1959). Smalleye pigmy shark. Oceanic.Squaliolus laticaudus (Smith and Radcliffe, 1912). Spined pygmy shark. Oceanic.

Most dogfi sh sharks are slope-dwellers, with some species ranging onto the shelves at higher latitudes. Most known species of the kitefi n shark family Dalatiidae are oceanic and include such small microceanic specialists as the cookiecutter sharks (Isistius), the pigmy sharks (Euprotomicrus and Heteroscymnoides), the taillight shark (Euprotomicroides zantedeschia), and the spined pygmy sharks (Squaliolus). Some of these little sharks are mesopelagic and bathypelagic, and may reach the ocean bottom during vertical migra-tions. The related sleeper shark family Somniosidae consists mostly of slope-dwellers, though some of the giant sleeper sharks (Somniosus, subgenus Somniosus) occur on the continental shelves at high latitudes. At least one somniosid species, Scymnodalatias albicauda, is oceanic and epipelagic, and is caught on pelagic longlines in high latitudes in the Southern Hemisphere; other members of Scymnodalatias may be oceanic, or slope-dwellers, or both. A small dalatiid, the pocket shark (Mollisquama parini), was collected on a submarine ridge off northern Chile and may be a slope-dweller or alternatively semi-pelagic or oceanic. The somniosid Zameus squamulosus is primarily a slope-dweller but is also semipelagic. There are several semipelagic or potentially semipelagic species in the lanternshark family Etmopteridae, which includes Etmopterus gracilispinis and E. pusillus. These are primarily slope-dwelling species that are also semipelagic.


Order Rajiformes: skates and rays (batoids)Suborder Torpedinoidei: electric raysFamily Torpedinidae: torpedo raysTorpedo nobiliana (Bonaparte, 1835). Great, Atlantic, or black torpedo. Benthic, littoral, and semipelagic.Torpedo semipelagica (Parin and Kotlyar, 1985). Semipelagic torpedo. Semipelagic.

Suborder Myliobatoidei: stingraysFamily Dasyatidae: whiptail stingraysDasyatis matsubarai (Miyosi, 1939). Pitted stingray. Benthic and semipelagic.Pteroplatytrygon violacea (Bonaparte, 1832). Pelagic stingray. Oceanic and littoral.

Family Myliobatidae: eagle raysAetobatus narinari (Euphrasen, 1790). Spotted eagle ray or bonnetray. Benthic, littoral, and semipelagic.Aetomylaeus vespertilio (Bleeker, 1852). Ornate or reticulate eagle ray. Benthic, littoral, and semipelagic.Myliobatis californicus (Gill, 1865). Bat ray. Benthic, littoral, and semipelagic.

Family Mobulidae: devilraysManta birostris (Walbaum, 1792). Manta. Littoral and oceanic.Mobula japanica (Müller and Henle, 1841). Spinetail devilray. Littoral and semipelagic or oceanic.Mobula mobular (Bonnaterre, 1788). Giant devilray or devil ray. Littoral and semipelagic or oceanic.Mobula tarapacana (Philippi, 1892). Sicklefi n devilray. Littoral and semipelagic or oceanic.

Most batoids are shelf- or slope-dwellers, and occur on or near the bottom, though some are more active littoral species in coastal waters. The stingray family Dasyatidae has one notable exception, the pelagic stingray (Pteroplatytrygon violacea), which is oceanic, while the pitted stingray (Dasyatis matsubarai) may be semipelagic as well as coastal-benthic. A few torpedo rays (Torpedo spp.: Torpedinidae) and eagle rays (Aetobatus, Aetomylaeus, and Myliobatis: Myliobatidae) may be semipelagic as well as shelf-dwellers, and the tor-pedinids may also be slope-dwellers. The devilray family Mobulidae has several litto-ral species, but the manta (Manta birostris) and possibly the sicklefi n devilray (Mobula tarapacana) are oceanic as well as littoral; the closely related species M. japanica and M. mobular are large, possibly semipelagic or oceanic species. Smaller species of appar-ently littoral mobulids are strong swimmers and also could make oceanic excursions.

Pelagic sharks in perspective

As with freshwater sharks and rays, the relatively low diversity of living pelagic elasmo-branchs and the low numbers of oceanic species suggest that the oceanic realms are rather marginal for cartilaginous fi shes as contrasted to the high diversity of oceanic teleost fi shes


and cephalopods (M. Roeleveld, personal communication). The few shark species that pen-etrate the open ocean are often large macropredators that feed high in the food chain (some carcharhinids and lamnoids), although blue and thresher sharks exploit medium-sized and small pelagic prey. Large fi lter-feeding sharks and rays include a handful of species in four families (whale, megamouth and basking sharks, and devilrays), but all of these readily penetrate the oceanic zone, and three are among the largest living fi shes (whale shark, basking shark, and manta). The larger oceanic macropredatory and fi lter-feeding sharks generally are strong swimmers. An anomaly among oceanic species is the pelagic stingray, a species that occupies an oceanic habitat with a body form clearly derived from that of benthic stingrays and that feeds on pelagic invertebrates and small fi shes.

Few oceanic elasmobranchs are of moderate size (the crocodile shark, whitetail dog-fi sh, and pelagic stingray are among the exceptions in the range of 60–150 cm). More oceanic species are large to gigantic (from 2 to nearly 20 m long) or small to dwarf (less than 50 cm), with some oceanic dogfi sh (pygmy sharks) among the smallest living sharks at less than 30 cm when mature. Some of these dwarf dogfi sh have unusual morphological adaptations, including luminous glands (taillight sharks) and specialized jaws and lips for removing plugs of fl esh from live prey (cookiecutter sharks). Large oceanic macropreda-tory and fi lter-feeding sharks and rays have rivals in the large and diverse oceanic teleosts and in oceanic cetaceans that share their epipelagic habitat and overlap in feeding habits, but the large sharks apparently are competitive with them and some even prey on them.

Although some oceanic sharks are uncommon to rare, others are rather common, while a few species (blue and silky sharks) are among the most abundant large sharks in the world and are very important as apical predators in the epipelagic zone. Apparently, low species diversity can coexist with success in abundance, ecological impact, and geographic dispersal, at least for the handful of large macropredatory oceanic sharks. The large fi lter-feeding oceanic and semi-oceanic families combine low species diver-sity with suffi cient abundance to support targeted fi sheries (except for the relatively rare megamouth shark) and have a wide geographic range, but some of these utilize feeding grounds in littoral waters. Moderate-sized oceanic species vary from rare to common (crocodile sharks and pelagic stingrays, respectively) but are not at all diverse. This is unlike cartilaginous fi shes on the shelves and slopes, which include many moderate-sized species. Dwarf oceanic species are even less diverse than their counterparts in continental waters, and are generally rare to uncommon and sporadically if widely distributed, though cookiecutter sharks and spined pygmy sharks can be locally abundant.

Pelagic sharks are best known from the epipelagic zone, with the greatest known diversity and biomass concentrated there. Less is known about the distribution of pelagic sharks in the mesopelagic and bathypelagic zones owing to diffi culties in collecting them in deepwater pelagic gear, although electronic tagging is yielding new data on the depth penetration of a few of the larger species.

Pelagic sharks capture our interest because of their size (both great and small), acces-sibility, beauty, and spectacular adaptations to their unusual (for a chondrichthyan) life-styles. Unfortunately, pelagic sharks, by virtue of their mostly “shallow” distribution in the world’s oceans, are easily accessible to huge fl eets of high-technology fi shing ves-sels that scour the open oceans in search of “marine resources.” Large pelagic sharks are commonly targeted for human utilization, but most often are swept up as bycatch in the


enormous fi sheries targeting high-value teleosts and cephalopods. Even the rarer dwarf species and plankton-feeding giants are not exempt from being caught in fi shing gear, and all pelagic sharks are subject to the deleterious effects of pollution and habitat modifi ca-tion from human activities.

The dramatic impact of high-seas fi sheries on pelagic sharks and the spectacular declines in their numbers have raised widespread concern for their long-term survival and for the negative ecological effects of their loss on the functioning of marine ecosystems. However, the more insidious consequences of environmental degradation may hasten the extinction of pelagic sharks despite future regulation or even cessation of direct human predation. At present, bycatch fi sheries on pelagic sharks are still largely operating with-out control, and environmental changes proceed apace. The effects of pollutants and glo-bal warming on pelagic sharks are not well understood, but those species that feed high on the food chain (such as soupfi n and white sharks) apparently can and do concentrate heavy metals such as mercury, and may do likewise for deleterious organic chemicals. Poisoned, degraded, and largely sharkless seas may be our gift to posterity.

Acknowledgments

Special thanks to Merry Camhi for inviting the present work and for having patience dur-ing its long gestation period. Thanks also to George Burgess and Elizabeth Babcock for reviewing versions of the manuscript.

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sharks of the open ocean || pelagic elasmobranch diversity

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