8. Oct, 2017

Jellyfish have drifted along the ocean currents for millions of years, even before dinosaurs lived on the Earth. They are found in every ocean, from the surface to the deep.  Jellyfish belong to the  Cnidaria  phylum that also  contains  the soft and stony corals and  sea anemones, collectively  known as Anthozoa.  The jellies start  their lives as  polyps with tentacles  on top,  like coral polyps and sea anemones, but then  end up as free swimming adults with  a medusa form, the reason why they are named Medusozoa.  Adult jellyfish are gelatinous and generally transparent or translucent,  with gelatinous umbrella-shaped bell and trailing tentacles. Despite their  respectable antiquity, jellyfish have long been ignored or misunderstood by mainstream science, dismissed as so much ‘mindless protoplasm with a mouth’.

Varieties Jellyfish consist of around  4000 different species belonging to various  families, that are  subdivided in the following four big classes:  

-Hydrozoa  are a mixed class  that contains  sessile (nonmoving) members, such as fire coral (Millepora) and the freshwater  polyps Hydra, but also  the marine hydrozoan  Portuguese man 'o war (Physalia physalis), nicknamed  the  ‘floating terror’ (see picture left).

-Scyphozoa are the most familiar jellyfish, including most of the bigger and more colorful jellies, also called "true jellyfish".  The at least 200 species of Scyphozoa spend most of their lives in the medusa body form. The remarkable  species of the Cassiopeidae  family,  known as the upside-down-jellyfish,  have their tentacles on top. The upside-down jellyfish  Cassiopea xamachana  which lives in the Carribbean, Hawaii and Florida, appears as a flower on the seafloor and tends to aggregate in large groups. Another beautiful Scyphozoan  species is the Mediterrean jellyfish   (Cotylorhiza tuberculata) from the Cepheidae  family. Its dome is  surrounded by a colourful gutter-like ring, often carrying  small horse mackerels. The large barrel jellyfish (Rhizostoma pulmo) from the Rhizostomatidae family, can  also be found in the Mediterranean and Adriatic. This jelly can become so large that young fish and small crabs seek shelter in its mildly stinging tentacles

-Cubozoa  -or box fellyfish jellyfish- have a more developed nervous system than other jellyfish, including complex eyes with lenses, corneas and retinas. Some species, such as the feared sea wasp (Chironex fleckeri), produce one of the most potent venoms  known.

-Staurazoa or stalked jellyfish   don't float through the water like other jellies, but rather live attached to rocks or seaweed. They are trumpet-shaped, and mostly live in cold water. There are around 50 staurozoan species, many notable for their unique combination of beauty and camouflage. 

Comb jellies  Comb jellies  are not Cnidaria but belong to another phylum called Ctenophora.Though they have much in common, jellyfish and comb jellies have very different  histories in the  tree of live. Comb jellies are named for their unique feature: the presence  of  cilia, known as combs, which run in eight rows up and down their bodies.  The combrows of cilia look like small paddles lining their bodies that propel them through the ocean.  They can also produce a rainbow effect. This is not bioluminescence, but occurs when light is scattered in different directions by the tissue of their skin. Most comb jellie havt two, often branched, tentacles.

Anatomy and reproduction  Jellyfish  vary greatly in size depending on the species. Most jellies range from less than half an inch (1 cm) wide to about 16 inches (40 cm), though the smallest are just one millimeter wide. On of the largest jellies is the Lion’s Mane Jellyfish (Cyanea capillata). They are highly distinguishable by their mass of long, thin and hair-like tentacles which can be almost 6 feet wide (1.8 m) with tentacles over 49 feet (15 m) long.

Most jellyfish do not have specialized digestive, osmoregulatory, respiratory or circulatory systems. The mouth/anus at the base of the bell opens into the gastrovascular cavity where digestion takes place and nutrients are absorbed. The mouth is surrounded by  tentacles that can stick, tangle or  sting and  are also meant to  bring food to the mouth. As jellyfish squirt water from their mouths they are propelled forward. Some jellyfish are colourless, but others are in vibrant colors such as pink, yellow, blue, and purple, and often are luminescent.

Polyps  can live and reproduce asexually for several years, or even decades. Medusa jellyfish reproduce sexually by spawning—the mass release of eggs and sperm into the open ocean—with entire populations sometimes spawning all together.  Adult jellyfish typically live for a few months, depending on the species, although some species can live for 2-3 years in captivity. One jellyfish species is almost immortal: Turritopsis dohrnii, a small hydrozoan can revert back to the polyp stage after reaching adult medusa stage through a process called transdifferentiation, meaning  a  process by which the state of cells differentiates and transforms into new types of cells.

Behavior With their modest oxygen requirements, jellies can grow in post-algal “dead zones” and other polluted waters where most marine life can’t — not surprising perhaps  for species that  has survived  over so many ages. Polyps feed on feed on zooplankton. All adult jellies are carnivorous, feeding on plankton, crustaceans, fish eggs, small fish and other jellyfish, ingesting and voiding through the same hole in the middle of the bell. Jellies do not actively hunt,  but instead use their tentacles as drift nets.

In turn, jellies are the favorite food of  seabirds, the ocean sunfish (Mola mola) and endangered leatherback turtle (Dermochelys coriacea). The abundance of plastic floating in the ocean is often the cause of death of these animals,  mistaking plastic particles  for  their favorite prey.  Humans have also been eating jellyfish,  for at least 1700 years. Some 425,000 tons (more than 900 million pounds) of jellyfish are caught each year by fisheries in 15 countries, and most are consumed in Southeast Asia.

Nervous system Are jellyfish more that a lump of mindless protoplasm with a mouth and venom?  Many  experts now say:  Yes.  Jellyfish belong to the earliest known animals to have organized tissues—their epidermis and gastrodermis—and a nervous system. Jellyfish  don’t have a brain like another famous invertebrate of the sea, the octopus (mte that the octopus does not have tentacles but arms.  The jelly's ‘brain' is in fact a network of nerve cells, that allows it to sense their environments, such as changes in water chemistry or the touch of another animal. The nerve net has some specialized structures such as statocysts, which are balance sensors that help jellies know whether they are facing up or down, and light-sensing organs called ocelli, which can sense the presence and absence of light.  The nervous system of box jelly fish is more complex. In its  visual system there is  an interactive matrix of 24 eyes of four distinct types — two of them very similar to our own eyes — allowing the jellies to navigate  through  rubbish or the mangrove swamps they inhabit.

Some however might say  that jellies are  intelligent, despite their lack of a brain (i.e. cortex), because of their refined adaptations to various conditions in the environment*. Instead of a brain some jellies have a central circuitry of  giant motorneurons controlling movement of tentacles and the bell**. Their behaviors include swimming up in response to somatosensory stimulation, swimming down in response to low salinity, diving in response to turbulence, avoiding rock walls, forming aggregations, and horizontal directional swimming.  In short, behaviors that go beyond simple reflexes and require sensory feedback during their execution.

Toxicity  Jellyfish and comb jellies both have tentacles with specialized cells to capture prey: nematocysts and colloblasts, respectively.  Jellyfishes' nematocysts are organelles within special cells (cnidocytes) that contain venom-bearing harpoons. The cell is activated upon touch or chemical cue, causing the harpoon to shoot  out of the cell and spear the prey or enemy, releasing toxin—a process that takes only 700 nanoseconds. The same nematocysts are  active on the hydrozoan firecoral.  A small number of jellyfish are very toxic to humans, such as the box jellyfish (Chironex fleckeri) mentioned  earlier,  and the Irukandji jellyfish (Carukia barnesi), which can cause severe reactions and even death in some people.

Jelly bloomings Some beaches can be temporarily  flooded by masses of jellyfish, often species of Scyphozoa like the Pelagia Noctiluca (nicknamed the mauve stinger) in the Mediterranean.  Occasionally with strong Westerly winds lots of blueish barrel jellyfish from the Northsea strand on Dutch beaches. Some believe that environmental factors like  warming and pollution  of seawater,  and the  decline of natural predators  are  responsible for the rapid growth of certain species. Massive  aggregations of  jellies,  known as "jellyfish blooms" or "jellyfish outbreaks," can cause a wide array of problems. Too many jellies in the water can be a danger to swimmers, forcing towns to close their beaches. Jellies have clogged up machinery at coastal power plants, causing power outages. They can interfere with fisheries by eating fish larvae, and fisherman catching jellies instead of the fish they want.

Sources and links:

Albert, D.J.(2011)*. What's on the mind of a jellyfish? A review of behavioural observations on Aurelia sp. jellyfish. Neurosci Biobehav Rev. 2011 Jan;35(3):474-82.

Satterlie, R.A. (2011). Do jellyfish have central nervous systems? J Exp Biol. 2011 Apr 15;214(Pt 8):1215-23

Kramp, P.L. (1961): Synopsis of the Medusae of the World. Order Rhizostomeae. Journal of the Marine Biological Association of the United Kingdom 40: 348–382.

Mackie, G. O. and Meech, R. W. (1995a)**. Central circuitry in the jellyfish Aglantha digitale. I. The relay system. J. Exp. Biol. 198, 2261–2270

https://en.wikipedia.org/wiki/Jellyfish

Jellyfish Take Over an Over-Fished Area". 21 July 2006

Jellyfish Gone Wild" (Text of Flash). National Science Foundation. 3 March 2009. Retrieved 17 November 2009

25. Sep, 2017

The Dugong (officially: Dugong, dugong) is a marine animal that lives  in warm coastal waters of Red Sea, East Africa, Australia, Japan and Philippines. Its name is derived from the Malay duyung, meaning "lady of the sea" or ‘mermaid’. It is the only living representative of the once-diverse family Dugongidae; its closest modern relative, Steller's sea cow (Hydrodamalis gigas), was hunted to extinction in the 18th century. The dugong is also the only marine  strictly vegetarian mammal.  

Dugong from the Philippines. Picture by Tanakit YanMo Suwanyangyaun

Manatees (Trichechidae) and dugongs are families classified in the order of Sirenia. According to molecular phylogeny they are more closely related to the elephants than to sea mammals. Both are gentle slowly-moving herbivores that are  found in areas of shallow waters along warm coastlines. Both families have mammary glands near their armpits. But there are also some important anatomical differences.  In summary:

-Manatees have a large, horizontal, paddle-shaped tail with only one lobe, which moves up and down when the animal swims. Dugongs have tail flukes with pointed projections, like a whale with a slightly concave trailing edge.

-Dugongs have a an undivided upper lip. The snout is flabby and downturned, an adaptation for grazing in benthic seagrass fields. This makes them fit for bottom-dwelling,  while manatees may also feed on floating vegetations (e.g. the Florida manatees)

-The dugong lacks nails on its flippers, which are relatively short.

-Manatees are generally larger than dugongs and can weight between 400 and 500 kg and grow to a length of 3.6 metres.  Dugongs rarely grow larger than 3 metres and weight is, on average 420 kg.

-Unlike the manatees, the dugong's teeth do not continually grow back via horizontal tooth replacement

-While the manatee also lives in fresh water like natural springs, the dugong  prefers salt water environments, but may  occasionally move inshore in brackish water of bays or mangroves channels

Behavior and habitat  Dugongs are long lived and  have few natural predators, although animals such as crocodiles, killer whales, and sharks pose a threat to the young. Like manatees they often suffer from injuries caused  by boat strike impact or by propellers of motor boats cruising  in their territories. Fast boats travelling in shallow water over sea grass beds pose the greatest threat.  Gatherings of hundreds of dugongs sometimes happen, but they last only for a short time. The exceptional large populations of Shark Bay and  Moreton Bay  along the Queensland coast of Western Australia are thought to be stable with over 10,000 dugongs  Large  populations are also found in Torres Strait south of New Guinea  and  in the Persian Gulf.  But in other areas their numbers are reported to decrease steadily due  to human interventions. The  main causes of population decline include fishing-related fatalities, collision with motor vessels, habitat degradation and hunting. With its long lifespan of 70 years or more, and slow rate of reproduction, the dugong is especially vulnerable to extinction.

Dugons have poor  eye-sight and depend on the bristles on their upper lip for orientation. Cows and their calves communicate by exchanging high pitched 'chirps'. Dugongs are  often escorted by  'pilot fish': actually these are juvenile Golden trevallys (Gnathanodon speciosus), attracted by the sediments stirred up by the dugong. For UW photographers with fish-eye lenses these bright yellow fishes  create a  nice photogenic contrast with the greyish bulky body of their host.  Similar to the black and white pilot fish (Naucrates ductor) escorting Oceanic sharks. 

Although dugongs are social animals, they are usually solitary or found in pairs. This might be due to the fact that seagrass beds do not support large populations and dugongs spread out to gather sufficient portions. The dugong is largely dependent on seagrass for its existence, and is thus restricted to  shallow sandy coastal habitats which support seagrass meadows.  Preferred species are the fast growing  Halophila and HaloduleBut sometimes they venture  further away from the shoreline where they dive to 30-40 meters in search of the seagrasses Halophila spinulosa. Just like cows in the meadows, their grazing stimulates the growth of new grass and may contribute to abundance of seagrass species that are preferred by dugongs, at the expense of less preferred species. They will dig up an entire plant and then shake it to remove the sand before eating it, using their  flexible and muscular upper lip  to dig out the plants. They may even collect a pile of plants in one area before eating them. Furrows in the sand often mark the path they have followed.  Conservation of the seagrass fields  remains an important condition for their survival, especially  in coastal areas where their growth is under pressure by human activities.

Source and links

Winger, Jennifer (2000). "What's in a Name: Manatees and Dugongs". National Zoological Park. Archived from the original on 2007-10-13. Retrieved 22 July 2007.

Preen, Anthony (1995). "Impacts of dugong foraging on seagrass habitats: observational and experimental evidence for cultivation grazing"Marine Ecology Progress Series124 (1/3): 201–213https://en.wikipedia.org/wiki/Sirenia

https://en.wikipedia.org/wiki/Dugong

http://www.int-res.com/articles/meps/124/m124p201.pdf

http://www.iucnredlist.org/details/6909/0

http://www.indigoscuba.com/the-difference-between-manatees-and-dugongs/

11. Sep, 2017

It is good to shift scenes  by time and while  from the world  under the waterline to the area where many species are now critically endangered:  the dry savannahs  of  the great  African continent. One such remarkable species is the African wild dog (Lycaon pictus: literally painted wolf), a splendid animal also known as African hunting dog,  Cape hunting dog, Painted hunting dog and ‘mbwa mwilu’ in Swahili.

Habitat  As Africa’s last tracts of wilderness are destroyed, wild dogs are forced into areas where they face unfamiliar threats, such as potentially fatal encounters with humans and exposure to the frequently lethal viruses carried by domestic dogs. Many wild dogs are still killed or poisoned by farmers as revenge for eating their chickens. In total now less than 7,000 remain in the wild containing roughly 39 subpopulations. The African wild dog is a canid native to Sub-Saharan Africa, where most species now live in the  savannahs and national parks of Southern East Africa or South Africa, for example  the Moremi Game Reserve  in Botswana, and the Niassa National Reserve in Mozambique. These reserves are relatively unexplored wilderness and  often form the last refuges for African wild dogs,  providing a large area of habitat and haven for  around 450 animals per reserve. The Okavango Delta in Botswana  is massive and unique in that all the water in the Delta does not flow into an ocean but ends in the desert. The farmers who are nearby just need to find ways to co-exist with the dogs and other animals. *

Behavior The African wild dog has long legs, large rounded ears and four toes per foot. Its muscled large ears can turn in in all directions to pick up the faintest sounds. Most species  have patched fur in black, brown, white and yellow. They are known for their strong family bonds and often gather in packs of around ten to 40 individuals. Packs have been known to share food and to assist weak or ill members. Each pack has a strict hierarchy with  an alpha breeding pair in charge of the group and the rest of the pack members as subordinates. Social interactions  like greeting ceremonies,  signs of  appeasement or submission etc. are common, and the dogs communicate by touch, actions, and vocalizations. The female has a litter of 2 to 20 pups, which are cared for by the entire pack.  

Hunting skills The African wild dogs are also fierce and opportunistic predators that hunt medium-sized prey such as antelopes and gazelles. Sometimes  they are after bigger game such as  the zebra and the wildebeest. Their high intelligence and teamwork allows them to adapt to changing scenarios during a hunt. For example when the pack chases a selected individual, some dogs will go after the target, while others will perform flanking movements to cut off any avenues of escape. When the leading dog gets tired, another will take its place. In a sprint, African wild dogs can reach speeds of more than 40 miles per hour. It hunts by approaching its prey silently then chasing it in a run lasting for 10 to 60 minutes. The average chase typically only goes as far as 2 km, during which time the prey animal, if large like a wildebeest,  is repeatedly bitten on the legs, belly and anus until it stops running, while smaller prey is simply pulled down and torn apart with their powerful  jaws and teeth.

The sneeze as signal Amongst the wild dogs’ vocal chatter, biologists have identified “audible, rapid exhalation of air through the nose”, also called sneezing.  After one  dog starts sneezing, the whole pack may take over  the sneeze. Later,  research teams realized that the number of sneezes predicted whether the pack set off to leave or to hunt: it reflected a kind of  collective  decision. Other social carnivores like meerkats (Suricata suricatta)  use a similar decision-making process. It also resembles the increased rate of ‘grunts’ that  occur before Mountain gorilla (G. g. beringei) groups move off from their resting sites.

A recent  study published by Reena Walker a.o.** discovered something interesting. They collected their data from five packs  of African wild dogs in and around the Moremi Game Reserve in the Okavango Delta in Botswana. They found that the number of sneezes needed for the group to depart (i.e. the ‘quorum’) was reduced whenever dominant individuals initiated rallies, suggesting that dominant participation increases the likelihood of a rally’s success. However it was  not an absolute prerequisite, because  the ‘will of the group’ could override dominant preferences when the consensus of subordinates is sufficiently great. The sneezes might either  act as a true voting mechanism or  it may reflect a purely physiological response to a consensus already achieved through other signals that researchers  did not observe.

*People that like to donate to a project devoted to conservation of the African  Dogs  should contact Botswana Predator Conservation Thrust

Sources and links

**Walker RH, King AJ, McNuttJW, Jordan NR. 2017 Sneeze to leave: African wild dogs (Lycaon pictus) use variable quorum thresholds facilitated by sneezes in collective decisions. Proc. R. Soc. B 284: 20170347.

Robbins RL. 2000 Vocal communication in freeranging African wild dogs (Lycaon pictus). Behaviour 137, 1271–1298.

Rutten S, Fleissner G. 2004 On the function of the greeting ceremony in social canids—exemplified by African wild dogs Lycaon pictus. Canid News, 7.3. Online. Estes R, Goddard J. 1967 Prey selection and hunting behavior of the African wild dog. J. Wildl. Manage.,31, 52–70.

https://africageographic.com/blog/10-interesting-facts-african-wild-dogs/

http://www.wildafrica.cz/en/animal/african-wild-dog/

http://www.npr.org/sections/thetwo-way/2017/09/07/549182066/a-quorum-of-sneezes-for-wild-dogs-it-s-not-just-snot-it-s-a-way-to-vote

https://www.nytimes.com/2017/09/06/science/wild-dogs-sneeze-hunt.html

https://www.youtube.com/watch?v=h4SlAc2U1A4

http://phenomena.nationalgeographic.com/2016/03/29/what-wildlife-shows-dont-tell-you-about-african-wild-dogs/

1. Sep, 2017

The name Great White shark (Carcharodon Carcharias)  comes from their snow-white ventral bellies.  The dorsal sides of their bodies are usually gray, which makes  them to blend in perfectly with deeper  darker layers of the water. Great whites (GWs) live in almost all coastal and offshore waters which have water temperature between 12 and 24 °C. Greater concentrations are found in the USA  (Northeast and California; e.g. Farallon Islands), Japan, Oceania, Chile, the Eastern Mediterranean and South Africa (with the most dense populations around Dyer Island)

Picture of Great White taken at Isla Guadalupe

Anatomy and behavior The GW is a viviparous epipelagic  fish, observed mostly in the presence of colonies of fur seals (Arctocephalus), sea lions, cetaceans, other sharks, and large bony fish species. The injuries seen at the often scarred  bodies of females  are mating scars, but could also result from  bites from  their prey, like the powerful  elephant seal  (Mirounga angustirostris). Male great whites on  the average measure 3.4 to 4.0 m long, females 4.6 to 4.9 m According to J. E. Randall, the largest white shark reliably measured was a 5.94 m individual reported from Ledge Point, Western Australia in 1987.

GWs  have the typical torpedo-shape of the of mackerel sharks (Lemnidae) which, combined with their muscular tails, can propel them through the water at speeds of 24 kmph, or even brake the water surface when they attempt a vertical attack on  prey swimming at the surface. A breach is the result of a high speed approach to the surface with the resulting momentum taking the shark partially or completely clear of the water. It is a hunting technique employed by GWs whilst hunting seals. Because the behavior is unpredictable, it has been very hard to put on film or video. It was first photographed by Chris Fallows and Rob Lawrence who developed the technique of towing a slow-moving seal decoy to trick the sharks to breach. Between April and September, scientists may observe around 600 breaches. Most successful attemps occur in the early morning.

GWs are known to migrate over large distances in the ocean: According to a recent study, California great whites have migrated to an area between Baja California Peninsula and Hawaii known as the White Shark Café to spend at least 100 days before migrating back to Baja. A similar  study tracked a different great white shark (nicknamed Nicole)  from South Africa swimming to Australia's northwestern coast and back, a journey of 20,000 km in nine months (see also my earlier Blog).  The GW is one of only a few sharks known to regularly lift its head above the sea surface to gaze at other objects such as prey, also known  as spy-hopping

Reputation Many superlatives haven   been used to characterize the dangerous side of GWs: ‘magnificent monster’, ‘terrifying creature’, ‘razor sharp teeth, beady black eyes’, ‘the ocean’s deadliest predator lurking beneath the waves’, ‘ ready to  strike without warning‘. But the bloodthirsty reputation  of this magnificent shark can be largely ascribed to films and sensation documentaries. GW attacks are extremely rare, even when people swim in close proximity to them.  Still, GWs are  held responsible for the relatively large number of reported and identified fatal shark attacks on humans. One obvious factor that is likely to trigger an attack is the  blood and vibrations spread in the water by a fish speared in its territory. Another factor is that GWs, like some other predator sharks, exhibit a behavior called test biting. Basically, they are interested in spotting what looks  like a food source. An initial bump may be followed by a test  bite on a human,  after which the shark will probably  never return again. Fatalities from shark bites in fact are quite rare  and caused  by blood loss from an initial test bite. Sometimes an attack might occur in  shallow and  turbulent water of the surf, where the shark mistakes the silhouette of a paddling surfer for its natural prey, the seal. Fatalities almost never result from critical organ loss or from whole consumption.

Cage diving  There are currently three  major locations for divers to visit the GW from inside a cage. Cage diving is most common at sites  were whites are frequent, including the  northern Neptune Islands in South Australia, Guadalupe Island in Baja California and off Seal IslandFalse Bay in South Africa.  At Isla Guadalupe several live aboards can be lined up during the high season, forming an avenue of boats. Each with cages underneath and  a couple of divers inside, shivering in the cold water filled with chum. Here the chances of getting a good shot of a GW are pretty good, but too many boats in the area may spread out the sharks and restrict the chances of a successful close passing for the individual UW photographer. I must admit that my visit to the island a couple of years ago did not result in a high ranking  on my list of favorite shark sites.

Cage-free diving with GWs. Man inside, animal outside cage is a slogan  that would probably receive much approval from members of Animal Lib.  Personally I dislike observing wild animals from behind bars (this holds for two directions).  Ever since the moment that divers felt safe to go out in open water to meet sharks, boundaries have been pushed and now include predators that were formerly seen as highly dangerous, such as Tiger Sharks and Oceanic white tips. The GW however was always considered as a special case, a more ‘raw’ and unpredictable species never to be approached in open water. But is there any biologically plausible reason why the GW should be treated as a rare exception? Free diving with GWs means that  a diver leaves a cage or boat to meet or swim with the sharks in open water, sometimes only with a snorkel, fins and mask.Wisely, these GW events are not open for the public, but performed  by experienced divers in favorable sea conditions, with relaxed sharks that often are accustomed to the visiting divers. Baiting could help in diverting the sharks attention from diver to bait, like it does with other species. For example Mike_Rutzen, a south African living in Gansbaai has spent a lifetime with GWs and taken a long time watching them and getting used to their behavior.   He regards cage-free diving with  GWs as something special to be done on unique occasions: a combination of a perfect day, perfect weather and a perfect shark.

Man swimming among apex predators without direct danger of physical injury could help to expose myths created by movies like like Jaws. Perhaps commercial movies and documentaries built upon the sensation side would then loose a part of their profit.  But  they would also receive something else in return.  Namely the opportunity to highlight for the public  the attempts to observe and preserve miraculous creatures that lived for millions of years, but that are now  on the verge  of extinction.   

Links:

https://en.wikipedia.org/wiki/Guadalupe_Island

https://www.facebook.com/gwcsr/

https://en.wikipedia.org/wiki/Rodney_Fox

https://en.wikipedia.org/wiki/Great_white_shark

http://www.rodneyfox.com.au/

 

18. Aug, 2017

The sandtiger shark (Carcharias taurus), also known  as Grey nurse shark or Spotted ragged-tooth shark, can be found in most moderate seas troughout the World. It is a typical social shark that congregates in smaller and larger groups above the sand, or prowls for bait in ship wrecks. Its ambiguous naming often leads to confusion. The sandtiger is not related to the tiger shark, nurse shark or the grey reef shark as the names above suggest.

Picture left: Sandtigers in North Carolina. Picture taken by Tanya Houppermans.

Phylogeny   Sandtigers belong to the order of  Lamniformes that are  distinguished by  eyes without nictitating membranes and a higher body temperature than the surrounding water. Many different families belong to the great order of Lamniformes. The tresher shark (Alopiidae), great white shark (Lamnidae) and mako shark (Isurus) are just some examples. The sandtiger belongs to the family of Odontaspididae, that splits up in two  genera: Carcharias and Odontaspis. The sandtiger was originally named Odontaspis taurus, but later changed to its current official name Carcharias taurus. It  is the only member of Carcharias, while Odontaspis contains two  extant species: the smalltooth sand tiger (Odontaspis ferox) and  the extremely rare  big-eye sand tiger (Odontaspis noronhai). So it appears that there are three different species of sand tigers. Notice however  that genetic markers suggest that the two Odontaspis species could be more closely related to sharks belonging to other families of Lamniformes, like the tresher shark (Alopius vulpinus) or even the crocodile shark (Pseudocarcharias kamoharai). Perhaps Odontaspis and Carcharias are the result of convergent  evolution, meaning a  process by which unrelated or distantly related organisms evolved similar body forms, coloration, organs, and adaptations.

Anatomy The sand tiger shark has a flattened, cone-shaped snout and a distinctive, oblong tail with a notched, upper lobe that is significantly longer than the lobe below. Individuals range in size from 6.5 to 10.5 feet in length. It usually swims with its mouth open displaying three rows of protruding, smooth-edged, sharp-pointed teeth. Adults tend to have reddish-brown spots scattered mostly on the hind part of the body. Its jagged protruding teeth certainly give the sand tiger a ferocious look: the sharp teeth seem to protrude in all directions, even when the mouth is shut. But looks can be deceiving, because sand tigers are a docile, non-aggressive species known to attack humans only when bothered first. They are also one of the few sharks that are tolerant of aquarium life.

Habitat and Behavior  Sandtiger encounters are common occurrences in shallow offshore waters. Ship wrecks are one of their favorite habitats, probably because of the  massive schools of small  bait fish. Along the coast of North Carolina sandtiger sharks congregate in numbers of more than one hundred in the shipwrecks lining the coast, many dating from World War II battles (see insert above). At the wrecks they can be seen swimming through dense schools of small fish (‘bait balls). The abundance of food obviates baiting by humans and allows UW photographers to close in to the sharks when  they glide their way through the bait balls.

Sandtigers are the only shark known to come to the surface and gulp air. They store the air in their stomachs, which allows them to float motionless in the water, seeking prey. When feeding at night  they become voracious predators, that generally stay close to the bottom. Their prey is small fish, but they will eat crustaceans and squid as well. They occasionally hunt in groups, and have even been known to attack full fishing nets.  During the day they are found near caves and ledges, hovering just above the surface either singly or in small groups.  In these conditions  the sand tiger is a relatively placid and slow-moving shark that will not cause any danger to humans. In fact the database of Shark Attack Survivors does not list any fatalities due to sandtiger sharks.

Survival. Two factors have made the survival status of the sandtiger critical. First,   the slow-moving, approachable sandtiger with a tendency to aggregate in large groups has been an easy target for spearfishers  for years. In  waters of New South Wales and southern Queensland in Australia the numbers of sandtigers have declined dramatically throughout the 1960s and 1970s, due to the combined effects of targeted spearfishing, incidental capture by commercial and recreational fishing and beach protective shark meshing. Although  the species have now become protected, accidental bycatch still occurs. A second factor is their low reproduction rate. Female sandtigers  are ovoviparous and only develop two embryos, one in each uterus. The young  embryos then feed on their mother eggs until only one pup is left in each uterus. Females don’t reach reproductive age until they’re nearly 10 years old, and then only produce one or two pups per litter every two years. Over nine months to a year, the pups grow within their mother feeding on hundreds to thousands of eggs and, by the time they are born, measure up to a meter long.

 

References:

http://wetpixel.com/full_frame/tanya-houppermans-sand-tiger-sharks

https://www.ehp.qld.gov.au/wildlife/threatened-species/endangered/endangered-animals/grey_nurse_shark.html

Amorim, A.F.; Arfelli, C.A.; Fagundes, L. (2005). "Odontaspis noronhai"IUCN Red List of Threatened Species. Version 2012.2International Union for Conservation of Nature