The whale shark (Rhincodon typus) is one of the many shark species I have never encountered. Perhaps one day when I am done with the Bahamian sharks (will I ever?), I shall cross the Atlantic to visit these gentle plankton gulping giants at Islas Mujeres. During the summer months hundreds of whale sharks gather just north of the island in a seven mile radius to take advantage of the plankton rich waters created by the joining of the Gulf of Mexico and the Caribbean Sea.
Left: The region behind the gills of whale sharks (above) exhibits suitable variation in spot pattern to enable individual recognition using image-matching software (below: see Arzoumanian et al. 2005).
Whale sharks have recently been upgraded from vulnerable to endangered under the IUCN Red List of Threatened Species. There are still numerous reports of individuals being killed for food, including having their fins removed for soup. Their characteristics – docility, being the largest of all extant sharks, and tendency to aggregate seasonally at several accessible locations worldwide – have encouraged ecotourism and citizen-science opportunities, but also mean they can easily be targeted by fisheries in jurisdictions where they are not afforded protection.
Adult’s whale sharks grow slowly and only reach sexual maturity when they are 30 years old, with their maximal age varying between 70 and 100 years. It seems that nobody has yet seen whale sharks mating or a female giving birth to her pups. A female species captured in 1996 appeared to have around 300 pups in her womb each about 40 cm long. There are several whale shark aggregation sites -or hotspots- spread over the world such as the Philippines, Islas Mujeres in Mexico, Belize, South Africa, Mozambique, Qatar and the Gulf States, Galapagos Islands and the Maldives.
Still little is known about the whale sharks migratory habits and habitats. A recent paper in Biomagazine* however described a large scale project of Australian researchers called EcoOcean. Central in the project is the Wildbook for whale sharks: a visual database of whale shark encounters of individually catalogued whale sharks. Here people can assist with whale shark research by submitting photos and sighting data. Since 1992 the library has collected around 30.000 pictures of whale sharks over 57 countries, many taken by snorkelers and scuba divers visiting one of their hotspots. This included in particular Ningaloo Reef and other sites along the along the extended Western Australian coastline. The ever growing data-base caused the number of 14 known hotspots to increase to 20. Top four sightings of whale sharks were from the locations Mexico-Atlantic, Western Australia (Ningaloo Marine Park), Mozambique and the Philippines (e.g. Cebu)
The project has contributed to a better understanding of the wale sharks mobility as well as their regular habitats. An unique aspect of the project is that creation of the picture library was realized with the help of Citizen Science: pictures taken by snorkelers or scuba divers while visiting popular sites of whale sharks. The technique used to identify the sharks in the EcoOcean project is straightforward, and based on pattern recognition software using the white dot patterns behind the gills of the whale shark as a standard (see picture above). This particular area on the shark’s skin needs to be photographed in the correct orientation. Participants also upload, when possible, other relevant sighting information for storage and future analysis, including sighting location, sex, and estimated total length
As noticed in my earlier Blog, using natural body markings is an alternative and shark-friendly way to identify individual sharks, although it does not allow real time tracking of the shark. But it is much cheaper than tagging of sharks with telemetric devices, a more invasive method that can only be applied on a relatively small number of species. An it also allows to search for individual sharks by comparing its unique pattern with a larger data base (somewhat like a fingerprint of a foreign visitor collected by the US customs at the airport). Another interesting side of this method is that UW photographers can contribute to collect such a data base.
The first whale shark that entered the library was a wale shark called Stumpy, because of the unusual shape of the upper lobe of its caudal fin. Stumpy’s unique patterning was later identified in 69 photographs submitted to the library between 1995 and 2016. During these two decades, Stumpy’s total length remained relatively unchanged at 7.40 m which closely matches the 7.44 m estimates calculated from lengths reported by citizen scientists between 2008 and 2015. Photographic evidence also showed that Stumpy’s claspers had become elongated for the first time in 1998 and 2001, respectively – an indication that they had attained maturity*. Other whale sharks from the data base are A-103, a faithful customer of Ningaloo reef in Australia estimated to be about 21 years, and BZ-011 that has been spotted over 5 years along the coast of Belize, but nowhere else. A surprising exception was A-424 who showed far more mobility. After been identified first in Australian waters, she was spotted later in Indonesia about 2.700 km further to the north-west. Smallest species were found in Indonesia (4 meter) largest in Galapagos (11 meter). In general, long distance migratory routes were extremely rare. However this did not rule out the possibility that the date base might cover only a small part of the world population, or that the identified whales sharks had visited other more remote and unknown locations.
Taking these cinsiderations in account, Citizen Science seems to have demonstrated that:
-Whale sharks are far less migratory than other shark species (like for example tiger sharks ak)
-Exhibit a high degree of site fidelity with longevity conservatively estimated to be at least 80 years
-Males and females are strongly segregated with a preference for their own distinct habitats (in accordance with other predator sharks ak). For example at the Galapagos, 99% of sexed individuals were female while in Maldives and South Africa, only 9.43% and 9.60%, respectively, of the sexed whale sharks that were submitted were females
Sources and links
*Norman B. et al. (2017) undersea constellations: the global biology of an endangered marine megavertebrate further informed through citizen science Bioscience xx: 1–15. Published by Oxford University Press on behalf of the American institute of Biological Sciences
Hsu HH, Joung SJ, Hueter RE, and Liu KM. 2014. Age and growth of the whale shark (Rhincodon typus) in the north-western Pacific. Mar Freshwater Res 65: 1145–54.
Norman B and Stevens J. 2007. Size and maturity status of the whale shark (Rhincodon typus) at Ningaloo Reef in Western Australia. Fish Res 84: 81–86
Arzoumanian Z, Holmberg J, and Norman B. 2005. An astronomical pattern-matching algorithm for computer-aided identification of whale sharks Rhincodon typus. J Appl Ecol 42: 999–1011.
Davies TK, Stevens G, Meekan MG, et al. 2013. Can citizen science monitor whale-shark aggregations? Investigating bias in mark–recapture modeling using identification photographs sourced from the public. Wildlife Res 39: 696–704
The Sperm whale (Physeter macrocephalus) is a highly social and gentle mammal. Its behavior certainly not justifies the reputation of ferocious monster created by Melville when the describes the deadly struggle between the white whale Moby Dick and the obsessed captain Ahab.
Left: skin pattern of a sperm male after exfoliation of the skin (author unkown)
Sperm whales are the largest predators on earth. They also have the largest brain in the animal kingdom, and can dive over 7000 feet deep to find their favorite food, the giant squids. It thanks its name to its enormous head, almost one third of the size of its body (15-18 meters). The large head of the sperm whale functions as one big sonar system. The sperm whale produces ‘clicks’ (sound burst of short duration) with a pair of phonic lips (also known as "monkey lips" or "museau de singe") at the front end of the nose, just below the blowhole. Sperm whales often swim in small social units or pods, although several pods may form larger groups or clans distributed over a much larger area. Recent studies have shown that families and clans have their own 'dialect': typical signatures of sound bursts called codas. Using these codas sperm whales recognize vocalizing individuals of other social units that share a similar dialect.
Although sperm whales usually swim in small groups there are periods when they gather in much greater numbers at certain locations. These periodical meetings have the character of ritual meetings or ‘get togethers’ were several clans meet members of other clans. Resembling the pow wows, the social gatherings held in the past by different Native American communities.
An area where these gatherings can be observed by snorkelers is Dominica in the Western Carribean. The ocean floor along Dominica's west coast drops steeply to several thousand feet very close to shore, providing a calm and sheltered area for a large group of resident sperm whales to feed, mate, and socialize. Year round, they can be spotted very close to shore, cruising up and down the island's coast. On occasions as many as 70 animals come together for hours or days at Dominica. Here some remarkable forms of behavior have been observed and photographed by UW photographers Keri Wilk from Canada and Tony Wu from the USA, which also has shed more light on the function of these gatherings. It appears that the major incentive for these cetaceans to gather periodically is highly practical, namely to groom each other by rubbing their massive bodies and itching skins together. The result of skin rubbing shows up in large chunks of skin floating on the surface giving the impression of large plastic bags. As a result the whales start to show white and black camouflage-like skin patterns, where patches of new skin show up against the old scraped of skin (see picture above).
According to Luke Rendell, a marine biologist at the University of St Andrews, UK. this shedding of skin is part of a natural antifouling mechanism to stop them being encrusted with other marine animals and parasites. “They love touching against each other and one of the rewards may be exfoliation,” says Rendell. Even more remarkable than the skin rubbing rituals (called ‘scratchatons’ by Tony Wu) are the concurrent defecations when the whales disperse clouds of liquid poop looking like chocolate milk in the water. The whales often show prolonged bowl movements, unlike the normal defecations when shark return to deeper water. One can only guess about the meaning of these Poonados (a term used by Keri Wilk). Pooping can occur as a a defensive reaction or a sign of anxiety, as sometimes observed in smaller whale species when feeling insecure. The clouds produced during defecation could also be a form of camouflage, like the ink-clouds emitted by the octopus. Another possibility is that adult bulls in the pod use these clouds to impress rivals or females from other clans. Finally, whales could simply enjoy the experience, with the reaction to empty their bowels triggered by with the pleasant sensation of skin rubbing. Whatever the reason, group defecation seems to be an integral part of large social gatherings for these animals...a group poop, so to speak. Another question that remains to be answered is if these mass gatherings perhaps also serve to swap between members of the pods, for example when females or young bulls hop over to other pods where their presence is more urgently needed to guarantee new offspring.
Source and links:
In Greek antiquity the Chimaera was a mythical monster, depicted as an incongruent animal with the head of a roaring lion spitting fire, the body of goat sticking out in the middle and a vicious snake-like tail. The word also stands for a delusion; like in the french chimère. In our oceans the chimaera fish (alias: ghost shark, rabbit fish, spook fish) earned its name because of its strange body shape: a composition of a birds beak, a fishlike body and long slender tail (see picture at left). The rat-like tail is the reason why certain species are called ratfishes. Chimaera seems to have diverged from its shark relatives around 400 million years ago. The ways of evolution are often mysterious and hard to unravel. Which holds also for its products, the creatures that have lived in the seas for millions of years, and probably even more so for the chimaera.
Considering the diversity of 'shark like species' it is good to start with a taxonomic classification. Chimaera, skates, rays and sharks are all cartilaginous fish, belonging to the class of Chondrichthyes with a cartilaginous skeleton, and claspers in the males. There are two subclasses of cartilaginous fishes: the Elasmobranchii (with sharks, rays and skates and the sawfish) and the Holocephali (chimaera), indicating that members of Elasmobranchii are more closely related to another than to the chimaera. Further down the taxonomy we have families and species. Chimaera, or rather the order of Chimaeriformes consists of many different species with different outlooks and habitats. Some species have several synonyms or aliases which make the naming process a bit messy. But taken together they end up in 50 accepted species that are assembled in three families:
Families and species
-Callorhinchidae (Plownose chimaera: alias elephant fish and ghostshark, only one accepted species) are the oldest clade in the evolution thee. They have an elongate and flexible snout bearing a hooklike structure and are mostly found on the Southern hemisphere.
-Chimaeridae (Shortnose chimaera or ratfishes; 40 species) have a short and rounded snout. They are found in Atlantic, Pacific, and Indian oceans in temperate to tropical waters, mostly below 200 m. Some popular species are Chimaera monstrosa alias rabbit fish (the only species in the Mediterranean), the spotted ratfish (Hydrolagus colliei) in the north-eastern Pacific Ocean, and the small-eyed rabbitfish (Hydrolagus affinis).
-Rhinochimaeridae (Longnosed chimaera; 8 species) have a long and pointed snout, lacking a hooklike process and living worldwide in temperate and tropical seas. Some weird looking species are: Narrownose chimaera, alias Harriotta raleighana alias spookfish, the long nosed Rhinochimaera pacifica and the paddlenose chimaera (Rhinochimaera africana) with its flattened paddle-like nose.
Anatomy and habitat Chimaeriformes have large rabbit-like eyes, and a large head along with a tapered body. The large translucent-green eyes are adaptions of the darkness of water at greater depths. Although chimaera has some characteristics in common with sharks and rays, there also big differences. One example is the head with the small mouth and lips, and upper jaws that are fused with their skulls. Chimaera also miss the row of sharp teeth of sharks, but posses three bony tooth plates with fused teeth, forming a ideal beak to break hard shells. Their diet consists mainly of bottom-dwelling invertebrates like sea urchins, crabs, shell fish, crustaceans and starfish.
Chimaera has two large dorsal fins, the first erectile high. with a short base and preceded by an erectile poisonous spine, the second nonerectile low, and with a long base. Their bird-like style of swimming with the spread out big pectoral fins resembles the propulsion of rays. Like sharks they are equipped with electroreceptor cells on their snout for orientation. But unlike sharks and rays, chimaera has a single external gill opening, covered by a flaps or opercula as in the bony fishes, on each side of the body. Breathing water chiefly occurs through the nostrils.
Chimaera species vary in size between 60 cm and 1.5 m and live in temperate oceans. They tend to dwell on muddy or sandy seafloors, often down to 2,600 m deep, with few occurring at depths shallower than 200 m. This is also the reason why pictures of these species are rare. Exceptions include the members of Chimaeridae, like the rabbit fish and the spotted ratfish, which locally or periodically can be found at relatively shallow depths. Many species that live on or just above seafloors at greater depth have become the victim of deep sea trawlers.
Reproduction Chimaera are oviparous. Male chimaeras have claspers formed from the posterior portion of their pelvic, one of which is used to inseminate the femal. In addition, they possess a supplemental clasping organ, the tenaculum on the forehead, which is thought to aid in holding the female during mating. It is only visible during copulation and then used to clamp onto the female’s pectoral fin. This remarkable stalked club structure with little hooks must have been a clever adaptation of their evolutionary ancestors that lacked the rows of sharp teeth that male sharks use to hold their mating partner in a steady position. The females lay eggs in spindle-shaped, leathery egg case. Finally, there is a second grasping structure, the pre-pelvic tenaculum, just before the pelvic fins that also allow the male to anchor into position.
Sources and links
Nelson, J.S., 1994. Fishes of the world. Third edition. John Wiley & Sons, Inc., New York. 600 p.
Didier, D. A., Kemper, J. M., & Ebert, D. A. (2012). Phylogeny, biology, and classification of extant holocephalans. Biology of sharks and their relatives, 2nd edn. CRC Press, New York, 97-124.
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
Jellyfish Take Over an Over-Fished Area". 21 July 2006
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 Halodule. But 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
Preen, Anthony (1995). "Impacts of dugong foraging on seagrass habitats: observational and experimental evidence for cultivation grazing". Marine Ecology Progress Series. 124 (1/3): 201–213https://en.wikipedia.org/wiki/Sirenia