4. Dec, 2017

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



15. Nov, 2017

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:









22. Oct, 2017

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.

Froese, Rainer, and Daniel Pauly, eds. (2014). "Chimaeriformes " in FishBase. November 2014 version

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.




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


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