The Gentle Giants of the Sea: Whale Sharks

I’ve sort of been neglecting this blog lately: I recently started school and a new job, and as a result writing in here has fallen by the wayside. BUT, I’m in the mood to write tonight!

The focus of today’s post is an animal that has fascinated me for years. That animal is the largest extant shark (and also the largest fish in the world), the whale shark, or Rhincodon typus. The name “shark” often evokes an image of a ferocious predator with rows of sharp teeth that you may not want to get too friendly with, but when it comes to the whale shark that image is misleading. Although they are a member of the Elasmobranchii group (which includes other sharks, as well as skates and rays), they are not nearly as intimidating as their other sharky brethren can be. At an average size of 12 m (40 ft), a maximum size believed to be 40 m (65ft), and weighing in at an average of 20.6 tonnes or more, the whale shark could make quite the formidable predator; however (perhaps fortunately for us) this shark only ingests plankton, as well as small fish, krill, squid, and crustaceans.

Oh, and it’s super gentle. In fact, scuba divers often swim with whale sharks in the open ocean.

The whale shark is one of three species of shark that filter feed, the other two species being the basking shark and the megamouth shark. In essence, the shark sucks water into its sizable mouth like a vacuum, where gill rakes then filter nourishment from the water before it is expelled through the gill slits. Although they do have about 3,000 tiny teeth in their mouths, these teeth seem to play no role in feeding. The sharks prefer feeding in the evenings near warm surface waters, where their prey is abundant. They may also congregate where food is plentiful, but they are generally solitary creatures.

Check out that huge maw.

Filter feeding

The whale shark mouth in action.

Whale sharks have a very large distribution throughout the world’s oceans, appearing in all tropical seas. It is not known where whale sharks go to reproduce, although they are known to be ovoviviparous (bearing live young).  There have been no studies which have validated growth rates, age of maturity, or maximum age, although a tentative hypothesis is that they can live to be 60-100 years old.  This species is also believed to be extremely migratory. In 2001, three sharks tagged near Seychelles in the Indian Ocean were then tracked traveling to Zanzibar, then Somalia, and over 5000 km to the coast of Thailand. More studies would need to be done to conclusively prove that they are indeed highly migratory animals.

Geographic distribution of Rhincodon typus

Whale sharks have distinct white markings on their bodies which form patterns that are unique to individuals, and scientists have used this to their advantage in tracking and identification.  Together, marine biologist Brad Norman, astrophysicist Zaven Arzoumanian, and computer programmer Jason Holmberg embarked on a mission in 2005 to devise a computer program that could identify whale sharks through images of their spots and stripes. They modified an existing algorithm that was originally written for the Hubble Space Telescope to identify patterns of stars in the sky. The new computer program they developed is now able to scan photographs of whale sharks and identify individuals based on the patterns of their markings. There now exists an online database of recorded whale shark encounters, the Wildbook for Whale Sharks, to which anybody can submit their photo to be identified and to help with research (http://www.whaleshark.org/).

White spots and stripes form patterns on the skin of the whale shark.

Thank you for reading, and I hope you enjoyed my post for today! I’ll end off with a video that sparked my original interest in these gentle giants, and although I’m not quite sure I support the captivity of such large, purportedly migratory animals, the video is nonetheless beautiful.

The Cetacean Brain and Hominid Perceptions of Cetacean Intelligence

Creature Feature in the spirit of Shark Week: The Greenland Shark

In the midst of Shark Week, it only seems fitting to do a post about sharks. The shark I’ve chosen to write about isn’t one of the most popular or well known species, and it’s generally not one that shows up in many television shows; however, it’s worth taking the time to learn about!

Somniosus microcephalus, also known as the Greenland shark or Sleeper shark, is one of the larger species of shark and can reach up to 24 feet in length. It is a member of the class elasmobranch, which is a group that includes sharks, rays and skates, and therefore it has a skeleton made of cartilage rather than bone. Sightings of this species of shark are quite rare. The first underwater photos of a Greenland shark weren’t taken until 1995, and the first video of one swimming in its natural habitat wasn’t taken until 2003. Because of this, not much is known about the Greenland shark, although research is now being conducted by a Canadian group known as the Greenland Shark and Elasmobranch Education and Research Group, or GEERG.

The Greenland shark is a pretty strange looking guy:

He does look kind of happy though.   

He is also reaalllly slow, averaging a cruising speed of 0.76 mph. This sluggish speed works well for these sharks due to their preferred habitat in the Northern Atlantic and Arctic regions, in cold waters ranging from -0.6 to 12°C. A slowed metabolism allows them to be the only species of shark that can tolerate cold temperatures year round. They also age slowly, with a growth rate estimated between 0.5 to 1.1 cm a year (which puts the 7 meter, or 24 foot long, individuals at an estimated age of over 200 years!)

Geographic distribution of the Greenland shark.

One very interesting fact is that the Greenland shark often plays host to a parasitic copepod (small crustacean) called Ommatokoita elongata. This parasite attaches to the eyes of the shark, feeding off of the tissue and rendering him blind. Fortunately, the shark seems unaffected by it since it relies much more on its sense of smell than on vision to search for food. The Greenland shark’s food of choice is fish, such as salmon and cod, and any carrion it can sink its teeth into. Marine mammals such as seals have been found in the contents of this shark’s stomach, however it is uncertain as to whether they are able to catch a live seal. Other remains of animals found in the Greenland shark’s stomach include polar bears, porpoises, moose, horses, and in one case an entire reindeer.

You can see the white, string-like parasite attached to the shark’s eye.   

The Greenland shark is ovoviviparous, meaning that the female carries eggs within her body until the embryos are full term. They then hatch within her body and are born live, with no placental attachment.

The flesh of the Greenland shark is toxic to humans due to the presence of TMAO (trimethylamine oxide) that is used by the shark to regulate osmotic pressure as well as working as a natural antifreeze. It is still eaten in Iceland as a delicacy: the meat is first compressed in large containers to remove the toxins and then allowed to dry outside for 2-4 months, after which it is safe to consume.

I will end this post with this brief video of a Greenland shark encounter. Thanks for stopping by!

 

Ridiculously cute ocean critter of the day: the La Plata river dolphin

Is this not one of the cutest things you have ever seen in your life?

Of all of the earth’s creatures, dolphins are arguably one of the most well known and well loved by humans. Playful, intelligent, and with a perpetual ‘smile’ on their faces, it’s hard not to fall in love with them. Today’s critter focus is on a less familiar member of the dolphin family: the La Plata river dolphin. Also known as the Franciscana dolphin, or by the more difficult to pronounce taxonomic name Pontoporia blainvillei, the La Plata dolphin is  one of the smaller, and rarer, species of dolphin. They belong to the order Cetacea, a group of marine mammals that includes all whales, dolphins and porpoises. Of the two suborders of Cetaceans, dolphins belong to the toothed whale group Odontoceti (as opposed to the suborder Mysticeti, or baleen whales).

Examples of Cetaceans

Pontoporia blainvillei skull

The La Plata dolphin is found in the shallow coastal waters off of Uruguay, Argentina, and Brazil. It also has the ability to move into estuaries and rivers such as (predictably, given the name) the La Plata river at high tide, in search of food. Of the four types of river dolphins, it is the only species that can also be found in saltwater environments. Characteristic of other river dolphins, it  has a very long and narrow beak and unfused vertebrae in the neck, allowing it the mobility to turn its head. This helps the dolphin to better navigate through rivers and catch prey. Mature dolphins have a light colouring and are sometimes referred to as “white ghosts” by local fisherman, although calves (juveniles) are darker in colour. They average in size of 1.2 – 1.8 m long (4 – 6 ft) and up to 50 kg (110 lbs).

Geographic distribution of Pontoporia blainvillei in South America

A mature La Plata dolphin

Like other dolphins, La Plata dolphins use echolocation to help visualize their surroundings. They emit a series of high frequency clicks, and a fatty organ called a melon in their forehead then focuses these clicks into a beam as they are emitted. The sounds then “echo” off the surroundings and are returned to the animal,  which receives them in the middle ear via the mandible (lower jaw). This is especially helpful in navigating and finding food in turbid river environments where visibility can be poor. The food the dolphins search for consists of fish, squid, and shrimp.

Sound production and reception in a toothed whale

La Plata dolphins travel in small familial groups, usually of 15 or less, and several of their populations do not mix. Because of the these small groups, reversed sexual dimorphism (males are smaller than females), small testis sizes of the males (which suggests sperm competition is not likely), and the fact that males have few scars from members of their own species, it is hypothesized that they form monogamous pairings.

Sightings of La Plata dolphins are rare. They are listed by the IUCN as a vulnerable species, and are threatened by commercial fishing because they are often caught as by-catch in gillnets. They are also at risk due to pollution, and ingestion of plastic and other debris in the ocean. There are many things each of us can do to help reduce the amount of plastic waste reaching the ocean, such as using eco-friendly shopping bags, making use of recycling programs, and sharing info!

The following images are of “Nipper”, a 7-day old La Plata dolphin baby that was rescued when it was found near a beach with some fishing net wounds. Heartbreakingly, Nipper was unable to survive his ordeal; however, there are many other animals like him, of all different species, in the wild that can use our help!

Thank you for stopping by Below the Surface, and I hope you enjoyed today’s post!

Starfish

For the first real post I’ve decided to dive right in (excuse the most obvious ever of puns), with a well known and recognizable sea creature: the starfish.

 

The leather sea star, Dermasterias imbricata

 

The starfish, or sea star, is not related to fish at all but is rather part of a group of organisms called echinoderms, which includes several other marine animals such as sea urchins, feather stars, and sea cucumbers. There are over a thousand species of sea star, which can be found in all of the world’s oceans. They are considered invertebrates, which means they do not have a backbone, although they do have a skeleton made of bony plates. Most people recognize  starfish by their five arms, radiating symmetrically outward from a central point; however, some species can grow more than five arms, and even numbering up to forty.  It is both fascinating and fairly common knowledge that some species of starfish have the ability to not only regenerate a new limb if one is removed, but also to regenerate a whole new starfish from said limb.

 

Crown-of-thorns starfish, Acanthaster planci

 

Nine-armed sea star, Luidia senegalensis

While the starfish does have a nervous system, it lacks a brain and complex sensing organs. Starfish also have a circulatory system but do not have any blood: they instead use a  network of tubes filled with filtered sea water. The sea water is used to transport nutrients and oxygen, and even provides hydraulics to move thousands of tiny tube feet located on the arms.

Fun Fact: Most starfish have the ability to consume prey outside of their body. Basically, the sack like cardiac stomach (see diagram) is pushed out through the mouth of the starfish (located at the centre of the main body portion) which then enters the shell of a clam or oyster, where it can begin its process of digestion. Yum!

 

Thanks for stopping by, guys, and I hope you enjoyed my first post!