Here There Be Giants

I've been wanting to check out the Giants exhibition at the National Museum of Scotland for a while now and, since the exhibition will be closing soon, I knew I needed to get a move on. Giants, designed and produced by the Royal Belgian Institute of Natural Sciences, exhibits life-sized sculptures of prehistoric megafauna (giant animals) that incorporate fossils into the design. The exhibition includes two marine species that I specifically want to talk about.

The real leviathan

Livyatan melvillei is an extinct relative of the sperm whale (Physeter macrocephalus) that lived about 9.9 million years ago to, potentially, 5 million years ago. At a body length of 13.5 to 17.5 metres, Livyatan was not much bigger than a modern sperm whale but, once you look at their teeth, you realise there's a clear winner on size. Just look how this Livyatan melvillei tooth compares to that of a sperm whale and a killer whale (Orcinus orca).

There's another clear difference between Livyatan and a sperm whale when we compare the jaws. Let's take a look at Moby's skull, another exhibit in the National Museum of Scotland's collection, pictured below. You'll notice Moby only has teeth in his lower jaw. When he was alive, these would have fit perfectly into sockets in his upper jaw, ideal for gripping squid so that it can't slip away.

The skull of a sperm whale (Moby) on display in the Grand Gallery at the National Museum of Scotland

Compare that with the skull of Livyatan melvillei pictured below and we see that Livyatan notably has large interlocking teeth in both the upper and lower jaws, providing the efficient bite it needed to prey on seals, dolphins, and other whales, with the tilted front teeth providing grip. The skull also gives some clues as to what Livyatan would have looked like, indicating a short, wide snout, with the large openings on the side suggestive of its incredible jaw muscles.

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L. melvillei fossils have been found in Chile and Peru but it is thought that the species would have had a wide global distribution. They were extinct by the early Pliocene around 5 million years ago, when there was a global cooling trend that caused baleen whales to increase in size and decrease in diversity, depleting Livyatan's main food source. This is a fate that parallels that of our next exhibit, Otodus megalodon.

The biggest shark that ever lived

Otodus megalodon, or simply megalodon (meaning big tooth), was a species of mackerel shark that lived from the early Miocene around 23 million years ago to the early Pliocene around 3.58 million years ago. As is the case with all sharks, megalodon had a skeleton made of cartilage rather than bone, making fossils (other than their teeth) rare. However, from the few vertebrae that have been found, their position in the spine can be plotted, like in the pictures, allowing scientists to estimate the shark's body length, with this particular individual being about 16 metres. To put that in perspective, the largest confirmed size for a great white shark (Carcharodon carcharias) is 6.1 metres.

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Reaching over 18 cm in slant height, megalodon had the largest teeth of any known shark species. These are serrated along both edges of the blade of the tooth, allowing them to cut through blubber and bone. As shown in the model below, like modern sharks, megalodon had rows of teeth in its jaw, ensuring that old and worn teeth were constantly being replaced with new ones. Maintaining a full set of teeth in this way, combined with its size and its powerful jaws, would have made megalodon a highly competitive apex predator in marine ecosystems.

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Megalodon fossils are widely distributed, with specimens found in Europe, Africa, Asia, Australia, and the Americas. This suggests the shark inhabited a range of marine habitats all over the world, making it likely that megalodon would have shared hunting grounds with Livyatan for a time, competing for similar prey of cetaceans. This means megalodon was faced with the same challenges as Livyatan during the early Pliocene, with food sources being insufficient to sustain Otodus megalodon populations. At this time, many marine giants died out, changing the oceans forever.

The Giants exhibition is open till Monday 14 September so you still have a little time if you want to see it for yourself. As well as the two marine species I've highlighted here, there are a wide variety of terrestrial animal exhibits on display, including Titanoboa (the largest snake to ever live), a giant ground sloth, and a woolly mammoth. The exhibits are definitely worth seeing in person to appreciate the sheer scale of the creatures that once lived here.

A Visit to Sea Life Loch Lomond

A dear friend of mine invited me along on a trip to the Sea Life centre at Loch Lomond recently. I hadn't visited since my first year at university as part of a practical for my first marine biology module. That was 17 years ago! I was interested to see what has changed and what has stayed the same in that time and the aquarium did not disappoint. I have lots to show you.

Lobsters galore

The aquarium has a collection of native lobster species, including European spiny lobster (Palinurus elephas) and European lobster (Homarus gammarus). In Scotland, European spiny lobsters live on the seabed around the western and northern coasts and, occasionally, the northeast coast. They prefer rocky seabeds at depths greater than 15 metres and like crevices and boulder holes they can hide in. They can grow to an impressive 60 cm in length and mature at around 10 cm. P. elephas numbers around Scotland have been in decline since the 1970s, likely due to a combination of factors, including fishing, environmental conditions, and water quality changes.

European spiny lobster (Palinurus elephas)

European lobsters are usually dark in colour, appearing almost black, but, like the individual at Sea Life Loch Lomond, some have a striking blue colour. They can be found around the entire Scottish coastline at depths of about 40 to 70 metres. They grow to around 25 cm in length and have an average lifespan of 45 to 50 years. Like the European spiny lobster, there are a number of pressures on H. gammarus populations, including fishing, habitat destruction, and pollution, and protection measures are in place in Scotland, with any lobster of a body length of less than 87 mm being protected under Scottish law.

European lobster (Homarus gammarus)

Like other decapod crustaceans, lobsters have several different types of appendages that they use for different purposes. Starting with the furthest back, the tail fan has the uropods, which are used for propulsion and steering when swimming. The underside of the abdomen (the tail section) also has four pairs of feathery paddle-shaped appendages called pleopods, which beat back and forth for swimming. The five pairs of walking legs, or pereiopods, on the thorax are how decapods get their name but lobsters are probably best known for their crushing and pinching claws, or chelipeds, which are classed as the first set of pereiopods. Modified appendages called maxillipeds are one of the components of the mouthparts, handling and processing food and passing it to the mandibles. Meanwhile, the maxillae, as well as playing a role in feeding, also serve a sensory function, beating to manipulate water currents and pass water over the chemoreceptors on the lobster's head, allowing it to smell and taste the surrounding environment, something that this individual I captured on video shows off well.

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As an aside, if lobsters interest you, the Forth Marine Hatchery in North Berwick is a conservation project that rears H. gammarus juveniles for release into the wild to restore population numbers. You can keep up with their work via their Facebook page.

A second chance for April

Sea Life Loch Lomond is also the home of a rescued olive ridley sea turtle (Lepidochelys olivacea) named April. Olive ridley sea turtles live in tropical waters, usually in the Pacific and Indian Oceans but they can also be found in the Atlantic. They are the second smallest of the seven species of sea turtle and the most abundant, though they are still vulnerable to a number of threats, including entanglement and ingestion of marine plastics.

April the olive ridley sea turtle (Lepidochelys olivacea)

April was found in the Maldives on the ocean surface entangled in ghost fishing gear and with a plastic bag caught around her neck. She had already lost her front right flipper and the left one was injured. She was brought to the Four Seasons Turtle Rehabilitation Centre, where she was cared for and her left flipper was able to heal. Unfortunately, April had also developed buoyancy syndrome, preventing her from diving, which meant she would no longer be able to survive in the wild.

With that in mind, Marine Savers and the Sea Life Trust teamed up to bring April to her new home at the coral reef area in Sea Life Loch Lomond, which mimics her natural habitat. This required an incredible 5000-mile journey, involving travel by speedboat, plane, and road before April could settle in at the Loch Lomond centre, where she now acts as an ambassador for the impacts of plastic pollution.

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What a mermaid keeps in her purse

Within the centre, there are a selection of eggs on display that have been collected from the various shark and ray species that call Sea Life Loch Lomond home. These are made of tough keratin casings with tendrils that allow them to anchor onto rocks and seaweed. They are known as mermaid purses. However, only around 40% of shark species actually lay eggs. Others, like the blacktip reef sharks (Carcharhinus melanopterus) on display at the aquarium, give birth to live young. And the aquarium is also home to a zebra shark (Stegostoma tigrinum) that is able to reproduce without a partner in a process called parthenogenesis, laying eggs that contain a genetic clone of itself.

Mermaid purse (shark egg), I believe from a lesser spotted dogfish

The lighting within the mermaid purse display means that the shark and ray embryos are able to be seen, even through the thick casing. In the video below, you can see this little lesser spotted dogfish as it moves around.

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The lesser spotted dogfish (Scyliorhinus canicula), despite its name, is actually a species of catshark, also known as the small-spotted catshark. It is one of the most abundant shark species in Scottish coastal waters and they grow to just under 1 metre in length. They can also be quite social creatures, as exhibited by this little huddle that I saw.

Lesser spotted dogfish (Scyliorhinus canicula)

The aquarium also has a rare albino individual that was caught by some fisherman and given to the Scottish Sealife Sanctuary in Oban, before eventually finding a new home at Sea Life Loch Lomond. You can partially see her tucked away in the picture below.

Albino lesser spotted dogfish

Little rays of sunshine

The cownose ray (Rhinoptera bonasus) is, in my opinion, one of the most unusual-looking ray species. They appear almost like they have two mouths, until feeding time comes and they open wide. Their diet consists of molluscs and crustaceans and they capture their prey via suction feeding by opening and closing their jaws, while the cephalic lobes, the flaps by the ray's mouth, unfurl to capture food and push it towards the mouth.

Cownose rays (Rhinoptera bonasus) showing mouth in closed and feeding positions (photo credit: Marcus Burnette)

Cownose rays are native to the eastern coast of the United States, Brazil, and the Gulf of Mexico. They are most commonly found in shallow estuaries at depths of up to about 60 metres and can tolerate a wide range of salinities, allowing them to migrate and change habitats in response to resource availability. The cownose ray is currently listed as vulnerable on the IUCN Red List of Threatened Species, with the ray's biggest threat being overfishing.

Sea Life Loch Lomond is home to the only cownose rays in Scotland, named Mary, Sarah, and Winnie, and they arrived at the aquarium in 2016. They are quite a lively group and move really gracefully through their tank, wouldn't you agree? Just look at this video.

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Sticking with elasmobranchs (sharks, rays, and their relatives), Sea Life Loch Lomond has the biggest variety of elasmobranch species in Scotland, as illustrated by this video I filmed in their tropical ocean tunnel. How many different species do you see?

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This is, of course, just a small sampling of what can be seen at Sea Life Loch Lomond. If you're ever in the area, I would recommend checking it out for yourself.

Species Spotlight: Ocean Sunfish

If I asked you to picture a fish, chances are that most of you reading this would come up with a similar mental image with the same basic form and features. But, although a majority of fish can fit into that classic template of what makes a fish, fishes have diverse forms, body shapes, and characteristics, with perhaps one of the most divergent being the strange-looking, slow-moving ocean sunfish, or Mola mola.

Description

The ocean sunfish is one of the largest bony fishes in the world. Adult individuals usually weigh between 247 and 1,000 kg, although the maximum weight recorded so far is 2,300 kg. In addition, they have an average length of 1.8 m and an average height (or fin-to-fin length) of 2.5 m, although individuals of 3.3 m in height have been recorded.

The ocean sunfish (Mola mola).

The fish's body is flattened laterally and lacks the typical fish tail (or caudal fin) , which is instead replaced by a rounded, lumpy structure known as the clavus. It has elongated dorsal and anal fins, and the pectoral fins are relatively small and fan-shaped. The fish has a beak-like mouth formed of four fused teeth. It also has quite thick skin, which can be up to 7.3 cm thick in some places, and, like a shark, its skin is covered in tooth-like denticles rather than scales.

Habitat

The ocean sunfish can be found in tropical and temperate waters all over the world. They spend a large portion of their time in the deep sea, reaching depths of greater than 200 m. They tend to be found in waters warmer than 10 °C; however, spending a prolonged amount of time in water temperatures of 12 °C or below can harm the fish, causing disorientation and potentially leading to death.

Many of you who have encountered an ocean sunfish for yourself will have likely done so during its somewhat bizarre-looking surface basking behaviour, where the fish swims on its side at the ocean's surface. It is thought that one of the reasons for this behaviour may be as a way for the fish to warm itself after its excursions to deeper, colder water to feed.

Ocean sunfish basking at the surface (photo credit: Laurel Parshall).

Diet

Ocean sunfish are generalist predators with a broad diet. Their beak-like mouth is perfect for catching and gripping soft-bodied prey but they also have pharyngeal teeth in the throat that grind their food into smaller pieces. They mostly eat small fish, fish larvae, squid and other molluscs, and crustaceans but they are also able to eat jellyfish and other soft-bodied animals like salps. The sunfish pulls in and spits out water through its mouth in order to tear apart the jellyfish.

The diverse range of prey items that the ocean sunfish consumes demonstrates that the fish feeds within several different ocean layers, from the surface to the deep seas. There is even evidence that the ocean sunfish will occasionally hunt on the seafloor in some areas.

Life Cycle

A number of spawning areas have been suggested for the ocean sunfish in the North Atlantic, South Atlantic, North Pacific, South Pacific, and Indian Oceans. The females can produce more eggs than any other vertebrate, up to 300 million at one time, which they release into the water to be fertilised externally by the males.

Once hatched, the larvae are only 2.5 mm long and weigh less than 1 g, so they have a lot of growing to do to before they reach their adult form. First, they develop into fry, which resemble mini pufferfish, complete with spines all over the body that they later lose as they grow.

An ocean sunfish larva (photo credit: G. David Johnson).

Young ocean sunfish tend to school together for protection. However, as they get older (and bigger), they go their separate ways, with the fully grown adults being mostly solitary. The fish's natural lifespan in the wild is not yet known.

How do ocean sunfish get around?

Contrary to popular perceptions, these fish are not passive drifters in the ocean. As we've already established, they spend a lot of time at depth, where they actively hunt for prey. Some ocean sunfish have been recorded swimming 26 km in a day, swimming at a speed of 3.2 km/h, not zippy by any stretch of the imagination but not sedentary either.

Due its body shape, the ocean sunfish has a different swimming style compared to most other fishes. The clavus is used much like a rudder, while the fish uses its dorsal and anal fins to propel itself. In terms of moving through the water column, the ocean sunfish has no swim bladder and so, to control buoyancy and move up or down, it instead has a stiff, gelatinous layer under its skin made up of water, collagen, and elastin.

Ocean sunfish have also been observed breaching and can clear the ocean surface by roughly 3 m, possibly in an attempt to dislodge parasites from their skin. Parasite removal is another possible reason for the fish's basking behaviour too, as doing so gives seabirds access to the fish's skin so that they can remove any unwanted guests.

What gives ocean sunfish their distinct shape?

The ocean sunfish's spine is shorter in relation to the body, with fewer vertebrae, than that of any other fish, contributing to the fish's distinct proportions, where some individuals are as tall as they are long. Another interesting detail about their skeleton is that, despite evolving from bony ancestors, much of the ocean sunfish's skeleton contains cartilaginous tissues instead of bone, which is lighter and enables these fish to grow to the sizes that they do.

Skeleton of an ocean sunfish on display at the Museum of Natural History Vienna (photo credit: Sandstein).

As alluded to already, perhaps the most notable feature of the ocean sunfish is its lack of a caudal (or tail) fin. The clavus was formed during the course of the fish's evolution by a converging of its dorsal fin and anal fin.

Biggest Threats

The ocean sunfish is currently listed as Vulnerable on the IUCN Redlist of Threatened Species and its numbers are declining. Full-sized adults have few natural predators but they are sometimes eaten by sea lions, sharks, and killer whales. Meanwhile, the younger fish are vulnerable to large predatory fishes like tuna and mahi-mahi.

Ocean sunfish are considered a delicacy in some parts of the world, including Japan, Korea, and Taiwan, so they are also targeted by fisheries. However, the sale of sunfish-derived products is banned in most of Europe under EU regulations.

Life-sized model of an ocean sunfish on display at the National Museum of Scotland.

The main threat to ocean sunfish is the use of drift gillnets and, as a result, being caught unintentionally as bycatch. Ocean sunfish make up almost 30% of the total catch in swordfish drift gillnet fisheries in California, while, in the Mediterranean, a shocking 71 to 93% of the total catch in swordfish fisheries is made up of ocean sunfish. Unfortunately, fishery bycatch of ocean sunfish is currently unregulated worldwide and, unless that changes, their numbers are likely to continue to decline.