Marine Life & Conservation Blogs
Creature Feature: Great Hammerhead Shark
In this series, the Shark Trust will be sharing amazing facts about different species of sharks and what you can do to help protect them.
We’re kicking off the new year with a classic shark – the Great Hammerhead. Known for their distinct silhouette, the Great Hammerhead is the largest species of hammerhead on the planet.
With a distinct notch on the centre of their head, the Great Hammerhead is light grey or brown above and white below. Reaching a maximum size of around six metres, this is a big shark. There are nine species of hammerhead. Identifying them often relies on looking at head shape. The Great Hammerhead has a much straighter front edge compared to the Scalloped and Smooth Hammerheads.
The unique hammer-shaped head (a.k.a. cephlafoil) improves their manoeuvrability. As the position of their eyes enable them to see 360°. And enhances their ability to detect electrical currents, a sixth sense that all sharks have. Sharks have lots of tiny pores covering their head and snout, called ampullae of Lorenzini. These are extremely sensitive and can detect even the faintest of electrical fields. Including those generated by the Earth’s geomagnetic field, or muscle contractions in prey. The broad flat head of a hammerhead provides a much larger surface area for these pores. Which is why they’re so good at finding prey – such as stingrays – completely buried beneath the sand.
Great Hammerheads are nomadic and seasonally migratory. They move towards the equator during the winter and then towards the poles during the summer. Unlike other species, they are solitary and migrate up to 1,200km (750 miles) alone.
The Great Hammerhead is Critically Endangered. Populations have declined in recent years. They are targeted and also caught as bycatch.
Although large and powerful, research has shown that Great Hammerheads are quite fragile. They are particularly vulnerable to the stress of capture. Recent research has found that individuals that are hooked have a 50% chance of dying following release. Combating the declining populations comes down to, as always, implementing scientifically informed catch limits and enforcing these.
The species is important for dive tourism in some areas. In the video below, head of marketing for the Shark Trust – Caroline, takes in the view as a Great Hammerheads cruise by. They are a true sight to behold.
SCIENTIFIC NAME: Sphyrna mokarran
FAMILY: Sphyrnidae
MAXIMUM SIZE: 610cm
DIET: Varied. Preference for stingrays, other rays and marine catfish.
DISTRIBUTION: Worldwide, tropical seas.
HABITAT: Coastal-pelagic and semi-oceanic. Close inshore to well offshore. 1-300m or more.
CONSERVATION STATUS: Critically Endangered
Images from Frogfish Photography
Distribution Map from Wikimedia Commons
For more amazing facts about sharks and what you can do to help the Shark Trust protect them visit the Shark Trust website by clicking here.
Marine Life & Conservation Blogs
Creature Feature: Dusky Shark
In this series, the Shark Trust will be sharing amazing facts about different species of sharks and what you can do to help protect them.
This month we’re taking a look at the Dusky Shark, a highly migratory species with a particularly slow growth rate and late age at maturity.
Dusky sharks are one of the largest species within the Carcharhinus genus, generally measuring 3 metres total length but able to reach up to 4.2 metres. They are grey to grey-brown on their dorsal side and their fins usually have dusky margins, with the darkest tips on the caudal fin.
Dusky Sharks can often be confused with other species of the Carcharhinus genus, particularly the Galapagos Shark (Carcharhinus galapagensis). They have very similar external morphology, so it can be easier to ID to species level by taking location into account as the two species occupy very different ecological niches – Galapagos Sharks prefer offshore seamounts and islets, whilst duskies prefer continental margins.
Hybridisation:
A 2019 study found that Dusky Sharks are hybridising with Galapagos Sharks on the Eastern Tropical Pacific (Pazmiño et al., 2019). Hybridisation is when an animal breeds with an individual of another species to produce offspring (a hybrid). Hybrids are often infertile, but this study found that the hybrids were able to produce second generation hybrids!
Long distance swimmers:
Dusky sharks are highly mobile species, undertaking long migrations to stay in warm waters throughout the winter. In the Northern Hemisphere, they head towards the poles in the summer and return southwards towards the equator in winter. The longest distance recorded was 2000 nautical miles!
Very slow to mature and reproduce:
The Dusky Shark are both targeted and caught as bycatch globally. We already know that elasmobranchs are inherently slow reproducers which means that they are heavily impacted by overfishing; it takes them so long to recover that they cannot keep up with the rate at which they are being fished. Dusky Sharks are particularly slow to reproduce – females are only ready to start breeding at roughly 20 years old, their gestation periods can last up to 22 months, and they only give birth every two to three years. This makes duskies one of the most vulnerable of all shark species.
The Dusky Shark is now listed on Appendix II of the Convention on the Conservation of Migratory Species (CMS), but further action is required to protect this important species.
Scientific Name: Carcharhinus obscurus
Family: Carcharhinidae
Maximum Size: 420cm (Total Length)
Diet: Bony fishes, cephalopods, can also eat crustaceans, and small sharks, skates and rays
Distribution: Patchy distribution in tropical and warm temperate seas; Atlantic, Indo-Pacific and Mediterranean.
Habitat: Ranges from inshore waters out to the edge of the continental shelf.
Conservation status: Endangered.
For more great shark information and conservation visit the Shark Trust Website
Images: Andy Murch
Diana A. Pazmiño, Lynne van Herderden, Colin A. Simpfendorfer, Claudia Junge, Stephen C. Donnellan, E. Mauricio Hoyos-Padilla, Clinton A.J. Duffy, Charlie Huveneers, Bronwyn M. Gillanders, Paul A. Butcher, Gregory E. Maes. (2019). Introgressive hybridisation between two widespread sharks in the east Pacific region, Molecular Phylogenetics and Evolution 136(119-127), https://doi.org/10.1016/j.ympev.2019.04.013.
Marine Life & Conservation Blogs
Creature Feature: Undulate Ray
In this series, the Shark Trust will be sharing amazing facts about different species of sharks and what you can do to help protect them.
This month we’re looking at the Undulate Ray. Easily identified by its beautiful, ornate pattern, the Undulate Ray gets its name from the undulating patterns of lines and spots on its dorsal side.
This skate is usually found on sandy or muddy sea floors, down to about 200 m deep, although it is more commonly found shallower. They can grow up to 90 cm total length. Depending on the size of the individual, their diet can range from shrimps to crabs.
Although sometimes called the Undulate Ray, this is actually a species of skate, meaning that, as all true skates do, they lay eggs. The eggs are contained in keratin eggcases – the same material that our hair and nails are made up of! These eggcases are also commonly called mermaid’s purses and can be found washed up on beaches all around the UK. If you find one, be sure to take a picture and upload your find to the Great Eggcase Hunt – the Shark Trust’s flagship citizen science project.
It is worth noting that on the south coasts, these eggcases can be confused with those of the Spotted Ray, especially as they look very similar and the ranges overlap, so we sometimes informally refer to them as ‘Spundulates’.
Scientific Name: Raja undulata
Family: Rajidae
Maximum Size: 90cm (total length)
Diet: shrimps and crabs
Distribution: found around the eastern Atlantic and in the Mediterranean Sea.
Habitat: shelf waters down to 200m deep.
Conservation Status : As a commercially exploited species, the Undulate Ray is a recovering species in some areas. The good thing is that they have some of the most comprehensive management measures of almost any elasmobranch species, with both minimum and maximum landing sizes as well as a closed season. Additionally, targeting is entirely prohibited in some areas. They are also often caught as bycatch in various fisheries – in some areas they can be landed whilst in others they must be discarded.
IUCN Red List Status: Endangered
For more great shark information and conservation visit the Shark Trust Website
Image Credits: Banner – Sheila Openshaw; Illustration – Marc Dando
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