THE SEALS OF DUBLIN BAY

Kate McGonagle

The Dublin Bay seals are arguably one of the more fun species to encounter on a dive in Dublin Bay, especially if you’re around Dalkey Island when the younger grey seals are in the mood to play.

Dublin Bay is home to two seal species: the Grey Seal (Halichoerus gryphus) and the Common Harbour Seal (Phoca vitulina). The harbour seals tend to favour sandy beaches like Bull Island, but you might see them occasionally the south side of the bay as well. Generally, when we’re talking about the seals we scuba dive with, we are talking about the grey seal colony of Dalkey Island. Grey seals like to haul out of the water to rest on the rockier shorelines, with seal bay in Dalkey Island being an amazing dive site to have some really cool interactions with them. These seals may come up to you and play around with your fins or blow bubbles if they are feeling curious, or you may just spot them gliding past you in the distance checking you out.

SEAL EVOLUTION

True seals belong to the Clade Pinnepedia, or the pinnipeds, within the Order Carnivora. The evolutionary history of pinnipeds is still debated, with two theories on whether they came from two different ancestors (one for the Otarioids, the sea lions and walruses, and one for the Phocoids, the true seals), or one common ancestor. Current evidence overwhelmingly favours the theory of one common ancestor, with the second proposed ancestor being a sister group (Paterson et al. 2020).

GREY VS HARBOUR SEALS

So, how can you identify which species of seal you are encountering? There are a few distinct differences between the two that make it easy to tell them apart once you know how. According to Seal Research Ireland, the first key difference is size – grey seals are a lot bigger than harbour seals, with male grey seals in particular growing much larger. Size differences are easier to spot when both seals are in the area, but I find it easier to distinguish them by their head shape – grey seals have a roman nose and longer head, whereas harbour seals have a more ‘puppy-like’ nose, sloped nose bridge and distinct forehead.  You can also pay attention to their body pattern, because harbour seals have smaller more distinct spots and in contrast grey seals have much more irregularly shaped spots. Male grey seals also tend to be much darker in colour.

THE BIOLOGY OF SEALS

Seal biology was an especially interesting area to learn about during my studies. If there is one thing you take away from this, it is that seals are built for the water. Firstly, they have a fusiform body shape, meaning they are streamlined and hydrodynamic in the water, in comparison to their clunky movement on land. Their limbs have evolved into flippers to propel them through the water, and their fur also helps with streamlining.

Beyond their body shape, all of a seal’s senses are adapted to make them excellent underwater hunters. Their long whiskers (vibrissae) can pick up vibrations in the water to tell the seal about fish swimming, currents, and surfaces they come in contact with (Reidenberg, 2007). In other words, when you are in the water with seals in the area, they can feel you there.

Even a seal’s eyesight and hearing has been perfected through evolution for aquatic environments. The lenses of their eyes are almost a perfectly spherical, and they have the ability to drastically change the size of their pupils depending on the light in the environment (Mass and Supin, 2007). The tapetum, the reflective layer of their eye, acts almost like a mirror, reflecting light back onto their eye a second time to maximise their vision in the water (Mass and Supin, 2007). Their eyes contain more rod photoreceptors for low light conditions, meaning they see more in greyscale (Reidenberg, 2007).  All of these adaptations shape them into excellent hunters that can hunt in deeper or darker water. Seal hearing is also strongly directional in water, whereas on land it is reduced (Reichmuth et al., 2013).

In dimmer light on land when they need to open their pupils more, images are quite blurry and they are more short-sighted, and their hearing is not as good. This makes seals an easy target for hunting, and they have sadly been hunted when on land resting by humans. This is an ongoing problem in Canada, Scandinavian Countries, Russia and the US, to name a few, and it is definitely something to be aware of as an advocate for these species when you’ve met them in their own back yard on a dive.

GUIDELINES FOR DIVING WITH SEALS

When diving with seals it is so important to remember that while they can be very curious and friendly, they are still wild animals that deserve our respect. If you encounter either of these species on your dives, relax, let the seals decide if they want to approach you or not, do not touch them or feed them, and let them do their thing (Guidance on Seal Interactions, BSAC).

If you are interested in learning more about our local seal populations, head down to Seal Rescue Ireland in Courtown, County Wexford to learn about their rehabilitation programs and see some of their current residents.

BIBLIOGRAPHY

British Sub-Aqua Club. Guidance on seal interactions. (n.d).  https://www.bsac.com/advice-and-support/protect-our-seas/guidance-on-seal-interactions/

Mass, A. M., & Supin, A. YA. (2007). Adaptive features of aquatic mammals’ eye. The Anatomical Record, 290(6), 701–715. https://doi.org/10.1002/ar.20529

Paterson, R. S., Rybczynski, N., Kohno, N., & Maddin, H. C. (2020). A Total Evidence Phylogenetic Analysis of Pinniped Phylogeny and the Possibility of Parallel Evolution Within a Monophyletic Framework. Frontiers in Ecology and Evolution, 7. https://doi.org/10.3389/fevo.2019.00457

Reichmuth, C., Holt, M. M., Mulsow, J., Sills, J. M., & Southall, B. L. (2013). Comparative assessment of amphibious hearing in pinnipeds. Journal of Comparative Physiology A, 199(6), 491–507. https://doi.org/10.1007/s00359-013-0813-y

Reidenberg, J. S. (2007). Anatomical adaptations of aquatic mammals. The Anatomical Record, 290(6), 507–513. https://doi.org/10.1002/ar.20541

SealResearchIreland. Seal identification. (n.d.). https://sealresearchireland.wixsite.com/srpi/seal-identification

A dried empty catshark egg case

Are there dogfish in Dublin Bay?

Kate McGonagle

At first you would probably say yes, right? There’s lesser-spotted dogfish and large-spotted dogfish all over Dublin Bay - but the scientific answer is a little bit more complicated. Scientifically speaking, these are both species of catfish, and the lesser-spotted dogfish is now more appropriately named the small-spotted catshark, and the large-spotted dogfish, or bull huss, named the nursehound. We’re not 100% sure why they are called dogfish, but this naming difference tells us more than you might think, as the change in name paints a very different picture on the evolutionary tree that these sharks have branched from.

“True” dogfish are in the family Squalidae, which includes sharks like spurdog, and are part of the superorder Squaliformes, which includes 126 species of sharks from a few different families. The spiny dogfish is probably the best example of a “True” Dogfish.

 Catsharks, on the other hand, are from the family Sciliorhinidae, in the superorder Galeomorphs. All living sharks are classified as either Squaliformes or Galeomorphs.

The Galeomorphs are older, branching off 273 million years ago, while the Squaliformes branched again 216 million years ago (Sorenson et al., 2014). Because of this, our local catshark species are actually more closely related to great whites than they are to true dogfish, which makes these common “nicknames” pretty confusing.

Most catsharks lay eggs, commonly called mermaid purses, and you can find these washed up on the shore and under the water. Mermaid purses have long tendrils at the ends, and a catshark will swim around and around what she is attaching her egg to in order to tie it down and stop it from drifting. Next time you’re out on a Dublin Bay dive, have a look around the kelp and the rocks, because you may be able to see the tiny catshark inside!

Catshark eggs on kelp

Redspotted Catshark—Shroederichthys chilensis. (2020, November 23) https://www.sharksandrays.com/redspotted-catshark/

So, are there any dogfish in Dublin Bay? Well, yes and no. Spiny dogfish are generally found out in the Atlantic or Pacific near the continental shelf, but they have been known to pass through coastal waters too (Veríssimo et al., 2010), so you’re not very likely to see them on a Dublin Bay dive. But you definitely could run into a catshark, or one of their eggs. Both of these species can be seen on your Dublin Bay dives around Dalkey Island and the Muglins Lighthouse. If you see any shark or ray egg cases you can report them using the shark trust app, and make sure not to disturb them.

Sorenson, L., Santini, F., & Alfaro, M. E. (2014). The effect of habitat on modern shark diversification. Journal of Evolutionary Biology, 27(8), 1536–1548. https://doi.org/10.1111/jeb.12405

Veríssimo, A., Mcdowell, J. R., & Graves, J. E. (2010). Global population structure of the spiny dogfishSqualus acanthias, a temperate shark with an antitropical distribution. Molecular Ecology, 19(8), 1651–1662. https://doi.org/10.1111/j.1365-294x.2010.04598.x