Rockpooling Trearddur

Rockpooling at Trearddur Bay

Trearddur Bay is the bay area around a small village called Treaddur on the west coast of Holy Island, Anglesey, in Wales. The bay used to be better known as Porth y Capel (bay of the chapel), but later became associated with the adjacent settlement of Treaddur, derived from a name meaning 'settlement of Larddur', with Larddur being an important person of the area from the Middle Ages.

This is an area I have visited quite a few times, though I will mostly describe some of my time in this area in Sepember 2020 for this piece. However, I first came to this area in 2007 with my family, when I was five, and rockpooled with my dad and sister. I don't remember much from this trip, but two years later I would return for an unforgettable trip. Over the course of two cloudy days in September 2009 I managed to find my first ever starfish in the form of loads of adorable cushion stars, my first ever lobster, and I also managed to catch a velvet swimming crab, with the species' typical evil-looking red eyes. These were among my first times rockpooling, so returning here in 2020 was somewhat surreal.

Back in September 2020 now, we arrived right at the centre of Trearddur Bay around 7pm, giving me some time to search the coast here as it was getting darker. This shows in my photos from the evening, which get gradually grainier as less light reached the rockpools, but at least it was a gorgeous sunset. At least, after the cloudy start

The first species I encountered here was one I find frequently in these rocky coastal environments. The juveniles are incredibly abundant in places, while I rarely see adults. These are shannies (Lipophrys pholis), a species of blenny fish. Blennies are generally small fish species, often superficially similar to gobies, with elongated bodies and a relatively large head area for the large eyes and mouth. The pectoral fins at either side near the front of the body are large and fan-like, and beneath them and slightly ahead are two pelvic fins which are short and slender and act almost like little stands for these frequently benthic fish. 'Benthic' means these fish spend most of their time by the sea floor, often resting on the bottom, hiding in tight crevices, or even burrowing in the substrate. The shanny fish, like most gobies, relies on cryptic colouring to camouflage and avoid predation, blending in to their environment. This often leads to populations of brownish individuals, but the ones here were particularly striking, with noticeable pinks and greens in their patterns. This is because the small tidal pools these juvenile fish live in are dominated by crustose coralline red algae species that are a pasty pink colour, and the lime green fronds of the green algae known as gutweed, both of which can be seen along with the fish in my photos. Larger and older individuals will likely darken in colour and head to deeper water, though this species is undeniably a coastal rockpool specialist. It is sometimes able to survive just among wet seaweed when the tide is out, and can even breath air through its mouth as well as using its gills for underwater breathing. A highly resilient species, designed for the extreme changes associated with living in the intertidal zone, and something which makes all creatures that live in this environment very cool.

Along with the shannies, one of the species I found in the rockpools here is Palaemon serratus, the common prawn. This species is very similar to the rockpool prawn (Palaemon elegans), both being what are known as glass prawns, named after their transparency, though many other prawns are also like this. The most reliable way to distinguish between these species is using features on the rostrum (the pointy nose at the front): Palaemon serratus' rostrum curves upward more while Palaemon elegans' tends to be straighter, and Palaemon serratus has 6-7 jagged rostral 'teeth' on the dorsal (upper) side of the rostrum (4-5 on the ventral (under) side), compared to Palaemon elegans' 7-9 ventral and 3 dorsal rostral teeth. I will admit it is hard to make out these features from my photos, but the prawn I managed to snap a photo of that evening was a particularly small one, and difficult to fit into focus when so close-up. However, one of the benefits of having a small prawn is that I always find that the smaller ones have the most prominent leg stripes. The yellow bands in this one were particularly beautiful .

Other decapod crustaceans were present here as well as the prawns. I managed to find a few small crabs hiding under stones and among the seaweed. Most abundant were young shore crabs (Carcinus maenas). This species is very common and often has a dark green-brown colour to the upper side of its carapace, as seen in the larger individual I found. However, while there is genetic determination to shore crabs' colour, it is also highly influenced by the environment, adapting to blend in and camouflage, and this is especially prevalent in very small individuals, who often have lots of patterning and may even seem marbled, as though to blend in among the variety of course sand grains.

Hermit crabs (Pagurus bernhardus) are also common here, using the discarded shells of gastropods, especially periwinkles and dog whelks, with the largest individuals making their homes in larger whelk shells, though I didn't find any so large on this trip. Different sea snail species have more or less comfortable shells, not to mention hermit crabs like to have the perfect sized shell for them and will change as they grow. As such, these little crabs will actually battle each other for shells to live in, though beyond this they are generally passive opportunistic scavengers of carrion that can also filter feed when necessary.

Unfortunately these hermit crabs weren't using the discarded shells of the more colourful sea snails found here. One of the coolest is the purple topshell snail (Steromphala umbilicalis), sometimes also known as the flat topshell as a distinguishing factor from Gibbula cineraria, which has a taller spire. S. umbilicalis also has fewer, more separated and defined stripes than G. cineraria which is often described as lineated rather than striped. Young individuals in particular show very clear and bright purple stripes, while oder ones become more cryptic.

There are also some very pretty flat periwinkles (Littorina obtusata), a species which thrives feeding on the brown seaweeds here. This species is common and colourful and is always a delight to find. However, I particularly liked some of the small ones here at Treardurr Bay, as they weren't just the typical bright yellow colour. There are many known colour variants, including these with an amazing orangish chequered pattern.

While the shady evening light made getting good quality photos difficult, I did get a particularly useful photo of a beadlet anemone (Actinia equina). These (usually) deep red anemones are very common on rocky tidal areas, being well adapted to resist desiccation as the tide goes out, able to retract it tentacles when exposed or threatened by tucking them inwards, leaving a low surface-area blob behind to face the elements. While I found a more brownish specimen here, and a particularly adorable and tiny young one, I especially enjoyed seeing a large red one which exposed its acrohagi better than I've seen before. Acrohagi are fighting structures that present as wart-like lumps hidden beneath the tentacles. They are used to fend-off other sea anemones during territorial disputes, because yes, sea anemones can mover around, peeling up and moving forward their basal substrate-adhering pedal disk in a wave-like motion (as seen here). These acrohagi are formed of nematocysts - elongated, or spherical capsules which contain a coiled, hollow, barbed thread. When stimulated by chemical or mechanical cues, a lid-like structure on the nematocyst opens, and the thread everts explosively with a twisting motion. The barbs act like a drill, penetrating the enemy, and, if a toxin is present, it passes through the hollow thread into victim’s tissues. That's some pretty insane territorial behaviour for an animal that moves at imperceivably slow speeds. On beadlet sea anemones, the acrohagi form what appear to be a ring of beads below the tentacles, and are fantastically blue

Not only was the baby beadlet anemone cute, but I also found some of the most adorable little snakelocks anemones (Anemonia viridis) too. This species grows significantly larger than beadlet anemones, not that you can tell from these babies. However, what you can see here is that snakelocks anemones have much longer tentacles relative to their bodies when compared to beadlet anemones. They can't tuck them in like beadlets do and so aren't so good at protecting from desiccation in the intertidal zone. Despite this, snakelocks anemones live coastally, often in shallow and exposed pools as they enjoy sunlight. This species has a symbiotic relationship with zooxanthellae, similar to photosynthetic corals, which are single-celled organisms within the dinoflagellate group which have enveloped a photosynthetic organism at some point in their evolutionary history (often a red-algae), eventually incorporating it as an organelle in its cells, forming a type of chloroplast, hence they are often described as a form of algae. Since Zooxanthellae can photosynthesise, snakelocks sea anemones like to be close to light to allow their symbionts to make food for them, while the algae benefits from the protection of living within the well-defended (stinging) anemone. Along with appearing a greenish-yellow colour with the help of its zooxanthellae and green fluorescent protein, snakelocks anemones also have pink tips to their tentacles for added coolness.

Speaking of creatures with tentacles, I did also find a starfish that evening at Treardurr Bay. At least, sort of. Brittle stars, also known as serpent stars or ophiuroids are echinoderms, like starfish, sea urchins, sea cucumbers, and crinoids. Like most echinoderms, they have pentameral radial symmetry (like a pentagon), and in particular are star-shaped, placing them in the Asterozoa superclass along with starfish. However, brittle stars diverged from all other extant starfish long ago, and are unrelated enough to form their own separate class (Ophiuroidea), apart from the typical starfish class (Asteroidea). The two classes are easily distinguishable, as starfish have their arms and main body all merge into one star-shaped body form, with the gut and other internal organs being spread through this body. Meanwhile, brittle stars have a very clearly defined central disk to the body where the internal organs are, and arms that are separate than mostly used just for crawling around and grabbing. The way the arms move is different too, as the tentacle-like tube feet of starfish are adhesive and can be used like loads of tiny feet so that the arms themselves don't move much and the starfish gracefully glides over the sea floor, while brittle stars' tube feet aren't adhesive, serving a more sensory function, and they instead move by using their flexible arms to crawl around, which looks clumsy and less graceful but is generally faster. I have found larger brittle stars elsewhere, but here on Anglesey I find a very small variety known as the small brittlestar (Amphipholis squamata), which is less than 5cm in overall diameter (3-5mm central disc, and up to 2cm arms). It is so small that, the first time I found this species, it was tucked beneath a common cockle shell, easily fitting within the tiny space.

It was getting darker now, and, as you can tell from the quality of the photos, the creatures hiding among the crevices and under rocks were in the gloom, so I headed to a larger pool with a shallow, pebbly bottom, and managed to find another fish species. This is a longspined bullhead (Taurulus bubalis), also known as a longspined scorpionfish. This is a benthic species, resting on the ground, with a large head, eyes, and mouth, and large fan-like pectoral fins, similar to blennies and gobies. However, as is evident in the name, the longspined bullhead is a sculpin fish, and a close relative of a freshwater species that I am more familiar with, often just called the bullhead (Cottus perifretum). This species is predatory, feeding on invertebrates, similar to the shanny as before, but is also large enough to east small gobies and blennies too, so the shanny really do have a reason for their camouflage, while the longspined bullhead's camouflage is also excellent for allowing it to stay hidden as an ambush predator.

By around 7:45 the darkness was really spreading, but I still managed to find two more weird organisms before heading back. The first was sliding over the underside of a rock I had looked beneath, appressed flat against it like a pancake. The elongated, flat body defined this as a flatworm, this one seemingly having had some damage to its rear end to leave a deep gash. It's wide, undulating, thin and flat sides, with two ocelli (simple eyes) near the front on top suggest that this might be in the common and often intertidal group of turbellarian flatworms known as Polycladida, and specifically I think it is Leptoplana tremellaris.

Under a different rock, clad with numerous barnacles of seemingly a variety of species, I also found the weird orange mass of a sea sponge. This orangish sponge is likely crumb-of-bread sponge (Hymeniacidon perlevis), though the similarly named breadcrumb sponge (Halichondria panicea) can also sometimes be this colour , though it is usually more greenish. These sponges have the recognisable towering tubes that sponges are often recognised for, in the form of pores known as oscula (singular - osculum), which are excretory passages through which water is expelled from the sponge after passing through the spongocoel (the main central cavity of sponges that gains water through tiny ostia pores, later excreting it through the larger oscula). Here, the oscula are small and inconspicuous, as this sheet-growing form is adapted for desiccation resistance in the intertidal zone, while specimens of this species found deeper down my form larger spires in a flanged or turreted form.

After this I decided to call it a day and decided to have a little sunset walk along the clifftops before bed. It's safe to say that it was an absolutely gorgeous sunset.

I returned to the coast here two days later, just a little further north than my original spot, for some rockpooling while the sun was out. It was a great day, and a flatter stretch of coast that had much deeper pools to search in. My first point of call was to try and get some better snakelocks anemone photos in the sunnier lighting conditions, but all the individuals I found here were very small still. Pretty nonetheless.

Besides the many marine animals, I also found many beautiful algae species in the rockpools here, beyond just the typical wrack seaweeds. As I brought up with the dinoflagellates earlier, 'algae' is a general term used to refer to most things photosynthetic that aren't true plants. This can range from the prokaryotic cyanobactera being called blue-green algae, to a variety of different eukaryotes which acquired chloroplasts for photosynthesis via multiple independent engulphing events to gain endosymbionts which became organelles.

To explain this requires a description of chloroplasts and their evolution. Chloroplasts are organelles, meaning they are similar to organs to organisms, except for individual cells. They are found within the cells of plants and eukaryotic algae and are what do photosynthesis for them, making sugars for food using water and carbon dioxide, and powered by sunlight. Similarly to mitochondria and some other organelles, chloroplasts actually have their own DNA, separate from that of the nucleus, which is generally seen as encompassing the full genome of the organism in each cell. Such DNA-holding organelles are though to have originated long long ago when an early cell engulphed another through phagocytosis, but didn't digest it, instead retaining it within itself as an endosymbiont, the two cells benefitting each other (e.g. photosynthetic ability in return for protection). Over time, these cells became inseparable, replicating together such that the endosymbiont and all its descendants will never leave the safety of the cells they are within. This fully reliant lifestyle led to the loss of many non-essential genes in the endosymbiont until it was basically just a functioning part of the larger cell and organism - an organelle. Early cyanobacteria were around in the fossil record at the time this likely happened, and DNA analysis does suggest that cyanobacteria are the closest relatives to chloroplasts, so it seems all higher photosynthetic organisms evolved by engulphing a cyanobacteria (or engulphing another cells which had themselves engulphed a cyanobacteria).

With this in mind, it seems sometime long ago, a cell engulphed a cyanobacteria and gained it as an organelle, using it for photosynthesis. Modern cyanobacteria photosythesise using the photosynthetic pigments chlorophyll a, and phycobiliproteins, so early photosynthetic eukaryotes that incorporated cyanobacteria likely shared this. In order to engulph the cyanobacteria, the cell also needed to use phagoctosis, stretching and pinching off parts of its own membrane to trap the cyanobacteria in a sort of bubble within the cell - a vesicle. This means that early chloroplasts would have had 2 layers of membranes, the cyanobacteria's membrane, and the vesicle's membrane. Modern red algae (rhodophytes) have chloroplasts with chlorophyll a and phycobiliproteins and two membranes, and likely represent something similar to early photosynthetic eukaryotes. Green algae and its derivatives (chlorophytes, charophytes, and true plants also have two-membraned chloroplasts, but with chlorophyll b instead of phycobiliprotein, so some evolutionary change led to that change as these algae diverged from red algae. Other photosynthetic eukaryotes are thought to have arisen from endisymbiotic events besides this one. Very rarely, there are species that seem to have gained new chloroplasts via an entirely new cyanobacteria engulphing, for example, as seen in paulinella chromatophora. However, most are actually thought to be secondary or even tertiary endosymbioses. One such event is thought to have included the engulphing of a red algae. This forms 4-membraned chloroplasts (2 from the original chloroplast, 1 from the membrane of the red algae, and one from the vesicle), and lead to the evolution of photosynthetic stramenopiles (including most typical seaweeds - brown algae), apicomplexans, and dinoflagellates. Meanwhile green algae (chlorophytes) are thought to have been engulphed and incorporated as secondary endosymbionts in two separete events - one time leading to the evolution of chlorarachniophytes, and another time giving rise to euglenids (which also have 3-membraned chloroplasts, so one membrane must have been lost along the way).

So yeah. Here are some interesting examples from the green, red, and brown algae groups that I found while rockpooling at Treardurr Bay:

The first is a beautiful mossy-looking green algae known as green branched weed (Cladophora rupestris), from a genus I am more familiar with in freshwater ecosystems, where I generally associate it with evidence of eutrophication in shallow streams and rivers. The second and third are a pair of similar green algae species known as gutweed (Ulva intestinalis), and broad-leaved sea lettuce (Ulva lactuca). Gutweed is one of the most common species I find at the coast, always filling small pools with its spindly lime-green fronds, and seen here giving a limpet (Patella vulgata) a fancy hairdo, and broad-leaved sea lettuce is a similar colour and smooth slimy texture, while being much larger, making them easily distinguished from each other. The fourth green algae here is an unusual one known as green sea fingers (Codium fragile), recognised by its cylindrcal, dark green, dichotomously branching fronds with a soft, hairy, velvety texture. Of the brown algae, there were many examples, as this group includes what we most commonly called seaweeds, such as all the wrack seaweeds. However, one that particularly caught my attention was the oyster-thief seaweed (Colpomenia peregrina), which is an algae species introduced to here from the Pacific ocean, and which is often epiphytic, growing on seaweeds like epiphytic plants grow on trees. It grows into a hollow sphere shape, requiring sheltered areas at the coast to not get washed away, and has thin enough walls that light passes through easily such that it often appeared a pretty golden-yellow colour. Of the red algae, despite the more typical coralline crusts of some species, I also found a much larger and darker species that I think might be Gelidium corneum, or a related algae, a thalloid red algae rather than the harder coralline ones nearby. All these aquatic flora, and more, filled the rockpools here and formed the basis of the habitat for the other rockpool creatures I would find.

Along with the flora, I also noticed all the different discarded shells that formed the substrate for some of the rockpools here. Along with old periwinkles and other gastropod snails were also some shells that once belonged to bivalves. Of particular beauty were the shells of dog cockle (Glycymeris glycymeris), with its thick shell and zig-zag pattern that reminds me of little mountain ranges. One specimen was also coated in the white spiral shells of what I would normally say is sinistral spiral tube worms (Spirorbis spirorbis), which are polychaete worms that build their shell around them, spiralling clockwise (unlike the similar Janua heterostropha, which is anticlockwise [I also think there are some small leftover examples of this species in among Spirorbis on my shell photos below]). They usually living on seaweed, but also sometimes rocks and, in this case, on a shell. However, the rock-growing habit, and some of the very ridged specimens on the left of the shell, rather than being a smooth spiral cylinder, may suggest that these are the less frequently recorded species Spirorbis tridentatus, though I can't find much info on this. Along with a few dog cockles, I also found an old and battered variegated scallop (Mimachlamys varia). Better examples of this species are often a brighter orangish red, with a large anterior 'ear' near the hinge, but I was still happy to find this eroded individual too.

Moving on from all these stationary finds, in one of the deepest rockpool I saw was a shoal of fish that I spent ages trying to get decent photos of as they swam around me. My aim was to do rockpooling without getting my pants wet, as this was just our morning plans before a hike of Holyhead Mountain in the afternoon that I wanted to have dry cloths for, but I was stood up nearly waist deep in this pool and it was totally worth it. These fish are two-spotted gobies (Pomatoschistus flavescens) [historically called, and still frequently referred to as Gobiusculus flavescens], an unusual member of the goby order (Gobiiformes) due to its ability to form free-swimming shoals, unlike the often more benthic, blenny and sculpin-like other gobies. To adapt to this different lifestyle, their eyes are on the sides of their head, rather than near the top, as the high position for sight from the seafloor is no longer required. This species can also be quite colourful, especially for males in the breeding season, whose pale blue spots along their sides become especially vibrant earlier in the summer. Despite living in the water-column, two-spotted gobies return to the seafloor like normal gobies in order to hide. Their topsides are less colourful than their sides, having a recognisable pattern of pale brown saddle-shaped markings along the dark brown back such that they camouflage in well with the substrate when buried.

One species I came across in surprising abundance here was Cancer pagurus, commonly known as the edible crab due to its position as the most heavily commercially exploited and commercially important crab in the world for food. In order to protect the species, there are now a variety of restriction on catching this species on a large scale. For example, it is illegal to catch 'berried' crabs - females with eggs, and there are minimum landing sizes to ensure growing juveniles aren't killed. When finding this species in the wild, juveniles can vary in colour, often having purplish tinges - all the smaller ones I found here were very pale, while older individuals settle on a orangish red-brown colour with a paler underside and black-tipped pincers. Its carapace is also edged with nine rounded and flattened lobes on each side at the front, often describe as a 'piecrust' edge.

I was excited to see the edible crabs, as I often just find shore crabs. However, I was even happier when I found two juvenile long-clawed porcelain crabs (Pisidia longicornis), one of which was missing a claw. These small orangish crabs are in the Porcellanidae family, a group of decapods (crustaceans including crabs, lobsters, and shrimp) which are not true crabs. True crabs are in the Brachyura infraorder, but many other crustacean groups have evolved to be crab like through convergent evolution due to the selective pressures and benefits of the overall crab-like body plan , including having a reduced abdomen and a broad cephalothorax to form the wide oval shape with claws. This process has occurred in so many different lineages of crustaceans that it has been given a name - carcinization, a term first brought about by evolutionary biologist Lancelot Alexander Borradaile who hilariously described it as "one of the many attempts of Nature to evolve a crab". Porcelain crabs such as these are the most closely related of the carcinized crustaceans to true crabs, being in the Anomura order along with squat-lobsters and hermit crabs.

Similarly to my trip here back in 2009, I also found a velvet swimming crab (Necora puber) in the rockpools here. The final crab species I found here, and the feistiest. This species, with its velvety texture, cobalt blue sections, and bright red eyes is, sometimes also known as the devil crab, or fighter crab. It is known for its astonishing speed, being the UK's largest member of a group of crabs known as the swimming crabs, which have modified segments on their fifth (back) pair of legs to be like flat and broad paddles used for fast swimming. This speed, along with its strong and sharp claws, make velvet swimming crabs very capable rockpool predators that the other creatures here need to watch out for.

Not only did I manage to find a velvet swimming crab like I was rockpooling in 2009 again, I also found cushion stars again. Asterina gibbosa, sometimes known as the starlet cushion star, is recognised from other common British starfish by its short, stubby, broad-based arms, which give it a pudgy and soft appearance, like a cushion. Despite its appearance, cushion stars are technically covered in spines, but they are short, stuff, and quite blunt and a colourful orange hue that stands out from the dull green-blue of the rest of the body. The underside is less colourful, which makes sense since it is rarely exposed, but includes central lines down each arm . These form an important part of the starfish' water vasculature system, a series of tubes called canals that run throughout the starfish' body (notably a canal down the centre of each arm and a ring around the middle, among other features). These canals are lined with cells that have cilia, small hair-like structures that extend into the inside of the tubes and waft/beat in order to move seawater through the tubes. Beneath the canal on each arm is a gap known as the ambulacral groove, which contains 2 rows of ampulla connected to the canal and leading down into foot-like tubes. By using valves to control the movement of water being pumped by cilia through the tubes, water can be pushed in and out of these tube feet to elongate and contract, allowing the starfish to move along the ground (elongated tube feet can be seen best on the bottom left arm of the upside-down photo). Of course, for this whole sea-water based vasculature system to work, the starfish needs to let seawater pass into its body in the first place, and that is where a feature on the top side of the starfish comes into play. A branch from the central ring canal leads up to a structure known as the madreporite, a calcareous plate with tiny radiating grooves and pores to let water in, but not much else. This structure can be seen just off-centre near the middle of the top of the starfish, in this case appearance as a little orange patch on the otherwise bluish body with orange spines.