July 2026
Disappearing Diatoms and Blooming Tintinnids
Summary
Every month there are guaranteed surprises, something new that I have not seen before. The June - August period is the end of the spring blooms and a gap before the start of the autumn rise in plankton. Yet this is the time I look forward to as it is never dull, always some new pattern developing. This month has had a great deal going on with the continuing decline in phytoplankton such that virtually all the spring diatoms had gone by mid-July. But on 30th June two new diatom species suddenly appeared and increased in numbers during July. Dinoflagellates made a noticeable appearance as well as showing a shift from spring to autumn populations. Zooplankton showed an increasing diversity including a sudden and prolific number of barnacle eggs and larvae of all stages. A marine heatwave (as well as the terrestrial one) was declared around the UK, record temperatures occurred and not surprisingly a huge bloom of Blue Jellyfish Cyanea lamarckii arrived on the south-west coast of Wales and England.
An underside view of Blue Jellyfish Cyanea lamarkii
Phytoplankton
Except for some needle-like Rhizostoma species of diatom there has been an almost complete collapse of diatoms in the plankton. The "hairy" Chaetoceros species have a few with resting spores inside but I have not seen such a diatom decline before at Dale. The other side of the Haven to the east at Burton has even less with a few estuarine species like Pleurosigma and Navicula. However, two species suddenly appeared at the end of June and then steadily increased in density through the month. Proboscia indica, a rod-like diatom with distinct ends rather like a proboscis was especially interesting as I have not recorded it before. The second is the bloom of a common diatom Striatella unipunctata. This is odd because it is primarily a benthic species usually growing attached to the substrate. Occurring occasionally in the plankton they are not generally thought of as phytoplankton. Whether it is through a lack of competition with other diatoms that now a huge bloom is possible, dominating the samples at Dale. I have never seen this happen before just odd single views of Striatella. At the end of July it was still blooming in the west of the Haven. Odd! Intriguingly, no sign of it on the other side at Burton.
Dinoflagellates are part of the phytoplankton and it is noticeable that often when the diatom component declines these increase. The spring genus Protoperidinium was still common but now several species of the late summer-autumn genus Tripos made an appearance. A dino that has been common off-shore for the last month or so, Noctiluca scintillans has now arrived in the Haven. It is a very large dinoflagellate and almost visible to the naked eye. It is bioluminescent at night and a predator. It has a long tentacle (impressive for a single-celled creature) for capturing prey. It was not abundant in the Dale sample but this may change as we come into August, a time when it is typical of the summer plankton.
The diatom Proboscia indica surrounded by other diatoms with a single tintinnid
The diatom Striatella unipunctata
Tripos furca increase
Protoperidimium decline
Side view of Noctiluca with extended tentacle for feeding
Zooplankton
Despite the significant drop in phytoplankton the zooplankton was prolific and dominated by tintinnids, a trend I often see at this time once the diatom numbers have declined. These are single-celled ciliates that secrete a coat around them (the lorica) and in smaller ones may be studded with minerals. Specialise cilia project from one end of the lorica to feed. These tiny little jewels formed such a huge bloom this month that with the naked eye it was like smoke swirling around in the sample. Most were the small, thimble type called Tintinnidium. However, just like June 2025 a larger species, Favella sp, appeared to become common in samples. While photographing the example here I noticed particular movement on the ciliate's side and higher magnification revealed the dinoflagellate parasite Eudobosquella. This goes on to multiply into two types of spore, mega and microspores, eventually killing the Favella. The tintinnid density was especially high in June/July 2025 such that more detail of the parasite could be seen. This link opens a new window for the June 26 blog.
The tintinnid Favella sp. The arrow indicates the presence of the dinoflagellate parasite Eudubosquella. This will create spores and ultimately kill the host.
Huge numbers of Tintinnidium
Barnacle egg ready to hatch
Besides the bloom of tintinnids, at the end of June a sudden release of barnacle eggs into the plankton triggered a huge numbers of barnacle nauplii to be present. The final stage larva, the cyprid, has been seen off and on for months (last peak in May) but by the end of July they were common in all samples across the Haven. The photo here shows the legs at the back that helps them manoeuvre across the substrate while the sensory antennae at the front searches for a suitable spot to glue itself to the hard surface. The nauplii of the parasitic barnacle, Sacculina, continued to be present (see photo in June below).
Barnacle cyprid
Several types of trematode cercaria larvae
Polychaete larvae: above, Capitella, below Polydora
Metatrochophore of (most probably) Pomatoceros. NB. the developing chaetae
Two crustacean populations continued to expand. The marine water flea Podon appeared last month as Synchaeta disappeared. By mid-July it had become common with many parthenogenetic females producing young. Copepods species are completely dominated by Euterpina acutifrons throughout the Haven. It is one of the very few harpacticoid copepods that live in the plankton. Most are found in sediment at the bottom. There were some present last month but they have really come into their own now with many examples of females carrying eggs. A highly specialised diatom species Sceptronema orientale lives on the outside of their bodies, not as a parasite but just for attachment and dispersal. As yet there is no sign of the diatom but as population ages through the summer I would expect this to make an appearance soon.
Tadpole larvae of sea squirts are still common in samples. They are a non-feeding larva, covered in abundant yolk cells, and remain in the plankton for only a few days.
There are many creatures that look like worms in the plankton but many are not true marine worms (polychaetes). For example the parasitic flatworms, Trematodes. There are many species and they pass through various larval stages and the cercaria is the most likely seen in the plankton. This month various forms of cercaria were found and these will be looking for secondary hosts like periwinkles and other molluscs. From these hosts they need to be eaten by a seabird or fish to pass pass into the primary host.
There were plenty of polychaete larvae present. Polydora is always the most abundant species, and this month it was joined by larvae of Capitella, which look very different. Some of the trochophores found last month were almost certainly of Capitella as well as Polydora. There was also a release of larvae into the plankton of a Serpulid worm, possibly Pomatoceros (they produce white, calcareous tubes on the rock). They are known to reproduce throughout the year but concentrate on spring summer. However, although they should be common I rarely seem to catch them
Marine Water Flea Podon
Harpacticoid copepod Euterpina acutifrons female with eggs
Head of a tadpole larva
All photos shown were taken in late June or July 2026
June 2026
Most Blooming Diatoms Decline and Rotifers Dominate
Summary
At the end of May the plankton in Milford Haven was like pea soup, thick and green, amongst which the biodiverse zooplankton thrived. By mid-June all was in decline except for a few new species that began to bloom. A repeat performance from last year, almost exactly in the same weeks. Phaeocystis, a very slimy colony within the phytoplankton that looked like becoming a nuisance virtually disappeared overnight.
Phytoplankton
By mid-June most of the diatom diversity was gone to be left dominated by two species. Most prolific was Cerataulina pelagica, a tiny diatom that rapidly grows into small chains, although at the end of June the chains were unusually long with twenty or more cells present. I always find it amazing that for many months there is no sign of a species like Cerataulina and overnight it suddenly becomes abundant. Last year, in early July, there was a period when it vanished and then reappeared blooming in August again.
The rotifer Synchaeta at the end of May with diatoms of Lauderia
Small chains of Cerataulina pelagica at the end of May x400 magnification
The diatom Lauderia, that had been abundant through the spring had almost disappeared by mid-June to be replaced by the long and needle-like species of Rhizosolenia setigera. Pseudo-nitzschia pungens forms long chains and was fairly common during the month. Ditylum has been abundant this spring and some individuals are still present but show signs of possible resting spores or sexual reproduction. The cell contents are reduced to single, spherical structure inside. This could be the oogonium or egg but it is so perfectly rounded (eggs are oval) I think it is more likely a resting spore, meaning it will drop to the bottom to pass a period low in nutrients, reappearing in early autumn.
A new diatom species for the Haven was found at the start of the month and there was just one single example: a ribbon-like chain of Fragilaria islandica. The latter name refers to Iceland as it is common in Arctic waters. Strange that with the water warming and subtropical species on the increase we see an example from a cold water region.
The phytoplankton has been interesting this month, despite a drop in biodiversity and blooming. Samples taken at the eastern end of the Haven, around Burton, show a much slower change and some winter diatoms like Odontella and Bacillaria are still present (low numbers).
Mid-June: longer chains of Cerataulina and bloom of the needle-like Rhizosolenia setigera x100. Note the Synchaeta egg
Ditylum, above, and Pseudo-nitzschia pungens chain below
Ditylum, with resting spores
Fragilaria islandica diatom chain. Lauderia below
Synchaeta female with eggs
Zooplankton
The story here is of a decline in biodiversity and huge increase in the marine rotifer Synchaeta. I have reported for the last six months the presence of the rotifer at Dale (west side of the Haven) but this month it completely dominates the water. It does this with only females present and reproducing parthenogenetically (unfertilised eggs). As well as the adults there were large numbers of eggs drifting amongst the diatoms (photo above) and developing rotifer juveniles could be seen inside. This bloom of Synchaeta seems to occur every other year. Huge blooms occurred May 2022 and again in 2024 although that was July/August. It is noticeable that when these blooms happen many other populations decline. For example, this year no water fleas (Podon) have been seen. At Burton there are no Synchaeta present but a small number of Podon are currently being sampled and biodiversity in mid-June was higher there.
The Gann Lagoon near Dale is a shallow, sheltered seawater pool that enables breeding of Synchaeta and I think that when populations explode beyond the pools capacity it spills out into the waters of Dale.
Podon
Burton tintinnid
Tintinnids are single-celled ciliates that build a lorica or case within which they live. Some species coat this with minerals forming a protective cover. Two quite different species have bloomed this month, one in Dale and the other at the other end of the Haven at Burton. The former has a long narrow tube and does not commonly occur. They can be tricky to identify to species despite the lorica being quite species-specific.
Dale tintinnid
Nectochaete larvae of Polydora
Trochophore of Polydora
Larvae of Polydora are probably the most abundant representative of the polychaetes in the plankton and thrived during late May and early June. To begin with it was the first stage or trochophore present. Tiny ciliated balls whirling around the sample. As segments are added at the posterior end so the bristles or chaete are added, at which point it is known as a nectochaete. As they continue to length baggy palps are added at the back of the head. Nectochaetes in various stages have been very common through the Haven this month. The larva of the Sand Mason worm Lanice has appeared in many samples this spring and recently there was a late larva, an aulophora, in a tube about a centimetre long probably around 60 days old. The tentacles have multiplied to at least 7. This is just prior to settling on a sand shore to burrow into the sand.
Late aulophora larva of Lanice
Like last month (see below) the barnacle nauplii larvae have metamorphosed into the last stage, the cypris larvae, that settle on to hard substrates to glue themselves to a permanent spot. One nauplius larva can be distinguished from the rest and is the larva of Sacculina, a parasitic form that enters the Shore Crab Carcinus maenas to become endoparasitic for many months. There were a number making an appearance mid-June.
Nauplius of Sacculina
Several specimens of fish yolk-sac larvae have occurred, especially at Burton. At this stage they are tricky to identify. Once the yolk-sac disappears the pigments and other structures become more distinct making ID easier.
Fish yolk-sac larva
The new book
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All photos shown were taken in late May or June 2026
April & May 2026
The Spring Bloom Occurs
Biodiversity High
Summary
Through March there had been a steady increase in the zooplankton but the phytoplankton had not changed significantly until the first sampling in April. By the end of that month there had been substantial increases in both diatoms and dinoflagellate biodiversity and volume. Going into May, with the huge increase in diatoms (food) a large rise in zooplankton occurred. Now there is a huge diatom bloom in the Haven with distinctly green water.
Bloom in early May
Phytoplankton
It has been fascinating to watch the change in diatoms over the two months. A gradual decline in the winter diatoms so now the larger species like Odontella sinensis and Bacillaria have all but disappeared. A sample taken in the narrow, furthest eastern region of the waterway (Burton) where salinity is lower shows these diatoms still present. The lovely species Corethron has now disappeared and at the end of April many were found with what looked like a resting spore inside. I cannot find a reference for this but many species have yet to be identified as having resting spores. These will drop to the bottom and "germinate" in more favourable conditions, later in the year. There is a possibility that they are sexual spermatogonia but the fact that the species disappeared from the water soon after high numbers of these being present suggests they are resting stages.
At the end of April the first diatom to bloom in large numbers was the chain-forming Lauderia followed by Thalassiosira rotula. The former species I last saw in July 2025 but is now completely dominant and is visible in the low magnification of the sample shown at the top of this blog. During May the genus Chaetoceros has exploded in numbers and variety. It is one of the diatom genera that shows the greatest biodiversity. C. socialis, a species with long chains coiled up in mucus, is common but one I have not seen for a few years now is C. decipens. The broad flat chain grows and lengthens to form a spiral.
Through March and April small colonies of Phaeocystis have occurred but by mid-May these had expanded (visible to the naked eye) into extensive, long green groups of cells embedded in thick mucus. As yet these are not too disruptive although many organisms become entrapped within the sticky mucilage. Much more growth and the Haven water will become greener as the thick, mucus-masses washup into the strandline where it will rot and smell.
Dinoflagellates have also bloomed, almost entirely Protopteridinium species, the typical spring genus.
Corethron with possible resting spore
Probable Bacteriastrum with resting spores
Lauderia chain with tintinnid
Long Thalassiosira chain
Chaetoceros socialis colony
Chaetoceros decipiens below right. Below left is C. curvisetus
Phaeocystis colony with many diatoms stuck in the mucus. Mid-May.
Small Phaeocystis colony from April. The animal is a trochophore larva
Cyprus larva of a barnacle
Zooplankton
March was dominated by the barnacle nauplii larvae and moving through April these declined as they matured into the cyprus stage. This is a non-feeding stage and fat deposits can be seen in the photo, sufficient stored energy to assist the larva find a place on a hard substrate to finally adhere to. The walking legs can be seen at the back with the antennae just visible protruding at the front. The latter are sensory to test the ground. Cyprids were common right through to mid-May. The nauplius has horns that release an anti-wetting agent so that they do not become caught in the surface tension. Missing from the cyprus larva they are most commonly seen at the top of the collected sample, stuck in the surface.
Other crustacean larvae were common like crabs (see March blog below) and mysid shrimps. So it was great to see the final larval stage of these in May, especially in the crab. I see these only on a few occasions as they quickly settle on the shore. This is the megalopa larva, just a few millimetres across, before it metamorphoses into the adult.
Megalopa larva in May above the diatom bloom
A mysid shrimp larva in April (4 mm long)
Polychaete nectochaete larvae have been prolific during this period, not surprising as the last blog was showing the huge numbers of trochophores present. Trochophores (first stage larva) have bands of cilia but as the worm develops with added segments at the rear so the bristles or chaetae, characteristic of polychaetes, begin to form. Once this happens the trochophore has become a nectochaete larva. As the weeks pass so more and more segments form to produce longer and longer larvae. The majority of these are of the spionid family and the palps that form on the side of the head are typical of Polydora. Larvae of Sand Mason worms (Lanice) were common along the coast both in a small tube or without. There were also nectochaetes of Capitella. Adults look like small earthworms and live in sand and mud.
Nectochaete larvae of Polydora, older one on the right
There have been plenty of other seashore larvae dominating the zooplankton like the veligers from periwinkles and other gastropod molluscs (see photo in March blog). However at the end of April huge numbers of bivalve veligers of the common cockle were present as well as plenty of eggs with developing veligers inside. During May it was great to see a few sea squirt larvae (Ascidiacea) called "tadpole" larvae. The photo of the head shows what looks like eyes but these are balance organs with an ocellus patch for detecting light. The projections at the front are used to stick the head to the substrate when they then metamorphose into the adult sea squirt.
Eggs with developing cockle veliger inside
Tadpole larva of a sea squirt
Lanice larva
Capitella nectochaete larva
Several developing eggs of starfish were in the mid-May sample. The embryos were only hours old and the developing gut can be clearly seen inside with yolk cells floating around the gut.
Egg of Starfish, Gastrula Stage
Developing Oikopleura, 20 hrs old
The tunicate, Oikopleura, witha fragment of the "house"
The rotifer, Synchaeta
During spring the tunicate Oikopleura starts to proliferate in the plankton. From seemly no where they arrive and are suddenly in abundance. One of the amazing facts about them is the speed of growth, sexual maturity in 5 days (depending on temperature). What looks like a head is actually the main body with a long, broad, wavy tail. The "hat" like structure at the top of the trunk-body is the gonad (in the photograph, a testes). They produce an extensive, membranous structure within which they live called a "house". By flexing the tail back and forth a current of water brings in bacteria and other minute particles that become caught. Several times a day they consume the house before starting again. During plankton collection the house is expelled, a reaction to possible predation. Growth is so rapid and individuals so abundant, they are essential food for fish.
The rotifer Synchaeta has been seen since the start of the year and several times I thought their numbers were going to sky rocket. They enjoy sheltered conditions and thrive in the Gann Lagoon nearby, spilling out into the bay at Dale. This is often the case in May when large blooms can occur, happening in 2022 when they dominated the zooplankton. In 2024 they reached moderate densities over May and June but suddenly exploded in numbers in July. So in 2026 numbers are still low but present in every sample. Maybe they are just waiting for the right moment.
Copepod with a parasitic cryptoniscus larva (isopod) attached. Common April.
Oncaeid copepods in precopulatory stage. The male at the front is guarding the female. Found in May.
Fish egg, early May. Note oil drop.
The new book
Seashore Plankton is due at the end of May/beginning of June
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All photos shown were taken in April and May 2026
March 2026
Larval Increases Dominated by a Sudden Barnacle Blizzard
Summary
At the turn of the month there was a proliferation of various larval stages dominated by the trochophores of marine worms. These tiny, pale ciliated balls of cells moved rapidly around the samples (see 3rd March photo of pale dots. Also the red arrows indicate crab zoea and the red ring is around the Phyllodoce trochophore shown in the Photo of the Month). There were many crab zoea larvae and some barnacle nauplii. At the end of the month the nauplii numbers had exploded, possibly due to the fine weather warming the barnacles on the shore. The density was so high that it was difficult to see anything else through the blizzard. (see 20th March photo. Circled is a single periwinkle veliger). No diatom bloom has started.
Phytoplankton
The numbers of winter diatoms, like Odontella sinensis and O. mobiliensis, are reducing; the sliding diatom Bacillaria paxillifer is still present in reasonable numbers along with some Helicotheca tamesis, Corethron and Paralia sulcata. The lovely spring diatom Ditylum is now increasing and my first Delphineis for five months has appeared. They are very small diatoms that adhere to sand grains to form tiny colonies. The freshwater desmid Staurastrum that was common over the winter after being flushed by the rivers down to the Haven have largely died so that while still in the plankton they are now dead. Their tough cell wall can remain for years. Green alga are rare in the plankton but the marine species, Halosphaera, is still present in very small numbers. Dinoflagellates are beginning to appear, a few Protoperidinium and a single Tripos furca. Rather unexpected at the end of the month after calm conditions there were a number of the resting stages of P. oblongum. (see last month below). These are normally present after rough weather in autumn not the beginning of spring.
3rd March:
20th March
Staurastrum sp x600
(above) and resting spore of Protoperidinium
O.sinensis. Possible development of spermatogonium
Two views of Shore Crab zoea larva
(3rd March)
Zooplankton
There was a continued presence of the species seen last month like the planula larvae of sea anemones, cyphonautes larvae (bryozoa) and even more actinotrochas of Phoronis (a close relative of the sea mats or bryozoans). As I said in February I have never seen so many before and they are still appearing in the samples.
As well as the actinotrocha another favourite turned up: a Muller's larva that live in plankton for just a day or two. This is a super little creature and I managed to film its antics as it swum slowly around, changing shape and probing the material it encountered. The photo shown here was quite red whereas most I have seen are bluey-grey. It was also quite large, 250 microns. Muller's larvae are a unique stage in the development of polycladid flatworms, an example of which is the gorgeous Candy-stripe Flatworm Prostheceraeus vittatus that lives on rocky shores. The photo of the adult flatworm was taken at Musselwick, a shore near Dale.
Without doubt the most abundant zooplankton members were crustacean larvae. At the start of the month it was decapod (crabs and prawns) zoea larvae with a few barnacle nauplii. Last month I was amazed at the number of Common Hermit Crab zoea (see the Photo of the Month) and two occurred this month but there were twenty or more zoea of the Common Shore Crab Carcinus maenas. I might find the odd one or two but this was probably the record! However, by the end of the month finding anything through the intense blizzard of barnacle nauplii was very difficult. Possibly the warm weather - it reached 20 degrees C in parts of Wales - warmed the rocky shore to synchronise the barnacle Semibalanus balanoides to breed. A few eggs occurred and plenty of shed egg cases but it was noticeable that all the nauplii larvae were of a similar age. With so many nauplii it was especially tricky to see other, smaller plankters although the slightly larger mollusc veligers, both bivalve and gastropods, did stand out
Actinotrocha larva of Phoronis
Candy-stripe Flatworm Prostheceraeus vittatus
Gastropod veliger larva
Muller's Larva
Barnacle nauplii 20th March
So, what were all those pale balls swimming in their thousands on 3rd March, shown at the top of this piece? They are trochophores, first stage larvae of marine worms (polychaetes). Last month there were many eggs of the marine worm Malacoceros and these must have hatched now. The adults live in mucus-lined tubes on the middle shore of sand and mud - plenty around Dale. The photo of a Malacoceros trochophore here does not show the two developing eyes but the ring of cilia around the middle is distinct as well as the sensory tuft at the top. A high proportion of the trochophores were this species although there were high numbers of a second worm, Sabellaria. The ball of cilia covered cells have long bristles (chaetae) flowing from the posterior end. Adults live in sandy tubes they adhere to rocks.
Trochophore of a Sabellaria worm
Trochophore of Malacoceros a polychaete
A harpacticoid copepod species
Last month I ended on an interesting harpacticoid copepod species and likewise here. A different species and certainly not a true planktonic one. It is very "worm-like" as rather than swimming it is adapted to crawl through sediment. Harpcticoids are typical of sand and mud and this one must have been brought into the water column by wave action and turbulence.
All photos shown were taken this month except the Candy Stipe Flatworm
February 2026
A Sudden Boost after a Poor Start
Summary
Bad weather has made sampling tricky this year and at the start of February it was no different. Rough weather produced samples high in detritus and material lifted from the sea bed, both dead and alive. In fact, continuing on from January, the dominant plankton community was of benthic (bottom dwelling) and estuarine species. A window of calm conditions occurred near the end of the month and a sample taken 17th February recorded a spectacular biodiversity, especially of zooplankton, with some unexpected results.
Phytoplankton
Although Odontella sinensis and O. mobiliensis were, again, fairly common, the most dominant (and annoying diatom because of its movement) was the sliding diatom Bacillaria paxillifer. What I call the sweet wrapper diatom, Helicotheca tamesis, is increasing in numbers with no Corethron found at all (common last month). A species that has made a huge increase is Paralia sulcata, reported last month. Next to Bacillaria this was probably the most common species found and in longer chains than a few weeks ago. High in silica and very compact they live on sediment surfaces but are easily taken into the plankton and often common at this time. The lovely spring diatom Ditylum has made an appearance with a small number present. Estuarine diatoms like Navicula were common along with plenty of the freshwater desmid Staurastrum. Dead cells can be common in turbulent conditions as the cell wall is remarkably resistant to decay and are easily lifted from the bottom. However, this month there were plenty with green chloroplasts inside suggesting that they are alive and washed down in the flooded rivers. Another green alga present was the marine species, Halosphaera. These green algal forms stand out in the plankton because, by contrast, diatoms are golden brown and part of the brown algae such as bladderwrack and kelp.
With the bad weather dinoflagellate numbers continue to be low with only resting forms present like those of Polykrikos and Protopteridinium. Of the latter the spores of P. oblongum are particularly striking with their red organelles of stored lipid, looking to me like a rich chocolate with raspberry filling. One very odd occurrence was a single Noctiluca cell (the only living dinoflagellate found) more often associated with autumn.
Chains of Paralia sulcata x400. An abundant benthic diatom species this month
Bacillaria paxiflifer - the dominant diatom
Staurastrum sp x600
(freshwater desmid)
Helicotheca tamesis
Ditylum brightwellii x400
Noctiluca x100
Resting spore Polykrikos x600
Resting spore P. oblongum
Zooplankton
Four examples of foraminifera this month
Less than 24 hour old actinotrocha larva of Phoronis
The samples taken early in the month had few zooplanktonic organisms and those present were benthic species like a diversity of foraminifera. These amoeba-like creatures secret a shell or test in which they live. Few are true plankton but in turbulent weather will be abundant. Some examples from 4th February are shown here. Planula larvae of sea anemones were still present but the most spectacular larval form was the actinotrocha of Phoronis (a close relative of the sea mats or bryozoans). I always like finding them, with every part of the body covered in beating cilia to drift the organism through the water. The most surprising aspect was that if I ever find them it will just be the odd one but here it was at least five. The number of tentacles increase with age and are visible in the photo here hanging down below the central region with the gut. Above this is the pre-oral lobe with the mouth at it's base. As the actinotrocha swims so this collapses forward over the mouth. These ones are the oldest I have found in shore samples. Last August was the previous best examples found. As they grow the larvae tend to move away from the shore developing many tentacles and one mature actinotrocha was found in plankton from off Skomer and was my photo of the month August 2025.
Other planktonic larvae consisted of veligers, both of bivalves and gastropods. With still large numbers of periwinkle eggs, especially the small periwinkle Melaraphe neritoides, it is not surprising. There were also plenty of young trochophores, the larvae of marine worms. They are near impossible to identify to species when they are so young. Malacoceros is a common polychaete worm found living in mucus-lined tubes on sediment seashores, in the middle shore. Here it feeds on organic matter. They have a distinctive egg which is currently becoming abundant in the plankton. I found one young larva but they are not the trochophores I mentioned earlier.
Melaraphe eggs
Malacoceros, a polychaete, developing egg
Periwinkle veliger larva
Arrow Worm Parasagitta elegans
Tomopteris helgolandica lives permanently in the plankton
T. helgolandica closeup of the head
A radiolarian (x600) becoming abundant
Probable harpacticoid species, Peltidium interruptum
There were several permanent planktonic organisms (called holoplankton) that did not require high magnification. The unexpected arrow worm Parasagitta elegans was 15 mm long. It was a surprise as I have never found one this close to the seashore in the winter, more likely to see them in summer coming in with the tide from the Atlantic. They are fast predators of copepods, which are not abundant at the moment. Arrow worms are transparent and quite difficult to see in samples although the master of this gelatinous invisibility is the beautiful polychaete worm Tomopteris helgolandica. The occasional one of these can appear at any time but are so difficult to see, even though they are several centimetres long. Staring at the sample in a dish under low magnification I could make out a disturbance at the bottom of the dish with material being moved around by an invisible culprit. I never fail to become excited by them. This species is the most common Tomopteris and has a tail at the end of the body. The gut is one of the few organs that can be seen inside the body and the pharynx behind the eyes is constantly moving, contracting and expanding.
Although copepods were few, especially the large calanoid types there were a reasonably large number of the small harpacticoid copepod Peltidium interruptum (ID has not been verified as yet). Again I have not seen these at this time before. It is a species that curls up in a ball, dark brown in colour and is widely distributed around the UK but not normally associated with the shore. Harpacticoids are typically benthic, associated more with sediment. Peltidium is found in off-shore sediment and possibly the turbulent water this year has brought them to the shore. Another crustacean that was common was a 3mm cumacean that lives in mud but at this time of year enters the plankton. Check out photo of the month a year ago.
Another species worth mentioning this month was the steady increase in a tiny radiolarian, a single-celled protist, probably Acanthostaurus. Requiring a high magnification, I have see the odd one or two this winter but now they have become commonplace in all samples. The cell is supported by a solid skeleton of spicules. They feed on bacteria and diatoms but also have symbiotic organisms that can photosynthesise.
All photos shown were taken this month
January 2026
Very Low Numbers So Far This Year
Summary
After a high diversity of zooplankton on 23rd December I expected great things for the New Year. A sample in the first week of January had a few different larval forms and diatoms but nothing like December. Then came the bad weather, snow, heavy rain and wind, one week after another. The only window to sample was 21st January. The conditions were still bad and was the worst I have ever known while sampling, with waves crashing over the top of the jetty. Samples were thick with organic debris, the count on diatoms low and zooplankton exceptionally low
Phytoplankton
The two species of Odontella, O. sinensis and O. mobiliensis, along with the sliding diatom Bacillaria paxillifer, remain common through the winter but are rapidly being joined by ever increasing numbers of the lovely species Corethron. The two frustule valves are slightly different and there are numerous spines. Paralia sulcata was also quite common and occurs in short chains. They have high concentrations of silica present in the frustule and the chains are very tightly compact. They live on the surface of sediment and easily lifted into the plankton during turbulent periods. This also applies to Pleurosigma and Gyrosigma diatoms which occurred in singles as they are non-chain diatoms.
Amongst the debris tiny star-like clusters of Thalassionema nitzschiodes occurred. The long needles of Rhizosolenia were quite common but I was a little surprised to find the very large species R. robusta. Until recently known as Neocalyptrella robusta, it is banana shaped and considered a warm water species.
The second component of phytoplankton is dinoflagellates, very diverse in December, but are not good survivors in rough weather. However, one genus, did occur in some numbers at the start of the month, Dinophysis.
I was surprised on 21st January to see several silicoflagellates (I saw none last year) and one was a rare species. There were Dictyocha speculum and D. fibula, respectively. They are very small (about 25 microns across) and easy to miss. They get their name from the skeleton of silica on which the cell then adheres to.
Dinophysis, probably D. caudata x600
Corethron dividing
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A chain of Paralia sulcata x400
Thalassionema nitzschiodes
Rhizosolenia (Neocalyptrella) robusta
Zooplankton
Biodiversity was low although early January a few interesting larvae were present: a pilidium and many planulae. The latter are the larvae of cnidarians, like sea anemones and they are delightful to watch cruising around. Common at the beginning of the month. The pilidium is the larva of a nemertean, a phylum group known as the ribbon worms, that are found on the shore. The most famous species is the bootlace worm that when unravelled (as it commonly is found in a slimy ball) can be many metres long. The pilidium is not that common to find. This one was probably a 4-day old larva and they can develop for weeks during which time they will disperse along the coast or off-shore.
What is particularly fascinating about the pilidium development is that after it is well formed it does not metamorphose into the adult but creates the juvenile inside the body of the pilidium. It can eat its way out or just break-out depending on the species.
One huge surprise was a single, mature Oikopleura, that occured at the start of the month. This is a tunicate (sea squirts) that lives permanently in the plankton but is a summer species, normally appearing at the end of April. Also unusual at this time was the group of rotifers, Synchaeta, that forms the Photo of the Month on the home page. If they occur in Dale the population develops in the late spring.
Barnacles laying eggs can occur at almost any time and it was interesting to see many developing eggs at the start of the new year in the plankton.
Planula larva of a sea anemone
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Silica skeleton of Dictyocha fibula
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Dictyocha speculum
4 day old pilidium larva (ribbon worm Nemertea)
Oikopleura
Barnacle egg with nauplius inside
Egg of the small periwinkle, Melaraphe neritoides, common in the 21st Jan sample
Egg capsule of Edible or Common Periwinkle