top of page

Crustaceans of the Holoplankton

The Crustacea are a huge class of Arthropods often completely dominating the plankton, especially in the open sea. Both adults and larval forms may live permanently (called holoplankton) or temporarily (meroplankton) in the animal plankton. Copepods and Cladocerans, known as water fleas (bottom of this page), are key groups within the holoplankton and are shown below. For the meroplankton crustaceans, such as crab and barnacle larvae click here.

Copepod Calanus

Calanus is a large copepod, a few millimetres long and so visible to the naked eye. Note the large diatom in front. One Calanus can consume over 300,000 small diatoms in 24 hours although they feed mainly at night.


Different Types of Copepod

Harpacticoid copepod  Euterpina acutifrons

Harpacticoid copepods are not typically planktonic in the open sea, more associated in sediments. In the Haven they can be as abundant as Calanoid types probably due to the shallow water and nearby sandy beaches.  Euterpina acutifrons is 700 microns long and one of the few that will live in the open sea.

 Porcellidium a bottom dweller common in the Haven.

 Euterpina acutifrons

Copepod Porcellidium


Commonly referred to as "Insects of the Sea" copepods are a keystone group, vital in the ecosystem as they convert the energy of the phytoplankton into food for the next level, primarily fish but also other carnivores. Copepods may constitute almost 75% by biomass of the zooplankton.

Copepod 05.jpg

An unknown species covered in a coat of diatoms

The eye of the copepod Caligus

Caligus species. This large (4-5mm), saucer shaped  copepod is parasitic on fish but occasionally ocurs in the plankton during the summer. Most years  find at least one specimen. Huge eye lenses are present (uncommon in copepods) and a closeup of one of them is shown here.  More on Caligus under Parasites. A member of the Sapphirinidae family see more below.

Copepod Caligus

Caligus sp

Spermatophore on a heavily red pigmented copepod
Copepod closeup of spermatophore

Copepods can often be found with red pigmentation but unusually this specimen is completely red. A number of possible function of red pigment have been suggested but it is essentially to do with the ability to absorb blue light. For example, hunting fish normally come from below and the copepod will not be so visible against a blue sky. Protection from UV is another function. This female copepod is carry a spermatophore, attached there by a male ready to fertilise her eggs. The close-up is enlarged x200 

Copepod egg sac and eggs carried by a female

Copepod with egg sac
Copepod egg sac
Hatching copepod eggs

Eggs hatching

Copepod egg with developed nauplius larva about to hatch

Egg with a developed larva about to hatch

Various nauplius larval stages. Note the presence of oil droplets inside some, for food and buoyancy

Copepod nauplius larva, note lipid oil drops inside
Copepod nauplius larva
Copepod nauplius larva, note lipid oil drops inside
Copepod nauplius larva, note lipid oil drops inside
Predatory copepod, Corycaeus with eye cone and lenses

Corycaeus sp. Side view and right, top view

Predatory copepod, Corycaeus with eye cone and lenses
Predatory copepod, Corycaeus with eye cone and lenses labels

The family Sapphirinidae contains copepods that are highly predaceous and with many being parasitic as adults or larvae (see Caligus above). I think that these copepod photos are of female Corycaeus with eggs (I originally thought it Ditrichcorycaeus). Most notable is the pair of huge lenses at the front that direct light back through a crystaline cone to a second lens and the "retina", called a rhabdome (found in other invertebrates). The second lens has the ability to move the focus very slightly through tiny muscles. This creates a field of view around 3 degrees and one imagines a kind of binocular vision for catching prey. Most of the specimens have pincer-like appendages. These are most likely modified antennae for grasping the opposite sex during copulation. 

Adults are occasionally found in summer and early autumn in Pembrokeshire.

Thanks to Charles B. Miller's excellent 2023 book "Oar Feet and Opal Teeth" all about Copepods and Copepodologists for above information.

Cladocerans,the water fleas, are a very common group in freshwater where they feed on bacteria, algae and detritus. Only a few genera are found in the marine environment and within the plankton samples in Milford Haven these are rarities. However, in the open sea they are common, especially in samples from St Bride's Bay. However, in the summer of 2023 Podon suddenly appeared in Dale after a strong easterly wind that may have push water from the open sea into Dale Roads. They remained for 6 weeks, during which, two specimens of a second species, Evadne, made an appearance. 

Marine water flea, Evadne (Cladocera)


Marine water flea, Podon (Cladocera)


Evadne can be very common during the spring in St Bride's Bay from where this specimen was found. Podon is common in the summer in the same water but this specimen was from a Haven sample in August 2023. Both are predators. Evadne photo was on a microscope using phase contrast while Podon was DIC lighting. Magnified x200 the detail was obtained by creating a videostack of 250 images that were combined to create one composite image. 

bottom of page