Crustaceans part 1 - the holoplankton forms
The Crustacea are a huge class of Arthropods often completely dominating the zooplankton, especially in the open sea. Both adults and larval forms may live permanently in the plankton (called holoplankton) or just temporarily (the meroplankton) in the zooplankton; the latter are are on page Crustaceans part 2.
The key holoplankton groups are copepods and cladocerans (known as water fleas (bottom of this page).
The Copepods
The large calanoid type copepods like Calanus are large copepods, a few millimetres long, and so visible to the naked eye. A single red eye can be seen and long antennae. One Calanus can consume many 1000s of diatoms in 24 hours, feeding mainly at night.
Calanus top view
Calanus side view showing red pigment at the tip of the oil sac
Hairs and sensors on the antennae pick up pressure movements in the water, particularly from potential predators like fish. The legs under the body can suddenly kick back and cause the copepod to "jump" rapidly forward and exceptional speeds.
Different Types of Copepod
The Harpacticoid copepods are not typically planktonic in the open sea and are more associated with sediment, living in the benthos and are common in damp places on land. They tend to be worm=like as they negotiate crawling around sediment. At times of water disturbance after strong wind they can be carried from sediment and mixed with the plankton before they gradually return to the mud. In the Haven they can be as abundant as Calanoid types probably due to the shallow water and nearby sandy beaches. There are some Harpacticoid species that do occur permanently in plankton, and one, Euterpina acutifrons, can be especially abundant in the summer. Porcellidium is a bottom dwelling harpacticoid but common in the Haven.
Euterpina acutifrons
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.
However, copepods are diverse and a significant number of them have a parasitic mode of life. Caligus spp. are large (4-5mm), saucer shaped copepods that are parasitic on fish and occasionally occur in the summer plankton searching for a host. 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.
Caligus sp
A fascinating copepod group are the monstrilloids. These copepods have adults that are non-feeding with empty space in the body where the gut should be. Often this is filled with fat. One common species in Milford Haven is Cymbasoma rigidum with often several being caught in the same sample. The females carry eggs attached in long streamers behind the body. These hatch into a larva that then is parasitic on seashore invertebrates like molluscs and polychaetes.
Copepod development
Copepod with spermatophore attached
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
Egg sac loose in the plankton
Eggs hatching
Egg with a developed larva about to hatch
The nauplius larva
Various nauplius larval stages. Note the presence of oil droplets inside some, for food and buoyancy
After five or six nauplius stages the larva becomes a juvenile copepod called a copepodite, that then passes through several instar stages until an adult.
A long tube-like structure runs for much of the body length, the oil sac. Used as an energy store a waxy lipid can fill this especially in autumn with the run up to winter when it may not feed (diapause). The posterior end is often tipped with a red pigment and Charles Miller believes this masks the effect seen from below of a pinprick of light that the oil sac can focus on to the rear of the body. This would be seen by predators hunting copepods from below. The red pigment in the middle is around the mouth. Eating diatoms can cause them to release light and so the red pigment again helps prevent this attracting attention.
Corycaeus sp. Side view and right, top view
A Predatory Copepod, Corycaeus
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 worldwide and these usually produce small spring populations in Milford Haven. In the open sea, particularly in in St Brides Bay off Skomer, they are common through much of the spring and summer just into autumn. Two genera are found, Podon and Evadne, with the former the most likely to be found in Dale. Both vary in size between 300 - 900 microns across and compared to freshwater species are quite robust predators.
Evadne
It is thought that the water fleas evolved in the sea but moved, via estuaries, into freshwater and then secondarily evolved back to become marine, and hence why there are so few species estimated at 11 across the globe.
Interestingly, a third species has appeared in recent years off Pembrokeshire. Penilia avirostris is the only marine filter feeding water flea, note the lack of large eyes. It is a tropical/subtropical species found in the Indian Ocean but has spread to the Mediterranean where a substantial population now exists. With climate warming the seas off the south-west of the UK there have been a number of marine heatwaves in recent years. Penilia uses the warm August water to migrate up to the North Sea and in the last few years has begun to establish large but temporary populations in the Celtic Sea, off Pembrokeshire. Occasional Skomer samples bring in specimens.
Quick Links:
Arrow & Acorn Worms
Bryozoa & Phoronids
Crustacea 1 - Copepods
Crustacea 2 - Larval Crab & Barnacle
Diatoms
Echinoderms
Foramens
Jellies - Cnidarians & Ctenophores
Molluscs
Parasites - flatworms & crustacea
Polychaetes - marine worms
Protists 1 - Dinoflagellates
Protists 2 - Ciliates
Rotifers
Tunicates
Decay, Oddities and Unknowns
Podon
Penilia avirostris female with eggs