Investigations into the little known and under-recorded

genus Jaera (ISOPODA) in Cornwall.

by David Fenwick


I have been interested in isopods for a while but have only recently found the time to check the status of members of the genus Jaera in Cornwall.

I started by gathering information and reading the section on Jaera in the Synopses of the British Fauna "Intertidal Marine Isopods" by Ernest Naylor

and Angelika Brandt, and looked at Cornish records on the ERICA database. There were twenty-six records of Jaera albifrons, one record for

J. forsmani, one record for J. praehirsuta and three records of J. nordmanni, Jaera ischiosetosa was absent from Cornwall.  On looking at the

records it appeared that there was probably a disproportionate amount of records for Jaera albifrons, which is now used as a collective name for

a group of four species. It was obvious that something needed to be done to try and improve recording.


I decided to look at highly detailed images that I'd already taken of Jaera to see if I could identify the species in the images. The results were shocking

as the only species that could be identified from detailed images was J. nordmanni, all the rest were obviously members of the Jaera albifrons group,

therefore Jaera sp., but I couldn't identify one to species level. Many images were also of the larger females, which cannot be used in identification

anyway. This means that any species included in the Jaera albifrons group can only be verified from images if they are taken using a microscope. I've

since found as a result of this brief study that details of habitat cannot be used to assist with species identification either and because all four species can

be found together!


Jaera albifrons was divided by Forsman in 1949, who split it into three distinct species; a fourth, Jaera forsmani was described by Bocquet in 1950

and named to honour Forsman. This means that in the UK, the Jaera albifrons group, is a group of four species, and the status remains to this day.

Members of the Jaera albifrons group include Jaera albifrons, J. forsmani, J. praehirsuta and J. ischiosetosa. The species Jaera. nordmanni is a

distinct species and not part of the group, it does not share the same shaped praeoperculum as members of the J. albifrons group.


The purpose of this article is mainly to report the differences between the species in the Jaera albifrons group  and to assist in the collection of higher

value data, Jaera sp. being acceptable for the purpose of recording, but in nearly 70 years we don't seem to have gotten away from recording of J. albifrons

for all species in this group.


Jaera albifrons male


Jaera nordmanni


I decided that I really needed to wipe the slate clean and have a fresh start with them. I already knew that Jaera could be abundant at certain sites so I

decided to sample and collect specimens to look at in more detail. I started by collecting Jaera that were found on rocks with wet bottoms and those

that fringed shallow pools in the middleshore, washing small rocks and stones in a bucket of seawater to remove specimens and retrieving the Jaera using

a small plastic sieve. This method was later improved and I found I didn't need to look for Jaera on the bottom of rocks and small stones to get a good

sample, I'd just pick up a suitable rock and wash it in a bucket before replacing it in the same position it came from, this saved time on visually looking for

them. All sites chosen were quite sheltered in terms of exposure.


A good sample is needed, ideally one over twenty specimens, I say this because at Hannafore, Looe, I collected about twenty specimens only to find all

were female. As I've already said, females cannot be used to identify the species, it is only the males that are useful. Males can be separated from females

visually, they are usually smaller and darker than the females, but juvenile females of similar size to males require a stereomicroscope to help remove them

from the sample. In any one sample, there will likely be more females than males, and many of the females collected will were berried, and will have either

eggs or developing young in their brood pouch, it is best practice to return females as soon as possible. A greenish tint can usually be seen on the dorsal

surface of sexually mature females, this is the colour of the eggs and developing young.


Green tinted berried female of Jaera albifrons group


Once males have been isolated, you can then start looking at specimens using a compound microscope at magnification of x100. A small number of male

specimens can be reviewed at a  time by placing individuals in a drop of seawater on a microscope slide, a small cover slip is usually enough to immobilize

animals without harm, the slide is then inverted and the pereopods on the ventral side of the animals studied. For a larger number of specimens, it is

advisable to dispatch specimens in ethanol then place them on a slide, turning specimens on their backs using a fine  acupuncture needle, and adding a

cover slip to help splay their pereopods.


Males of the species within the Jaera albifrons group are defined by the shape of the praeoperculum, this is located at the ventral posterior of the animal,

under the telson, this is often described as T shaped, but I think of it as looking rather like an old-fashioned moustache. J. nordmanni has a much different

praeoperculum but it is a species that could appear in samples because it often occurs with members of the albifrons group, because the species can tolerate

low levels of salinity.


If a male specimen has a praeoperculum as seen in the J. albifrons group, then pereopods 1-2 (legs, front)  and 6-7 (legs, rear) must be looked at to

determine the species. The position of setae, the sparsity of curved setae, number and position of spines, and the shape of parts that make up the pereopod

(carpus, propodus, ischium and merus) is used to determine the species. Figures below show the notable differences between the pereopods of the species

within the albifrons group.


Praeoperculum of Jaera albifrons group


Praeoperculum of Jaera nordmanni


In early December 2017 J. forsmani was identified, it occurred in a sample of washings taken from a rock crevice at Little London, Marazion, Cornwall.

It was the finding of this species that encouraged me to look for other species of the genus. In January 2018, a sample was collected from under stones

next to where a stream flows down the shore, at Wherry Town, Penzance, Cornwall. Males were examined and all were found to be Jaera ischiosetosa,

a species not listed on the Cornish ERICA database. This species was found to be abundant at the site, and at other sites where water flows down the

shore, at one site on the River Fal it was found to be superabundant.


Jaera forsmani - Little London reef, Marazion.


In late January a friend sent me a Jaera from Poole Harbour, this was identified as the true Jaera albifrons. Just days later and on washing stones from

pools under the bridge that crosses Penzance Harbour, I found Jaera albifrons myself. This species  also occurs at Restronguet Passage in large numbers.


Jaera albifrons - Penzance Harbour


In early February during a visit to Hannafore I decided to look at Jaera again, and I took a sample from the same site where I had previously collected

twenty females. The site is beside the storm drain, where a stream runs down the shore. Males were collected and these were identified as Jaera

ischiosetosa.  Jaera ischiosetosa was also found to occur on the lowershore at Hannafore.


Numerous other sites were looked at around Penzance, Cornwall and the most common species was without any doubt Jaera ischiosetosa, but it must

be noted that collection was a result of stone washing and not weed washing. Weed washing at different levels of the shore might give different results with

other species found to be equally abundant. I use the words "equally abundant" because Jaera are not rare, they are often very common indeed. It's only

their small size that probably puts people off recording them, males are smaller than females and are only around 2mm in length, so specimens need to be

taken away from the shore to be identified using a microscope.


Jaera ischiosetosa - Wherry Town, Penzance.


There was just one record for Jaera praehirsuta on the ERICA database, it was found at Restronguet Passage, Mylor Bridge, near Penryn, Cornwall

in September 1990, no other Jaera were recorded as being present at this site at that time. I decided that if I was to find and photograph this species in

detail that this should be my starting point. One the 22nd February 2018 I had to visit the Tremough Campus of Exeter University at Penryn, I decided to

go on to Restronguet Passage after the visit to look for Jaera praehirsuta. The same technique was used to collect specimens, the washing or small rocks

and stones, anything less then 3/4 diameter of a bucket considered, some stones had fucoid algae attached. On straining the sample it was easy to see that

numerous individuals had been collected and that Jaera were potentially superabundant at the collection site which was just in front of the Pandora Inn.


Once home, a lot of time was spent in separating males from females visually, and then juvenile females removed from the males using a stereomicroscope.

Males were then turned on to their backs using a fine acupuncture needle and their pereopods examined using a compound microscope. I was surprised to

find Jaera ischiosetosa and Jaera albifrons dominating the sample, a single Jaera forsmani was also found. I had to go through nearly all the male

specimens collected before finding Jaera praehirsuta, which probably represented approximately 2% of the entire sample.


All four species of the Jaera albifrons group were found at Restronguet Passage. Re-finding a single species like Jaera praehirsuta is one thing, but finding

all four species together in a small area means that one cannot guarantee how many species any one sample will contain. On determining the first specimen one

 should not assume that all the rest will be the same, a lot more patience and time is required to go through a sample to determine if it contains multiple species.


Jaera praehirsuta - Restronguet Passage, near Penryn.


It appears a lot needs to be done to record this common but overlooked genus successfully, and we really need to look closely at specimens from areas

such as the Fal, Camel, Helford, Lynher, Tamar and East Looe rivers. We need to look in brackish pools on and above high water, in freshwater streams

running down the shore, in estuaries and fully saline areas of sheltered coasts to get a better picture of the distribution of the species. Although in estuaries

and probably some very sheltered shores it is likely all will occur together, like at Restronguet Passage.


Jaera can be found on algae such as Saw Wrack, Fucus serratus (Jaera praehirsuta); and Bladder Wrack, Fucus vesiculosus (Jaera albifrons s.s),

but specimens can be found in the same zones under pebbles, stones or rocks with damp / wet bottoms. It is easier to wash stones than cut and wash algae,

and is by far less destructive.  In essence Jaera are a bit like barnacles, and like barnacles, species of Jaera tend to have a preference for different zones

on the shore, although it would seem that this is not a hard fast rule.


One species not dealt with here is Jaera hopeana, which I have never seen, it has been found living as an ectocommensal on the isopod Sphaeroma serratum.

It has occurred at Wembury in Devon since the 1930s, it should therefore be looked for in Cornwall especially towards the east of the county.



Members of the genus Jaera are extremely easy to collect and commonly occur on the shore in wet areas, in areas of either freshwater or seawater seepage.


The reason they are not recorded more often is due to the fact that Jaera cannot reliably be identified in-situ on the shore, but sexing individuals 'might' be

possible with a magnifying glass, loupe or head magnifier. Specimens really need to be taken away, sorted, and examined under a microscope/s.

The process of identification is therefore complex and beyond the scope of Citizen Scientists / recorders who do not own microscopes, but this should not

prohibit their collection and sending to people who can identify them.


In terms of identification, verification, photography, only male specimens should be used, good images of the praeoperculum is a primary consideration,

this would prove if a specimen was in the J. albifrons group or not. If within the J. albifrons group then pereopods 6 and or 7 are important for J. forsmani,

J. ischiosetosa and J. albifrons; pereopods 1 and 2 are important for the determination of Jaera praehirsuta. In some examples of J. ischiosetosa the

setae on pereopod 7 had worn off, they were found to be in better condition and easier to see on pereopod 6.


 To get microscope images for validation, pereopods need to be carefully dissected from the specimen, placed on a slide, covered with a cover slip and

photographed, making sure that the setae on the pereopod are in focus and their position obvious.



A quick guide to Jaera pereopods

J. nordmanni - Oval shaped body, praeoperculum parallel sided, acutely pointed.

J. albifrons group - praeoperculum T / stalked moustache shaped.

J. praehirsuta - Pereopods 1-4 with curved setae on propodus, carpus and merus.

J. albifrons - Pereopods 6 and 7, distal region of carpus with spined lobe. 

J. ischiosetosa - Pereopods 6 and 7, curved setae on the distal portion of the ischium.

J. forsmani - Pereopods 1-4 are sparsely covered with curved setae on the propus, carpus and merus segments; pereopods 6 and 7 have well developed carpal spines.




Ventral anatomy - position of pereopods and praeoperculum


Pereopod anatomy


Jaera praehirsuta pereopod


Jaera albifrons - distal region of carpus with spined lobe on pereopod 6 and 7


Jaera forsmani - sparsely covered with curved setae on pereopods 1 and 2


Jaera forsmani - carpal spine on pereopod 6


Jaera ischiosetosa pereopod 6 - curved setae on distal portion of ischium


References used:

E.Naylor and A. Brandt. Synopses of the British Fauna, "Intertidal Marine Isopods"

Haywood and Ryland. Handbook of the Marine Fauna of North-west Europe.

Bocquet, C. (1950) Le probleme taxonomique des Jaera marina (Fabr.) (J. albifrons Leach). Comptes rendus hebdomadaires des Seances de l'Academie

des Sciences, Paris 230:132-134.


Online resources


Marine Species Identification Portal, Macrobenthos of the North Sea - Crustacea