Pea crabs

I had a bit of a surprise in my dinner over the weekend... after boiling up a bag of mussels, I found several odd-shaped crabs at the bottom of the water. Pea crabs, as it turns out... a not exactly uncommon but rarely seen member of Englands aquatic community.

Pea crabs and their relatives (Family Pinnotheridae) are best known as parasites of clams and mussels, although they have also been found affecting sea urchins, sea cucumbers, and tube worms. Most of what is known about these crabs comes from a few species that infest edible mussels and oysters, and because of this, the literature on the life history of pea crabs can be confusing and contradictory. The adult female pea crab is the best studied life stage, for the obvious reason that this is the stage most frequently encountered. Much less is known about the males and the early free-living stages, and what is known comes from captive animals observed in aquaria.

The general opinion is that pea crabs pass through several planktonic stages before moulting into a recognizable crab shape, although some species of pea crabs may hatch skip the planktonic stage. The young crab stage is characterized by a hard shell and flattened hairy legs for swimming. In this stage, both males and females have a free-living existance while looking for suitable hosts. Following entry into a host mussel, the female undergoes a transition (over several molts) into the familiar pea-shaped crab with a soft shell and limited mobility. Following these molts the female grows too large to leave its host, and its legs become smaller and weaker, sufficient to move around inside the mussel but not to live outside.

The male will continue to grow through several more molts, but is less dependant on a host mussel. In the British Pea Crab, the males remain small and can alternate between a soft-shelled form adapted to the inside of a host and a free-living hard-shelled form adapted for moving between hosts, with the season guiding their decision on whether to be soft-shelled or hard-shelled at any given time. Its less clear whether males of other pea crabs can switch between hard and soft shelled forms, however. The hard exoskeleton helps protect the crab both in the outside world and during the dangerous process of entering a mussel - the mussel can close down on a crab, potentially crushing it or snipping off legs, and males missing a leg or two show up with regularity inside mussels.


Female pea crabs are obligatory parasites once they settle in a host. The female doesn't actually feed on its host, however, rather it steals food from its host - though this has the effect of damaging the gills, causing local irritation of the mantle lining and reducing the growth rate of the host. The male can presumably also steal food from its host, but its less clear how dependent it is on the host mussel in the wild, and it may prove to have a largely free-living existance. The male sports fringes of hairs on its (flattened) legs which enable it to swim and hard shelled forms are restless, frequently leaving and re-entering hosts when in captivity. As all of the behavioral studies are based on captive individuals kept in aquaria with lots of mussels (and probably not much else), its not clear how much of their time hard-shelled males actually spend in hosts - it is probable that they are only part-time parasites.

Shrimp

The spanish have a reputation for being friendly, and that seems to extend to their shrimp....  these small shrimp on Tabarca Island are happy to be hand-fed pieces of bread.



Of course, not all were so friendly... this one tried to 'attack' the camera.

A well stocked table

The mediterranean isn't known for its diversity of life... you won't find the vast array of colorful fish that inhabit the reefs of the Red Sea, nor will you see the complex habitats of the North Atlantic kelp forests.  Much of the inshore western mediterranean is composed of vast lawns of sea grass interspersed with rocks and the occasional wreck, and supports a limited range of sea life.  The med has undergone several cycles of drying out, and the sea life within it represent the descendents of a relatively recent recolonization over the last 5 million years.

One advantage of this reduced diversity is that when you are in the med, rather than being distracted by a variety of fish, you can pay more attention to what they are actually doing.  One interesting behavior I noted was of clusters of seabream and wrasse feeding on the rocks... the smaller Rainbow Wrasse and Ornate Wrasse would hover around the larger seabream  and Ocellated Wrasse, letting them do the heavy lifting of tearing chunks of algae and encrustations off the rocks, before diving at the debris cloud to grab any interesting food morsels that were shaken free.

You can see some of this behavior in the first half of the clip below, where off to the right several small wrasse are diving through one such cloud of debris, from where a seabream has pulled away some of the encrustations.  Near the end of the clip is a different behavior - a watchful Painted Comber assessing my intentions before deciding that discretion is the better part of valour and fleeing into a nearby cranny.



For those interested in which fish is which, the two larger fish in the above photos are the White Seabream (oval with one tail spot) and Two-Banded Seabream (oval with two stripes), while the Ocellated Wrasse is the large wrasse with a small spot near the tail. There are two species of smaller wrasse in the photos and movies - the wrasse with a red stripe interrupted by a black bar is a male Mediterranean Rainbow Wrasse, while the female of the species has the dark upper body and light colored underside.  The orange wrasse with the light bands and black spot near its back is an Ornate Wrasse.

Symbiosis

Every dive is an adventure, and for a (former) biologist like myself, the sea is a wealth of interesting and unusual creatures, behaviors, and relationships.

While snorkeling near Tabarca Island in the med, I came across a mauve stinger jellyfish (Pelagia noctiluca) that was swimming in close association with a tiny fish.  If you watch through the movie clip (its a bit jerky, as I was both trying to keep the camera on the jellyfish and avoid getting too close) you'll see the little fish swimming around the bell of the jellyfish.




If you are having trouble seeing him (he was very quick) you can spot him in this still from the video, just to the left of the jellyfish. Throughout the whole encounter, he never strayed more than an inch or two from the bell of the jellyfish.

Notice something else about the jellyfish? Its missing its tentacles! (Compare with the picture on the wikipedia page). This starts to give a clue as to the relationship between these animals.

It took a bit of digging to find anything on these relationships, but I came across an old paper talking about similar symbiosis between young fish and an atlantic jellyfish species (Copeia, Vol. 1963, No. 1 (Mar. 30, 1963), pp. 40-80).  The paper describes how larval fish associate with jellyfish Chrysaora quinquecirrha when they are young, using the jellyfish first as protection, then as a ready food supply, nibbling pieces of their host until they are large enough to eat them entirely and live an independent life.  That paper also mentions an even earlier 1915 paper that describes how young scad seek shelter among jellyfish, using them initially for protection before consuming their gonads and tenticles as they get older.  The unwillingness of the larval fish to leave the proximity of the jellyfish and the lack of tentacles on its host suggest that what I filmed is one of these symbiotic relationships, and that it won't be much longer before this larval fish is ready to leave its host for an independent life.  

I would be curious if anyone else has filmed this relationship between a fish and a mauve stinger... there doesn't appear to be much in the scientific literature and given the age of the papers I found, I doubt anyone in the 60's (never mind 1915) was plunking a camera into the mediterranean to chase jellyfish.  So I guess what you are seeing here is a scientific first...  in my museum of curiosities.

Seabream

I have to wonder what was going through the mind of this saddled seabream as I came up behind it.

These fish were surprisingly calm around divers, though if you got too close they would move away.  I suppose there hasn't been much natural selection favouring a flight response from large, ungainly sea creatures streaming bubbles and making regular rasping noises, but more likely it just isn't worth the effort to flee from every large creature in the ocean... a truly timid pelagic fish would be a very exhausted fish and not one to contribute to the next generation.

A side view of one of these fish.  The latin name is Oblada melanura. (as an aside, I need to get some better identification guides for the mediterranean... I have one very old guide, and the pictures in it don't do the live fish justice.)

Shore diving

Diving the british coast can be an amazing experience, with a diversity of life unlike anything you'll find in the tropics.  You just have to be willing to brave cold waters, bad weather, and a visibility measured in inches instead of meters.

This weekend I was at St. Mary's Lighthouse, north of Newcastle.  This is how the tourists see St. Mary's:

and this is how I saw it:

Probably due to the shallowness of the dive site and the proximity of a sandy beach, there was not as much life as at places farther north up the coast, but there were still crabs, lobsters, turban snails, and sea anemones.  The high point of the dive was finding a small blenny curled in an S shape, hoping not to be noticed, then darting off when it realized it had an audience!  The low point (or sad point, really) was coming across a large lobster trapped in an abandoned lobster pot... if I had thought to bring a dive knife I could have cut it free, but as it was, I had to leave it behind.  There something about seeing these large, beautiful crustaceans when they are alive and in their native habitat that makes it seem like such waste to trap and eat them.

The visibility in the water was too poor for most of my attempts at photography, but I did manage a short film clip of a crab scuttling away from us.  It also gives you a bit of an idea of the visibility I had to deal with.  Getting lost was a constant risk, even with a compass,  and we had to surface several times just to figure out where we were!

A good day for butterflies

A recurring theme of mine is how few insects there are in the UK, compared not only with areas of similar climate in North America, but also with the UK of 100 or even 50 years ago.  A quick glance at old identification guides or museum collections show just how much diversity has been lost, and how the numbers of many species have crashed.  No surprise given the changing land use in this country... hedgerows, for example, have been lost at an alarming rate, despite their role as wildlife refuges in the otherwise monoculture environments of England.

This weekend was one of those rare days when I've seen a lot of insect life.  The Yorkshire Dales are one place you would think would be a good place for seeing butterflies and moths, given the diversity of habitats and the smaller scale of farming, but years of bad weather have, I suspect, knocked down the numbers of leps in this region.  Yet this weekend, on a nice warm day near Hawes, I saw more butterflies (and more numbers of butterflies) than at any other time in the years I've lived here.

On top of the more common Small White and Green-veined White, there were the less-seen Large Whites, Small Tortoiseshells, Peacock butterflies and even a Red Admiral, but the prize for the day was seeing a Fritillary, a rare sight at the best of times.  Unfortunately, I was too slow with my camera (and the butterfly itself was in no great shape) so I don't know what type it was, but if I had to guess I would say it was a Silver Washed Fritillary.  Hopefully when I can look up some range information I'll have a better idea.

Some of the butterflies I saw:

Small Tortoiseshell

Large White

Peacock

Red Admiral