Identification
Location
Habitat and behaviour
Photography techniques
Focussing techniques
Strobe techniques
Camera and strobe settings

Identification:
Identifying the Thor Amboinensis shrimp, otherwise known as the squat shrimp, is very easy. It is up to 2cm (0.5 inch.) in length and have very distinct coloration. Its body is light brown with clearly defined white patches.
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Location:
Finding the Thor shrimp is also not that difficult. Distribution of the Thor shrimp is widespread and it can be found in all tropical seas. They have been found in the Indo-Pacific: E-Africa to the Central Pacific and also in the Atlantic.
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Habitat and behaviour:
The Thor shrimp lives commensal on corals and various anemones e.g. Heteractis. Commensalism is defined as the relationship between members of different species in which one member benefits while the other is unaffected.

If you look closely at the base of the host anemones you will find Thor shrimps usually in pairs. The Thor shrimps display sexual dimorphism meaning that there are structural or size differences between the species. In the case of the Thor shrimp females are almost twice the size of the males.

It is not unuasual to find groups of six or more individuals alongside other partner shrimps of the Periclimenes sp. When the Thor shrimp is excited or agitated it stretches its tail up and waves it about. At times the abdomen is held up almost vertically.

Thor amboinensis is not normally classsified as a cleaner shrimp. However, in his book Crustacea - Guide of the World (p289), Helmut Debelius has a photograph showing Thor Amboinensis possibly supplying cleaning services to the mantis shrimp of the Lysiosquillina species. As these mantis shrimps are quite capable of reaching all parts of their bodies for cleaning purposes you would have thought there would be slim pickings for the Thor shrimp.

It is thought that the shrimp may be attracted by the mucus that the mantis uses to line its burrow or by the non-digestible food that the mantis shrimp regurgitates after it has eaten.

The question still remains, what is the advantage for the mantis shrimp and why does it not just eat the Thor shrimp? A question that still remains to be answered. Watch this space!
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Photography techniques:
As it is rare to find these squat shrimp too far from their host anemone the use of a housed camera with a macro lens is almost essential. The small size of the crustaceans requiring a lens capable of reproducing life size images on film.

I have had some success when using the Nexus Wet Lens adaptor on the front of my Subal 105mm flat port. This diopter increase the magnification factor by approximately a further one third. The advantage of this diopter is that it can be fitted and removed underwater unlike the use of extension tubes or a diopter that is fitted to the front of the lens itself, which must remain in position for the duration of the dive.
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Focusing techniques:
The depth of field (range of acceptable focus in front and behind the subject) is extremely shallow when when using the 60mm or 105mm macro lens. It is even shallower when using an internal or external diopter.

As the entire subject is unlikely to be in focus due to the shallow depth of field it is critical that you focus on the most important part of the subject which in this case will be the eye of the Thor shrimp. By adopting a camera position in which the film plane is parallel to the body of the Thor shrimp it is usually possible to produce an image in which most of the shrimp is in focus.
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Strobe techniques:
Ensure that you position your strobe as close as possible to the subject in order to be able to use as small an aperture (large f number) as possible thereby increasing the depth of field to the maximum amount.

With the subject filling the viewfinder, setting the strobe and camera to TTL will give perfectly acceptable exposures. Be careful not to use too large an aperture (small f number) as some strobes will not be able too quench the strobe ouput sufficiently quick enough to prevent over-exposure.

In most cases it will be necessary to position the strobe right next to the lens in order to prevent shadows from being cast by the anemone or its tentacles onto the subject. This is not normally an ideal position for a strobe in murky water as it creates a lot of backscatter but as we are so close to the subject the water column will be very small between the strobe and the subject, backscatter will be limited.

Camera and strobe settings:
The images below were all taken with my Subal housed Nikon F90x camera with 105mm F2.8D lens with the AF/MF 105mm flat port. The strobe used was the Subtronic Alpha Pro Macro on an Ultralight arm.

Film used was Fuji Velvia 50ASA with the camera film speed selector set to 40ASA.

Camera and strobe settings were as follows:

Lens aperture f22-f45
Len focus distance set to minimum
Shutter speed 1/125th
Strobe set to TTL
Strobe to subject distance set to 6 inches
Film speed 40ASA

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In this shot taken in Grand Turk, a group of Thor Amboinensis were living in and among the tentacles of a tube anemone. - CD48

Thor Amboinensis is more commonly found living in association with anemones of the Heteractis species like the one in this shot taken in Lembeh Straits, North Sulawesi. - CG9

It seems likely that although Thor Amboinensis is not known to be a cleaner shrimp, it may provide cleaning services to the Spearer Mantis Shrimp Lysiosquillina sp. - DD32

In order to obtain a further level of magnification beyond the life size produced by the 105mm lens I have fitted a Nexus Wet Lens to the front of the port in order to increase the magnification by approx one third more. - DE38