Sometimes it takes a little time before I get around to writing about something I’ve read elsewhere.

A couple of weeks ago PZed wrote a very interesting article describing the moulting of crustaceans.Most people are aware that crustaceans moult. As they grow, their shells become too small for them and they have to leave their old shell and develop a new larger shell. Amongst the various problems associated with the moulting process is that of orientation awareness. Some crustaceans such as shrimp – have their balance mechanisms (statocysts) external to their bodies. Lose the shell, lose the Statocyst

A little about balance…
If we are inside a sealed box – say a ship at sea or a rocket in space – we cannot detect uniform motion. We cannot detect when we are moving with a constant velocity (or if our constant velocity is zero). We (vertebrates) have motion detectors in our ears, but these are only capable of detecting acceleration, and not constant velocity. We have two sets of accelerometers. The first is what people usually associate with balance – the semi-circular canals. If you have a bowl of water and rotate it, the bowl wil turn, but – through inertia – the fluid will lag behind, taking a moment or two for the motion of the fluid to catch up. The SCC are fluid filled tubes and as we move the fluid lags behind our movement. The relative current deflects clusters of hair cells lining pockets in the canals, and these deflections trigger nerve impulses to the brain. The SCC are angular acceleration detectors. they can only detect rotational accelertion about a mid-line.
The second set – the Utricle and the Saccule (aka the otolithic organ) – is the detector of linear acceleration and as we are always (unless in far off in space) accelerating downwards at 9.8ms-2, these tell us where local ‘down’ is located, as well as detecting if we are accelerating forwards / backwards and up / down. These consist of a lump of a jelly-like substance embedded on hair cells. Within the jelly are lots of little particles of calcium, and as we accelerate the particles lag behind, bending the hair-cells

Homologous to the Utricle and Saccule are the Statocycts of the Crustacea. These contain similar jelly-like masses embedded on hair cells. The difference is that these are outside the body, and when the crustacean moults it loses its statocysts along with the rest of the shell. The crustacean cannot produce the otoliths which need to be present to deflect the hair cells, so instead uses sand particles which are introduced into the statocysts through small apertures.

In an early experiment into physiology (1893), Kriedl substituted iron filings for the sand in the bottom of an aquarium, just prior to the crustaceans’ moulting. The crustaceans then introduced the iron filings into their new statocysts. Under normal conditions these functioned exactly the same as the sand particles and fell under gravity, indicating for the crustacean where “down” was. However, Kriedl could alter the orientation of “down” by the simple act of brining a magnet along side the aquarium. The iron filings were pulled towards the magnet and the crustacean interpreted this as being due to gravity, and they altered their orientation within the tank accordingly. When the magnet was removed, the iron filings again responded to gravity and the shrimp adjusted its position.