124 LOWER COASTAL ANIMALS 



tion known as mangrove, and here we frequently made interesting obser- 

 vations. Quantities of crabs inhabit the extensive mud-flats between the 

 aerial roots of mangrove trees. The fiddler-crabs (Uca) menacingly raise 

 their single, gigantic claw, closing up their hole with it if forced to retreat 

 into the mud. Many kinds of snail draw their trails across the surface. 

 The fan mussel (Pinna), anything up to half a metre in length, lies 

 buried, its paper-thin shell flush with the top of the mud, giving a nasty 

 cut to the foot that treads on it. 



We visited a similar, though tree-less, locality a little south of Sydney. 

 Here we found a dense growth of grasswrack and various algae. Large 

 bivalves {Area among others) lay scattered over the flats, where big 

 snails of the previously mentioned Cerithium type were crawling. There 

 were also sea hares (Aplysia, Dolabella), which are large snails with a 

 very small, almost internal shell. The easiest way of finding these was to 

 tread on them. They are rather soft and slimy and it was like putting 

 one's foot down on a firm, wet sponge. A purplish pigmentation, secreted 

 by special glands, would then appear in the water. A closer inspection 

 would reveal the snail, which might weigh as much as two kilograms. We 

 carefully froze a few specimens and succeeded by this means in preser- 

 ving one of the biggest for display in the Zoological Museum. 



On another occasion — near Auckland - — we had the good fortune 

 to be invited to join an expedition to gather scallops (Pecten medius), 

 a great delicacy in New Zealand. It was an extremeh' interesting excur- 

 sion on an extensive flat, though there was not enough time to get far 

 while the tide was still out. We started as soon as the bottom began to 

 appear, making the best of the half hour at our disposal before it was 

 again covered. The scallops were half buried and easy to see in the flat 

 mud, where the covering of grasswrack was too open to conceal them 

 (Fig. p. 125). 



Often in rock pools, where there is shallow water at low tide, or on 

 the lowermost reach of the tidal zone itself when there is a specially low 

 tide, it is possible to get a glimpse of the immensely rich fauna which 

 characterizes the next marine zone, the shallow-water zone with a depth 

 down to 10 — 30 metres. Desiccation plays no part here at all. Here there 

 is light and here is produced the food on which, ultimately, all marine 

 animals live; here the temperature is warmest, and here, finally, there is 

 often sufficient movement in the water to enable many animals to sit 

 and eat their fill of small marine organisms. Nevertheless, there are certain 

 variations in this zone, due principally to the nature of the bottom. We 

 can best illustrate this with an example taken from the tidal zone. Peri- 



