460 LAUOHTON [chap. 18 



in the shallow coastal waters where so much work has already been done. 

 However, from photographs alone it is extremely difficult to identify the fauna, 

 and detailed work must depend on the collection of samples. 



D. Disturbance of the Sediment-Water Interface by the Benthos 



Most of the benthic fauna leave some marks on the soft sediments of the 

 bottom where they move from place to jjlace, when they search for food or 

 when they seek seclusion or cover. The tracks, mounds and burrows that are 

 produced are often preserved in the geological strata as the surface becomes 

 biu-ied by subsequent sedimentation. Furthermore, the sequence of layering of 

 sediment in chronological order is disturbed over the range of burrowing 

 activity, and the fine detail of stratification is often blurred. An extensive study 

 of the fossilized traces of animal activity and their palaeobiological significance 

 was made by Lessertisseur (1955) from the examination of sedimentary rocks. 

 Many of the problems discussed by Lessertisseur can be illuminated by the 

 study of contemporary animal traces on free sediment surfaces, and in some 

 cases it is possible to observe the animals responsible for them. 



Lessertisseur has classified these traces into exogene traces, comprising trails 

 and tracks made by an animal on the surface (epifauna), and endogene traces, 

 comprising the galleries and burrows of animals below the surface (infauna). 

 This classification cannot always be rigorously adhered to since an exogene 

 trace may suddenly be transformed into an endogene trace if a worm on the 

 surface turns downward, and, furthermore, burrowing just beneath the surface 

 may make an appreciable disturbance of the surface itself. When a burrow is 

 formed, the sediment removed from below often accumulates on the surface as 

 a mound, or else an empty burrow may subside and become filled with surface 

 material leaving a depression. 



The exogene traces can further be classified according to the nature of the 

 operation which made them. Locomotion tracks are usually long and narrow, 

 or in the form of a series of depressions if made by feet. Feeding tracks are 

 characterized by spiral, radial or helminthoid patterns. (A helminthoid pattern 

 consists of a long continuous track regularly folded back on itself with alternate 

 left and right turns so that it efficiently covers a well defined area.) Resting 

 marks are roughly equidimensional depressions. Similarly, the endogene traces 

 can be divided into those of locomotion, feeding, dwelling and rest with some- 

 what similar characteristics. 



Sea-floor photography can only observe the exogene traces (Emery, 1953) 

 although there is sometimes evidence of further subsurface activity from the 

 abrupt termination of tracks. The disturbances can be roughly grouped into 

 foiu' categories although they are often interrelated. Most prominent are 

 the momids which are found almost everywhere on the sea fioor. They are 

 roughly conical and vary in size from a few millimetres to 50 cm. Any suggestion 

 that they represent a sediment covering over erratic rocks must be dismissed 

 since dredging in the sediment basins has never produced such rocks. Some of 

 the mounds appear to be due to a protective covering of sediment built over an 



