some cases to be distinguished from one another 

 as separate faunistic regions . For example, 

 between the deep, cool, saline water of the lower 

 estuary and shallow, warmer (in summer), 

 fresher water of the upper, the physical differ- 

 ences are very noticeably reflected in the faunas . 

 The number of species is far greater in the 

 lower estuary, even when an area of a size com- 

 parable to that of the upper estuary is considered. 

 The lower estuary contains populations of such 

 stenohaline creatures as lobsters, rock crabs, 

 sea scallops, sponges and starfish which are 

 absent in the upper. Besides a few euryhaUne 

 species (e.g. soft -shell clams or mud shrimp) 

 the upper esmary contains numerous organisms 

 which are limited to it by their requirements of 

 higher temperatures or less saline water. The 

 temperature differential, which may be 5°C or 

 more in the summer, particularly through limit- 

 ations on larval survival, spawning activity or 

 gametogenesis, is an effective barrier to the 

 spread of certain species. These species may 

 be called "relict", that is, they appear to be 

 remnants of a more universal distribution at 

 some past time when the coastal waters were 

 warmer than at present . Such relict populations 

 are common on the strongly dissected coast of 

 Maine . Those of Casco Bay, for instance, have 

 been discussed by Verrill (1873). 



Marine populations along the coast of 

 Maine belong to a zoogeographical sub-unit to 

 which the term "Acadian" is sometimes applied. 

 Those farther south are sometimes called 

 "Virginian" . Three species in the Sheepscot 

 estuary which are more typically Virginian than 

 Acadian are the American oyster ( Grassostrea 

 virginica) and the two mud crabs Neopanope 

 texana and Rhithropanopeus harrisi . These are 

 limited to the upper estuary. Another species 

 which is typically more southern in its distribu- 

 tion is the horseshoe crab ( Limulus polyphemus ), 

 and this species is most common in the upper 

 estuary. Its ecological requirements, however, 

 seem to be less restrictive than those of the 

 other three species mentioned, and its distribu- 

 tion along the coast of Maine in general is more 

 widespread. 



Oysters are less abundant now than form- 

 erly in the Sheepscot. During the early part of 

 this century, a dam was maintained across the 

 narrows at Sheepscot Falls. This partially 



impounded the tidal water above it, and excellent 

 sets of oysters are said to have occurred there 

 each year. The maintenance of the oyster popu- 

 lation is dependent, among other things, on 

 favorable temperatures for spawning and larval 

 survival . With the high tidal exchange ratio in 

 the upper estuary, more than half of the oyster 

 larvae in the water of the upper estuary during 

 the period of a tidal cycle would be flushed out, 

 their surrounding water mixed with the colder 

 water of the lower estuary, and a consequent 

 loss of large numbers of them would take place 

 with each tide, either by simple dilution or 

 through mortality due to unfavorable temperature . 

 This procedure, taking place day after day would 

 tax the reproductive potential of the oyster popu- 

 lation even though conditions around the beds 

 were suitable for gametogenesis and spawning. 

 When the dam was in place, the tidal exchange 

 was considerably reduced and more larvae were 

 able to remain in favorable water long enough to 

 reach setting size . 



At present, small populations of mostly 

 older oysters are all that remain . The largest 

 beds are located in the inner extremity of Marsh 

 River. Setting of young occurs but seldom (Tax- 

 iarchis, Dow, and Baird, 1954) although in unusu- 

 ally warm years a small number of larvae appar- 

 ently survive in numbers sufficient to produce a 

 small set. Such an event took place in 1955, for 

 example . 



The foregoing discussion refers primar- 

 ily to temperature barriers. A few species are 

 quite definitely limited to the upper esmary by a 

 salinity barrier. One of the most interesting of 

 these is the hydroid Cordylophora lacustris , the 

 salinity tolerance of which is a very narrow 

 range near the fresh water end of the scale. 

 This hydroid is known from coastal estuaries 

 and also from inland rivers. The amphipod 

 Gammarus tigrinus occupies only the portion of 

 the Sheepscot esmary from the lower limit of 

 continuous fresh water to the level of Station 13. 



Other species are more characteristic of 

 the upper estuary than the lower, although by no 

 means confined to it. The blood worm, Glycera 

 dibranchiata, is an example. 



The distribution of organisms is divided 

 on another plane by the depth of water and the 



11 



