CHAPTER 3: THE BENTHIC INVERTEBRATES 



3.1 BENTHIC EPIFAUNA 



Benthic animals are those which live in close 

 association with the bottom. This catagory in- 

 cludes animals which live in more or less fixed 

 positions on top of the substrate (epifauna), those 

 that live buried in the substrate (infauna), and 

 those which move readily about the bottom for- 

 aging both on and in the sediments (mobile epi- 

 benthos). Marine epifauna and infauna are almost 

 solely composed of invertebrate species, whereas 

 both invertebrates and vertebrates are commonly 

 represented among the mobile epibenthos. 



On most sand and mud flats the epifauna is 

 apparent, but sparsely distributed (Moore et al. 

 1968). In the intertidal flats of Europe and along 

 the Pacific coast of North America, the blue mus- 

 sel, Mytilus eduHs, is the most apparent epifauna! 

 species. Along the southeast Atlantic and Gulf 

 coasts of North America, the oyster, Crassostrea 

 virginica, is the most obvious epifaunal inverte- 

 brate. In North Carolina, oysters are often found 

 in a dense band in the mid to high intertidal zone 

 at the lower edge of the salt marsh (Figure 1). Be- 

 low this band of oysters is usually an open mud or 

 sand flat which may contain more oysters ("an 

 oyster reef") at a lower level oi the shoreline. In 

 North Carolina, however, most of these oyster 

 reefs tend to be subtidal. 



Oyster beds contain numerous plants attached 

 to the shell surfaces, and a whole community of 

 associated animals. In North Carolina, the inter- 

 tidal oyster shells are colonized by various marine 

 algae and epifaunal invertebrates characteristic of 

 intertidal hard substrates. Tunicates (such as 

 Styela), branching bryozoans (Bugula), hydroids 

 (Pennaria), barnacles [Balanus eburneus and B. 

 amphitrite), encrusting bryozoans (Schizoporella), 

 sponges {Hymeniacidon, Cliona), soft coral 

 (Leptogorgia), and small gastropods (like Bittium) 

 are common epifaunal elements associated with 

 oyster beds. 



Oysters feed upon suspended algae in the 

 water column. Haines's (1976a) data demonstrate 



this trophic link, as do numerous examinations of 

 gut contents of adult oysters (Nelson 1921). The 

 most important single source of energy is prob- 

 ably phytoplankton, but resuspcnded benthic 

 diatoms and other microalgae may contribute a 

 significant proportion. No study has successfully 

 separated the planktonic and benthic components 

 of the oyster's diet. 



Occasional empty shells from dead clams or snails 

 provide hard substrate for attachment by the local 

 array of hard-surface epifauna. Except for these 

 ephemeral substrates, there is no basis for firm 

 attachment in a soft-sediment environment. .\s a 

 result, the epifauna, other than oysters and mus- 

 sels, are not important components of intertidal 

 flat habitats. Oysters and mussels succeed in this 

 environment, in part, by forming heavy clumps 

 which are more stable than isolated individuals. A 

 few other epifaunal species can be found in 

 depressions on mud and sand flats where they 

 have been carried by the currents. For instance, 

 Styela plicata is often found in such depressions 

 on the intertidal sand flats at Beaufort, North 

 Carolina (Williams and Thomas 1967). Neverthe- 

 less, the oyster is the most common epifaunal 

 resident of this habitat and certainly the most im- 

 portant commercially. 



On relatively high-energy sand flats in North 

 Carolina and elsewhere, various species of sea 

 pansies (Renilla) are often abundant. These ani- 

 mals are common on sand flats at the mouths of 

 inlets in North Carolina. They are colonial coelen- 

 terates which form a purple disk about the size of 

 a silver dollar when fully developed. Sea pansies 

 are not really epifaunal in that, although the disk 

 is visible at the sediment surface, an appendage 

 called the peduncle extends vertically into the 

 sediments and anchors the colony in the sand. 

 Some authors have called sea pansies semi-infaunal 

 because of this. These colonial coelenterates are 

 suspension feeders. They are most numerous on 

 the ocean floor just outside the surf zone, but 

 they are also typical inhabitants of the high-energy 

 sand flats of sounds and lagoons. 



19 



