The river-influenced assemblage contains a small group of common bay species, including the 
bivalve Mulinia lateralis, the polychaetes Capitella capita ta, Streblospio benedictiand Mediomas- 
tus spp., and brackish-water mollusks such as Macoma mitchelli, Texadina sphinctostoma and 
Rangia flexuosa. These species occur in parts of estuaries where salinities vary from fresh to brackish 
over long periods of time. Average salinities in Trinity Bay range from less than 5 ppt to about 10 ppt 
(15). However, over relatively short periods of time, the river-influenced assemblage is subjected to 
greater natural salinity fluctuations (0-33 ppt) than are other bay assemblages. 
In contrast to the river-influenced assemblage, the inlet-influenced assemblage contains the 
highest number of species, partly because of more stable salinities. This assemblage, composed 
primarily of mollusks, contains some species that are restricted to the area of Galveston and East Bays 
near Bolivar Roads and Rollover Pass and to West Bay near San Luis Pass. Common species include 
mollusks such as Mulinia lateralis, Lyonsia hyalina, Mysella planulata,Turbonilla sp., Acteocina 
canaliculata and Nassarius acutus and polychaetes such as Owenia fusiformis, Paraprionospio 
pinnata, Clymenella torquata and Mediomastus californiensis. 
The oyster reef assemblage is found primarily on or near reefs and is dominated by the American 
oyster Crassostrea virginica and the mollusks Ischadium recurvum, Brachidontes exustus and 
Mulinia lateralis. The common polychaetes Mediomastus californiensis and Streblospio 
benedicti are also abundant. 
The bay margin assemblage is limited to shallow, sandy stations in East and West Bays and lower 
Galveston Bay. Most stations are less than 2 km from shore and less than 1 meter deep. Crustaceans 
such as Ampelisca spp., Cerapus tubularis and Oxyurostylis salinoi are more abundant in the bay 
margin assemblage than in any other assemblage except the grassflat assemblage. 
Crustaceans are dominant in the grassflat assemblage and include such species as Ampelisca 
abdita, Acanthohaustorius sp. and Cymadusa compta. Bivalves such as Amygdalum papyrium, 
Lyonsia hyalina and Laevicardium mortoni and polychaetes such as Aricidea fragilis and Scol- 
oplos fragilis are common. Grassflats are of limited distribution in the Galveston Bay system and 
occur principally in patches along the margin of the Trinity River delta and Christmas Bay. 
The open bay center assemblage occurs in lower Galveston Bay and East and West Bays in muddy 
sediments and in relatively deep water. Polychaetes are the predominant group and are character¬ 
ized by Paraprionospio pinnata, Parandalia fauveli and Podarkeopsis levifuscina. 
A 12-month study of the benthos of Trinity Bay (1) indicated that polychaetes were the most 
speciose group collected (35 species), followed by crustaceans (18 species), mollusks (14 species), and 
bryozoans, rhynchocoels and chordates (5 species). Seventy-four percent of all individuals collected 
were polychaetes, primarily Mediomastus californiensis and other capitellids. Other abundant 
species were the mollusks Macoma sp., Amnicola sp. and Texadina sphinctostoma. Densities of 
benthic organisms exhibited spring and late summer peaks. 
Macroinvertebrates—These mobile and sessile species are rarely encountered using the plank¬ 
ton or benthic sampling methods involved in prior sections except as larval or early juvenile forms. 
No synoptic surveys of macroinvertebrates in the Galveston Bay system (other than oysters, 
Crassostrea virginica) have been conducted. The public oyster reefs within the estuary have been 
described (19, 20). The reefs are typically long and narrow, are oriented perpendicular to water 
currents, and are densest in the mid-bay region and across the mouth of East Bay. Settlement of spat 
(free-swimming larvae) generally occurs during April to November, primarily in the summer 
months. Oysters reach market size in 13 to 18 months. The distribution of oyster reefs depends on the 
interactions of temperature, salinity, predation and disease (19). High salinities allow an increased 
predation by oyster drills (Thais haemastoma) and increased infection by Perkinsus marinus 
("dermo"). Extensive periods of low salinity can also kill oysters, so most of the viable reefs are 
located in areas characterized by 10 to 20 ppt mean annual salinity. Since 1975, the areal distribution 
of oyster reefs has been stable. 
Although not well documented, there are numerous species of mobile macroinvertebrates in the 
estuary (13,21-24) (Table 2.5). All of these species were collected in western West Bay (but are found 
elsewhere) and many of these species are probably limited to submerged vegetation or oyster reef 
habitats, rarely caught elsewhere. In shallow, fringing habitats Palaemonetes spp. (grass shrimp) are 
most common and reach maximum abundance in March through July. Macrobrachium ohione 
29 
