Variability In space and time. Species data were 

 organized according to thie salinity zone boundaries 

 developed for each estuary in the NEI data atlas- 

 Volume 1 (NOAA 1985). However, division of an 

 estuary on the basis of salinity is highly variable due to 

 the many interacting factors that affect salinity, such as 

 variations in freshwater inflow, wind, and tides. To 

 compile information on species distribution according 

 to these zones, it is assumed that if a particular salinity 

 zone expands or contracts, the distribution of a mobile 

 species in that zone will correspond to the shift. For 

 example, if increased freshwater inflow shifts the tidal 

 fresh zone further down the estuary, the distribution of 

 a species confined to that zone increases to include the 

 new area. If a species exhibits a wide range of salinity 

 tolerance, a shift may or may not occur. The placement 

 of species in a salinity zone was ultimately determined 

 by where they have been observed or captured. 



Species temporal distributions are often dependent on 

 annualclimatic conditions andwatercurrents. Monthly 

 distributional patterns were derived based on the con- 

 sistent presence of a life stage within a particular 

 month. If a species is only present in an estuary in 

 unusual years (e.g., drought), this was not portrayed as 

 part of that species' spatial or temporal distriljution. 

 However, if a species usually occurs, even during a 

 restricted time period, it was considered present for the 

 specific month(s). Greater temporal resolution, such 

 as on a biweekly rather than on a ninthly basis, was 

 not possible. 



below 25 ppt. Not comrrxjn in the south Florida and 

 south Texas estuaries, which have relatively high sa- 

 linities. 



• Hard clam: Also known as quahog (Turgeon et al. 

 1988). Most life stagesoccur in salinities atxjve 20 ppt. 

 Two species occur in the Gulf of Mexico, and hybrid- 

 ization may occur. The northern quahog (Mercenaria 

 mercenaria) is generally found in intertidal and subtidal 

 waters to 15 m, and the southern quahog {Mercenaria 

 campechiensis) in deeper, more saline waters. The 

 two species are considered together in this report 

 because most fisheries data do not distinguish be- 

 tween them. 



• Bay squid: Also known as Atlantic brief squid (Turgeon 

 et al. 1988). The lower lethal salinity limit is approxi- 

 mately 17 ppt, and bay squid actively avoid salinities 

 that are lower than this. Therefore, the distribution of 

 juveniles and adults will only be from the lower mixing 

 zone to the seawater zone, and out to the nearshore 

 waters of the Gulf of Mexico. 



• Penaeid shrimp: Postlarvae and juveniles are the 

 main life stages utilizing the estuaries. Adultsgenerally 

 move to nearshore spawning grounds, where spawn- 

 ing, egg development, and most of the larval develop- 

 ment occur. Brown and white shrimp are generally 

 more abundant in the central and western Gulf of 

 Mexico, whereas pink shrimp are generally more abun- 

 dant in the eastern Gulf of Mexico. 



Life history notes. Because of the complex life histo- 

 ries of some species, the following comments are 

 provided below to clarify and supplement information 

 presented in the data summary tables. 



Invertebrates. Sessile invertebrates, such as clams 

 and oysters, usually have a patchy rather than a 

 uniform distribution. Therefore, the ELMR framewoii< 

 may overestimate the areal distribution of these organ- 

 isms, but identify the salinity zones of colonization. 

 Specific areas may contain acceptable salinity re- 

 gimes, but suitable bottom habitat for colonization may 

 not exist. Specific habitat requirements and life history 

 characteristics of a number of invertebrate species are 

 provided below: 



• Grass shrimp: Also known as daggerblade grass 

 shrimp (Williams et al. 1989). Most abundant in veg- 

 etated or oyster reef habitat. Fertilized eggs are held 

 on the female's pleopods until hatching. In higher 

 salinities, Palaemonetes pugio is often replaced by 

 brackish grass shrimp (P. intermedius) and/or marsh 

 grass shrimp (P. vulgaris). 



• Spiny lobster: Also known as Caribbean spiny 

 lobster (Williams et al. 1989). Found in the Gulf of 

 Mexico estuaries of southern Florida and southern 

 Texas. Juveniles do not mature into adults until 6-8 

 years of age. Life stages considered in this report are 

 adults, mating (instead of spawning), juveniles, larvae, 

 and eggs. 



• Bay scallop: Usually associated with seagrass beds 

 and salinities greater than 25 ppt. 



• American oyster: Also known as eastern oyster 

 (Turgeon et al. 1988). Prefers hard substrate in inter- 

 tidal and subtidal estuarine waters. 



• Common rangia: Also known as Atlantic rangia 

 (Turgeon et al. 1988). All life stages occur in salinities 



• Blue crab: Mating usually takes place in the low 

 salinities of the tidal fresh to the upper region of the 

 mixing zone. After mating, females move to the seawa- 

 ter zone, while males often remain in the upper reaches 

 of the estuary. Females brood the eggs (sponge 

 females), and larvae are released in higher salinities. 

 Development through the late zoeal stages occurs 

 offshore. Megalopae are transported back into the 

 estuary and disperse throughout the salinity zones. As 



