An Introduction to the Zoogeography of the Northwestern Gulf of Mexico with Reference to the Invertebrate Fauna

Hedgpeth, Joel

The estuarine and neritic waters of the northern Gulf of Mexico, especially along the coasts of Texas and Louisiana, are characterized by broad ranges of environmental factors providing conditions favorable to temperate organisms in winter and tropical organisms in summer. The fauna is a mixture of temperate Atlantic and tropical Caribbean elements, with a very low endemic component. The distribution of many of the Atlantic species occurring in the northern Gulf is characteristically disjunct. they being absent from southern Florida or represented there by stunted individuals or reduced populations. Many of these species were apparently continuous in distribution across northern Florida during high stands of the sea is late interglacial periods of the Pleistocene. The small number of endemic species-about 10% in most invertebrate groups may be a reflection of the environmental extremes which enable the development of large populations of wide-ranging adaptable species at the expense of the development of indigenous forms. Similar conditions apparently prevailed well back intothe Tertiary, and the evidence of the Cretaceous suggests that the region now bounded by the Gulf Coastal Plain has been in an area of transition between northern and southern environments since that time. The pronounced range of environmental factors has many effects on the communities of the region, and fluctuations in populations are pronounced., Populations in bay waters in particular undergo wide variations as a result of killing cold, high salinities during droughts, and excess drainage of fresh water during floods. Both droughts and killing cold spells llIay be increasing in frequency, especially in south Texas. These waters are nevertheless productive. Extensive oyster reefs occur in the bays, and the shallow bottom of the neritic Gulf is inhabited by large colonies of sea pansies and sea whips, sand.loving echinoderms and various bivalves. One of the largest invertebrate populations is that of the various species of penaeid shrimp. This population is one of the key-if not the key-components in a complex major ecosystem which includes the marine and estuarine environments.

An Introduction to the Zoogeography of the Northwestern Gulf of Mexico with Reference to the Invertebrate Fauna

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