the offshore current flowing counter to the 

 shore current flows at about 11 miles per day. 

 The northern eddy is by far the most important 

 to both the present and future distribution of 

 the sponges to the deeper water areas of the 

 Gulf. 



Several other features of the current pattern 

 over the sponging grounds are important for 

 sponge distribution. In the area just west and 

 north of Indian Rocks, 15 miles north of Tampa 

 Bay, the set of the current is generally shore- 

 wards. Lack of commercial sponges in any 

 area upcurrent from Indian Rocks and the con- 

 stant shoreward set of the current appear to be 

 the primary causes in limiting the occurrence 

 of commercial sponges to the Indian Rocks 

 area and downcurrent from this area. In this 

 case the movement of the permanent current 

 is possibly preventing the spread of commer- 

 cial sponges to a large area just north of Tampa 

 Bay which is now supporting a very healthy 

 population of noncommercial species and which 

 appears to have all the essential ecological 

 characteristics necessary for the growth of 

 commercial types of sponges. 



Lack of active circulation of the water by the 

 semipermanent currents in the area just north 

 of St. Martins Reef Light and Anclote Key may 

 be responsible for the present scarcity of 

 sponges in the shallow water of this area. The 

 few sponges found here on the field trip were 

 aU more or less the same size, indicating that 

 larvae had been brought into the area during one 

 or two short periods when the current had 

 swung towards shore. No sponges of less than 

 legal size were seen in the 10- to 2 5- foot 

 depths north of St. Martins Reef Light. 



Calculated Rate of Dispersion 



The only way an area can become repopulated 

 naturally by commercially valuable sponges is 

 by the drifting of their planktonic larvae into 

 the area. These larval stages last several days 

 at most (Duboscq and Tuzet, 1937), which limits 

 the distance sponges are dispersed. No work 

 has been done to date with life span of com- 

 mercial sponge larvae. To calculate the rate 

 of dispersion of the sponges, I have made an 

 arbitrary estimate that the larval stage lasts 

 a maximum of 4 days. 



If it requires 4 years for a sponge to mature 

 in the upper Gulf, the annual dispersion rate is 

 approximately equal to the current flow per day 

 during the reproductive season. 



STRUCTURE OF THE OCEAN 

 BOTTOM IN SPONGING AREAS 



Bottom Slope and Sediment Zones 



The composition of the bottom sediments 

 within the sponging area of the Gulf south of 

 Cedar Keys and the slope of the bottom give 

 considerable information with regard to the 

 general type of sponge habitat. Fortunately a 

 detailed report of the area has been made 

 (Society of Economic Paleontologists and Min- 

 eralogists, 1955), and most of the following 

 description is based on this report, with 

 specific details added from personal observa- 

 tion. 



The bottom west of Tarpon Springs is fairly 

 representative of the entire northern sponging 

 area from Tampa Bay to Carrabelle. The aver- 

 age slope is 2.6 feet per mile. This slope is 

 maintained to the 180-foot depth and comprises 

 the inner shelf. 



From the shore to the deeper water, the un- 

 consolidated sediments can be divided into a 

 series of zones roughly parallel to the coast. 

 In the inshore zone , about 20 miles in width, 

 the sediments are relatively uniform in struc- 

 ture, composed chiefly of quartz sand, with 

 phosphorite grains abundant locally. The sedi- 

 ment also contains shell, coral fragments, 

 calcareous algae, and sponge spicules. The 

 quartz particles contribute more than 50 per- 

 cent to the makeup of these unconsolidated 

 sediments and are for the most part fine to 

 medium in texture, similar to the material 

 brought down by the rivers or found along the 

 shore. 



Many local concentrations of medium and 

 coarse quartz and phosphorite sands occur 

 within the shore zone, often mixed with coarse 

 sandy limestone fragments and foraminiferal 

 tests, primarily those of Archaias angulatus. 

 The sandy limestone and the fossil foramini- 

 feral tests probably originated from the out- 

 croppings of Pleistocene limestone exposed on 



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