Low Water Of Spring Tides Research Articles

Observations on the biology of Donax trunculus and D. vittatus at Ile d'Ol�ron (French Atlantic coast)

Observations have been made on the biology of two species of the bivalve genus Donax, on two beaches, Vertbois and St. Trojan, on the Ile d'Oleron, French Atlantic coast. The two species displayed different vertical distribution: D. trunculus Linne in the intertidal; and D. vittatus (da Costa) between low-water spring tide (LWST) and 5 to 6m depth, with only the fringe of the population extending to the shore. D. trunculus also showed differential distribution by size or age, the youngest individuals being highest on the shore, and the oldest near LWST. The two species exhibited a similar pattern of seasonal recruitment and growth, but in both there were differences in growth characteristics between the two beaches, individuals at St. Trojan growing to a larger size than those at Vertbois. The mean pattern of mortality in the two areas was similar, and productivity per 100 recruits surviving to the first winter following settlement was greater at St. Trojan than at Vertbois. For D. trunculus the mean population density at Vertbois was higher and the production per unit area on the two beaches was similar. Production to biomass (B) and elimination to biomass (B) ratios were similar for the two beaches for both species, and while production was greater for D. trunculus at both beaches, the B and B ratios were higher for D. vittatus. Comparisons with other published data reveal that the rate of growth of D. trunculus is similar throughout its range from Brittany to the Mediterranean Sea, while that of D. vittatus shows more variation. In both species, there is a trend of increasing P/B and E/B ratios from north to south throughout the geographical range.

The effect of intertidal exposure on the growth and survival of hatchery-reared Pacific oysters ( Crassostrea gigas Thunberg) kept in trays during their first ongrowing season

In a number of trials during 1971 and 1972 small populations of hatchery-reared Pacific oyster spat ( Crassostrea gigas Thunberg) 0.07–4.1 g mean live weight, were grown intertidally in trays to measure the effect of exposure to air on growth and survival. In the Menai Straits, N. Wales, and at Paglesham Pool, R. Roach, Essex, several stations were selected above low water of spring tides (LWST) to cover the range of 0–30% exposure to air. Live weight, dry meat weight and dry shell weight were reduced with increased exposure to air, but the ratio of dry meat weight to dry shell weight (shell condition index), the gross biochemical composition, and survival were not affected. By extrapolation the point of no growth was found to be at 36 and 47% exposure to air at the sites in the Menai Straits and R. Roach, respectively.

Distribution of Sabellaria vulgaris Verrill (Polychaeta: Sabellariidae) on a Sandflat in Delaware Bay

The vertical distribution of the polychaete Sabellaria vulgaris Verrill and its sandgrain tube material was studied on a sandflat near the mouth of the Mispillion River in Delaware Bay. Numbers of live worms, and amounts of tube material were determined for 13 exposure time zones on the sandflat. Exposure zones were established according to measurements made during an extreme low-water spring tide. Numbers of live worms declined sharply above the 126–150-minute exposure zone. Almost no live worms were found above 175 minutes of exposure. The largest amount of tube (colony) material was found in the area characterized as the 101–150-minute exposure zone. There was less colony material both above and below this level on the sandflat. An explanation as to why the 101–150-minute exposure zone is optimal for tube construction is proposed, and a comparision with the zonation of Sabellaria alveolata in England is made.

An Ivory Trumpet from Sofala, Mozambique

The trumpet was found in association with the ruins of the Portuguese fort at Sofala, some 75 miles south of Beira on the Mozambique coast. Sofala was an important trading centre for many centuries of the African Iron Age. Trade routes from the gold and ivory regions of the Rhodesian interior converged on the Sofala region, one of the southernmost outposts of the East Coast trade. The exact date at which Sofala itself was first occupied is unknown, but Arab traders are said to have discovered the gold trade of Sofala by the tenth century A.D. (Freeman-Grenville 1962:15)o This trade, and that of ivory and base metals, prospered in Moslem hands, and reached its peak in the fifteenth century. The Portuguese annexed Sofala in 1505 and founded a settlement on the north side of the estuary. This small village--it was little more when compared with the large towns of the East Coast-was occupied continuously until the middle of the nineteenth century, when the bay began to silt up as a result of a change in the course of the river. The height of its prosperity was in the sixteenth and seventeenth centuries, when the Portuguese were receiving some 51,000 pounds of ivory and 12,500 miticals of gold annually, not counting the large quantities of gold and ivory which were secretly smuggled by Moslem traders (Axelson 1960:1). Little remains of the Portuguese settlement today. Most of the stone from the fortress was removed to Beira to construct the cathedral there; only a few blocks remain, washed by the waters of the Indian Ocean, for the site now lies well below the high-water level. The outline of the fort can be traced at low-water spring tides, when the walls are marked by lines of rubble and midden debris. Pottery, imported china, beads, elephant tusks, fishing weights, human bones, and other traces of human activity are scattered throughout the ruins and along the high-water mark for several hundred yards on either side of the fort. Surface collections from the site have now yielded a considerable quantity of cultural material, but unfortunately, owing to the waterlogged situation of the site, excavations are quite impossible without enormous expenditure on shuttering and breakwater works.

A fungus in Ectocarpus granulosus C. Agardh near Plymouth.

DURING April and May 1949, Ectocarpus granulosus C. Agardh growing at mean low-water spring tide at Saltstone, which is about 1½ miles up the River Sal, from Salcombe, was heavily infected with a parasitic fungus. The Saltstone habitat is a purely marine one and is sheltered in the river estuary. The Ectocarpus grows attached to small stones and to other algae in the Vaucheria-community. By July most of the Ectocarpus plants had disappeared from the locality.

Observations on Patella uulgata. Part III. Habitat and Habits

SUMMARYThe inter-tidal habitats of the common limpet, Patella vulgata L,, have been investigated in relation to constant lunar tidal levels and tidal zones. The habitats of some common shore organisms which occur at approximately definite tidal levels or zones—and can therefore be used with discretion for zoning the habitats of Patella—are shown in a chart.A study of the distribution of the common limpet on the shores of Devon and Cornwall has shown that limpets are absent or rare on those parts of the foreshore above high-water neaps which are subjected to prolonged periods of direct insolation. In the same locality limpets may be totally absent from the high-water region on cliff-faces or rocks facing southerly, but present on adjacent ones facing northerly or north-westerly. Strongly insolated habitats appear to be unviable as a result of the direct effect of the insolation on the animal, but the apparent deficiency of food in such habitats is probably also a limiting factor of importance.The importance of the splash-line of any tidal level is emphasised. It is shown that in Devon and Cornwall the occurrence of low-water spring tides at about midday (and midnight) results in exposing to strong sunshine (and correlated factors) those regions of the shore above high-water neaps which face in a southerly direction. In other localities where high-water spring tides occur about midday, equivalent regions of the foreshore are not so strongly nor so persistently insolated as in the former.Viable habitats may therefore occur in different regions on the shore in different localities or at different latitudes according to the local orientation of the substratum and local tidal phenomena.

Recent researches in Bench Cavern, Brixham, Devon

About the end of March 1861, nearly two years after I had completed the exploration of the famous Brixham Cavern, more correctly known as ‶Windmill Hill Cavern, Brixham,″ information was brought to me that ‶another bone cave″ had just been discovered very near that town. I at once proceeded thither, and ascertained that the new ‶find″ was in the northern or Torbay slope of Furzeham Hill, between which and Windmill Hill the town of Lower Brixham mainly stands. Considerable limestone quarries have from time to time been worked in this slope, and one of them, known as Bench Quarry, had been all but abandoned as long ago as 1839; but quarrying had been resumed there in 1861, and had led to the discovery about which I had gone to inquire. The workmen had laid bare a vertical wall-like mass or dyke, composed of red earth, angular pieces of limestone, and bones, all cemented firmly together and to the back wall of the quarry. The base of the dyke was 96 feet above spring-tide low water, and its summit 123 feet above the same level. The dyke itself was 27 feet high, 12 feet long, and from 2 feet to 3 inches thick. It was obvious that the materials composing it had from time to time fallen into a narrow fissure in the limestone hill. Mr Wolston, the proprietor, informed me that, as early as 1839, the whole of the outer surface of the dyke was laid bare, with the exception