Burrowing amphipods and Eurydice pulchra in well-drained clean sand shores
Ecological and functional relationships
Patterns of distribution and abundance in exposed sandy beaches have been assumed to be primarily controlled by specific species responses to the hydrodynamic climate and sediment characteristics which are intimately linked, a scenario where biological interactions do not appear to play a critical role (McLachlan, 1983). There is a conspicuous lack of information concerning the effects of biotic factors e.g. competition, on the structure and distribution of sandy beach populations, as it is likely that detection of intra- and interspecific competition in such a dynamic environment is very complex (Branch, 1984). Consequently the ecology of exposed sandy beaches remains relatively poorly understood in comparison to rocky shores (Schoeman et al., 2000).
The macrofauna of sandy beaches and the meiofauna (and microfauna) of the interstices between sand grains, comprise two entirely separate faunal components with limited overlap or exchanges of energy (McLachlan, 1983). This is because the meiofauna are extremely small interstitial forms while the macrofauna are several orders of magnitude larger.
The meiofauna are likely to be important consumers of the microphytobenthic productivity. The dominant components of sandy beach meiofauna are nematodes and harpacticoid copepods with several other taxa of variable importance (McLachlan, 1983). There is a well established relationship between the relative proportions of nematodes and harpaticoids and grain size. Nematodes tend to dominate in finer sediments, harpaticoids in coarser sediments and in sediments with a median grain size of 0.3-0.35 mm they are both equally important (Gray, 1971; McLachlan et al., 1981).
The macrofauna of sandy beaches are often abundant. A common feature is the high degree of mobility displayed by all species with shifting distributions in the intertidal. The crustaceans, polychaetes and molluscs are the most conspicuous taxa on sandy beaches, the Crustacea e.g. Eurydice pulchra and Bathyporeia pelagica being most abundant on exposed sandy shores.
The sandy beach comprises an unusual ecosystem in that the customary food chain of plants-herbivores-carnivores is not clearly discernible (Eltringham, 1971). The absence of macroalgae means that herbivorous macrofauna either feed on the biogenic film, on and in the deposit, or on phytoplankton from the overlying seawater during high tide, or on plant debris carried by currents to the area from elsewhere.
The isopod Eurydice pulchra is a carnivore feeding on a wide range of invertebrates found on sandy shores.
The sandy intertidal zone is utilized by juvenile flatfish as a feeding ground. Sole, Solea solea, dab, Limanda limanda, flounder, Platichythys flesus and plaice, Pleuronectes platessa migrate inshore on the flood tide to feed upon tidally active crustaceans, such as Bathyporeia and Eurydice spp., polychaetes and young bivalves and their siphons (Elliott et al., 1998).
The biotope complexes represented by this key information review are used by important wintering and passage birds for feeding and roosting and consequently are important visiting predators. Particularly dependent species are brent geese, shelduck, pintail, oystercatcher, ringed and grey plovers, bar-tailed and black-tailed godwits, curlew, redshank, knot, dunlin and sanderling (Jones & Key, 1989; Davidson et al., 1991).
Seasonal and longer term change
- Vertical migrations from the substratum into the overlying sea water are made by the dominant crustaceans e.g. Eurydice pulchra and Bathyporeia pelagica. Such behaviour is endogenously controlled and has a circatidal rhythm that is coupled to a circasemilunar pattern of emergence (Alheit & Naylor, 1976; Fish & Fish, 1972; Preece, 1971; Fincham, 1970a & 1970b; Jones & Naylor, 1970; Watkin, 1939b).
- Fish & Preece (1970) observed the disappearance of Bathyporeia pelagica from their sampling site in west Wales in March 1967 and specimens were not recorded again until October. In subsequent years the disappearance of Bathyporeia pelagica was sudden and characterized by the movement of a large proportion of the population to the lowest levels of the shore. The seasonal change was believed to be influenced by salinity-temperature fluctuations to which the species is intolerant.
- Seasonal storm events can change sediment distribution and composition significantly e.g. the removal of the top 20 cm of sand has been reported (Dolphin et al., 1995).
- Seasonal change has been documented for the meiofauna of sandy shores in temperate regions, with the meiofauna occurring in lower abundance and moving deeper into the sediment in winter (citations in McLachlan, 1983). Vertical migrations other than seasonal have been reported in response to heavy rain, wave disturbance, tidal factors and changes in moisture and oxygen over the tidal cycle.
Habitat structure and complexity
The hydrodynamic regime (tides, waves and residual currents) together with the underlying physiography and geology create the conditions for a given substratum to develop. In terms of ecological relationships the interstices of the sand are significant owing to influences on the physical and chemical properties of the substratum to which the infauna are sensitive. Most physical properties of the interstitial system are directly determined by the sediment properties which in turn are related to the wave and current regime. Grain size, shape and degree of sorting are most important in determining porosity and permeability which influence drainage. Drainage is critical in determining the moisture content, oxygen saturation, organic content and the depth of the reducing layer (if present). Permeability increases with coarse substrate and better sorting, and drainage also increases on steeper beaches. The macrophyte community is poor owing to the lack of stable substrata, consequently the presence of occasional stones or an artificial substratum (e.g. coastal defences) would allow the establishment of some species of macrophyte e.g. Chorda filum.
The primary production of sandy shores is very low if the sand is particularly clean, i.e. low in organic matter. Steele & Baird (1968) gave a figure of 5 g of carbon per m² per year for the productivity of a moderately exposed sandy beach. In most situations, diatoms are the primary producers of the depositing shore, and are confined to the illuminated sediment surface layers. The role of sulphur reducing bacteria is limited in clean sandy shore environments owing to the lack of an anoxic black sub-surface layer (under normal conditions). The phytoplankton of the sea becomes a temporary part of the sandy shore ecosystem when the tide is in and primary producers from other environments appear on the shore. These are invariably macroalgae that have become detached from rocky substrata and have been washed up, eventually they decompose on the beach and contribute to the energy budget of the shore system. Consequently most productivity on the depositing sandy shore may be categorised as secondary, derived from detritus and allochthonous organic matter, which is utilized by the fauna.
- Eurydice pulchra breeds between April and August once sea temperatures rise above 10°C, and the highest number of juveniles occur around the periods of maximum summer temperatures. Males and females pair during their nightly swimming on falling spring tides and mating occurs in the sand once the female has completed her moult. Incubation of the embryo in the brood pouch takes some 7-8 weeks and after release of the young, the female returns to the non-breeding condition (J. Fish, pers. comm.). Juvenile Eurydice pulchra first appear in July, the minimum length being 1.7 mm (J. Fish, pers. comm.). Although the first juveniles may reach sexual maturity before the onset of winter, they begin breeding in the following spring and die during their second autumn after a total life span of approximately 15 months. Mid-summer juveniles also mature to breed the following summer and only reached 12 months of age before dying. In contrast, the last broods appearing as late as October, do not mature until late the following summer. They breed in their second October and then over-winter for a second time, producing a second brood in the spring before dying of at 18-20 months old (Hayward, 1994; Jones, 1970; Fish, 1970).
- Bathyporeia pelagica may breed throughout the year, but the greatest reproductive activity occurs during spring and late summer/autumn. Males and females pair whilst swimming and mate on the night-time ebb tides following each new and full moon. Development of an egg to the stage when it is released as a juvenile takes about 15 days to complete. The over-wintering population of Bathyporeia pelagica consists largely of juvenile animals. These mature in spring to form the majority of the next breeding population and eventually die in June and July, after a life span of about one year (Fish & Preece, 1970). Bathyporeia pilosa has a similar recruitment cycle.
- In Pontocrates arenarius from Irish Sea coasts breeding has been recorded throughout the year (Fish & Fish, 1996).
- Little is known of the breeding patterns of Haustorius arenarius populations in Britain, but females with eggs are found during the summer months and longevity is believed to be two, possibly three years (Fish & Fish, 1996).
- Scolelepis squamata from the south coast of England bred from March to at least July. The sexually mature worms are not pelagic. Fertilization is external and the larva is free swimming for about five weeks before settlement (Fish & Fish, 1996).
- Important meiofaunal nematodes and harpacticoid copepods of the sandy shore are reported to have year round reproduction with generation times ranging from 1-3 months (McIntyre, 1969).
Time for community to reach maturity
Little evidence concerning community development was found and consequently information on the key species recruitment processes and longevity has been used to infer a time period of 1 to 2 years. One of the important characterizing species, Bathyporeia pelagica, produces a sequence of broods throughout the spring and summer which reach maturity within a year to produce subsequent generations. The meiofaunal community produces several generations within a year.
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This review can be cited as follows:
Burrowing amphipods and Eurydice pulchra in well-drained clean sand shores.
Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line].
Plymouth: Marine Biological Association of the United Kingdom.
Available from: <http://www.marlin.ac.uk/habitatecology.php?habitatid=344&code=2004>