Nephtys cirrosa and Bathyporeia spp. in infralittoral sand
Ecological and functional relationships
Communities in wave exposed sand habitats and, by extension, any sediments subject to hydrodynamic disturbance 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). Consequently mean macrobenthic diversity and species richness of clean mobile sands is generally lower than that of the surrounding seabed, reflecting greater stresses inherent in such environments (Elliott et al., 1998).
Intertidal and subtidal sandy biotopes comprise an unusual ecosystem in that the customary food chain of plants-herbivores-carnivores is not clearly discernible (Eltringham, 1971), the physical environment being too harsh for vegetation to become established. The absence of macroalgae means that herbivorous macrofauna either feed on the biogenic film, on and in the deposit (e.g. Bathyporeia pelagica which is an epistrate feeder) or on phytoplankton from the overlying seawater.
The meiofauna are likely to be important consumers of the microphytobenthic productivity. The dominant components of sandbank 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, 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).
Polychaete worms are dominant infaunal predators, they are opportunistic and actively pursue prey, so that their numbers may be closely related to that of their prey which includes other polychaetes and small crustaceans (Meire et al., 1994). Bamber (1993) found a significant linear correlation between declining densities of Scoloplos armiger and increasing densities of Nephtys cirrosa in a psammophilous polychaete community from the Solent Coast, Hampshire. Nephtys species also scavenge, as does the isopod, Eurydice pulchra, which also actively preys upon the amphipod Bathyporeia pelagica.
Conspicuous epibenthic species that may be encountered within the biotope include shrimps (Crangon crangon), crabs (Carcinus maenas, Cancer pagurus and Pagurus bernhardus), starfish (Asterias rubens). Sand eels, Ammodytes sp., may be locally abundant, whilst juvenile gadoids (Gadus morhua & Pollachius virens), adult and juvenile flatfish (Pleuronectes platessa, Limanda limanda & Platichthys flesus) frequent the biotope to feed upon the epi- and infauna.
Seasonal and longer term change
- A seasonal pattern of abundance is demonstrated by many species, and is characterized by annual recruitment of species increasing their density typically in late summer/autumn. For instance, common cumaceans recorded within the biotope, Pseudocuma longicornis and Cumopsis goodsiri, are almost entirely restricted in their presence to late summer-autumn months (Bamber, 1993). Two reproductive peaks for Bathyporeia pelagica occur in spring and autumn suggesting that an over-wintering population matures slowly and reproduces in the spring, and their progeny mature rapidly over five months to reproduce in the autumn of the same year (Watkin, 1939b).
- Warmer summers may cause temporary declines in the abundance of some species as a result of recruitment failure (juveniles being potentially more sensitive). For instance in a sandy shore community following the warm summer of 1989, Bamber (1993) recorded a significant decrease in the Bathyporeia sarsi population, a species which shows its southern limit of distribution in the English Channel.
- Mortality of some of the infaunal and epifaunal population may be expected as a result of any winter storms that cause suspension of the substratum.
- Seasonal changes 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.
- Vertical migrations from the substratum into the overlying sea water are made by the dominant crustaceans e.g. Eurydice pulchra and Bathyporeia pelagica on nearly every night of the year. Such behaviour is endogenously controlled and has a circatidal rhythm that is coupled to a circasemilunar pattern of emergence (Alheit & Naylor, 1976; Jones & Naylor, 1970; Preece, 1971; Fincham, 1970a & 1970b; Watkin, 1939b).
Habitat structure and complexity
Superficially the habitat may appear to be rather homogenous, but within the sand a variety of niches are probably available for colonization. For instance, sandbanks may show a gradient from finer sediments to coarser sediments resulting from the prevailing current pattern. The upper sand layers may be characterized by sand waves and ripples occurring on a variety scales, which are continually destroyed and rebuilt by currents, a process visible at the waters surface by the appearance of patches of suspended sediment. In other instances the distribution of different grades of sandy sediment may be very patchy and at the bottom of depressions finer sediments, more stable deposits, enriched with some mud might be found (Vanosmael et al., 1982).
The macrobenthic infauna of the biotope consists of animals which feed largely on particulate matter in or on the sand, and which are themselves preyed upon by populations of juvenile flatfish (McIntyre & Eleftheriou, 1968). Owing to the lack of stable substrata, benthic microalgae constitute the main primary producers of the biotope and the quality of light (as critical depth for primary production) reaching sandbanks in the sublittoral will determine the type of microalgae colonizing the sediment. Owing to turbulence created by tidal flow and wave action, overlying water may be particularly turbid, limiting primary production further. Steele & Baird (1968) estimated microphytobenthic production to be 4-9 g C/m²/yr a figure they considered to be inadequate for the macrofauna and rich meiofauna (assuming an ecological efficiency of 10%, the infaunal biota of this biotope would probably have an annual requirement in the region of 25 g C/m²). To support the infauna the biotope must be subsidised to a high degree by organic matter produced in the water column and other environments and transported to the biotope, consequently productivity of this biotope is mostly secondary (McLachlan, 1980). Primary production in the water column was estimated to be in the region of 95 g C/m² annually (Steele & Baird, 1968) and some of this is directly available or may reach the benthos indirectly after intervening trophic levels. Baird & Steele (1968) demonstrated that only 3-5% of the organic matter in the sand was unattached, so that a continuous supply of material and rapid filtering of water through the sand are essential if the requirements of the benthos are to be met.
Characterizing macrofauna of the biotope are iteroparous, meaning that they breed several times per lifetime. Some species have a brooding and benthic mode of reproduction whilst others are broadcast spawners with a planktonic phase of development.
- 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).
- 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 just 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).
- 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. 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).
- Polychaete worms such as Nephtys spp. and spionid worms release their eggs and sperm into the water where, after fertilization and a relatively prolonged planktonic phase of development, they metamorphose and commence a benthic habit. Recruitment of Nephtys species seems related to environmental conditions in central parts of the species range, marginal populations exhibit occasional reproductive failures, e.g. Nephtys cirrosa, which is a temperate species and reaches the northern limit of its range in the north of the British Isles. Populations of Nephtys cirrosa on the east and west coasts of northern Britain exhibit different reproductive patterns. In south-west Scotland gravid adults breed every year in early autumn, whilst those on the east coast experience periods (e.g. over three years) of reproductive failure (Olive & Morgan, 1991).
Time for community to reach maturity
As a consequence of the dynamic nature of the habitat the faunal component of the biotope is very sparse and low in species richness. Therefore, the community might be considered 'mature' (in terms of representative species present) only a few days or weeks after the last disturbance, as displaced polychaetes and crustaceans re-enter the substratum.
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This review can be cited as follows:
Nephtys cirrosa and Bathyporeia spp. in infralittoral sand.
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=154&code=1997>