Recruitment and lifecycles
The main predators in intertidal and subtidal areas are birds, fish and epifaunal
crustacea such as crabs and shrimps (Meire et al, 1994). These aspects of mediating
relationships have been detailed above.
The faunistic variation in these physically controlled environments reflects the
species tolerance and sensitivity to those conditions. Competition between organisms
occurs in response to a limitation of resources - the abundance of reproductive mates
(intra-specific competition) and food and space (inter- and intra-specific competition).
Competition for space and food is unlikely to be a limiting feature in the high energy
sedimentary environments (sandbanks). This is because the populations are small, due to
the harsh conditions, and many organisms swim and feed in the water column at high tide
and only shelter temporarily in the sediment at low tide (Peterson, 1991). Densities are
kept low by the disturbance of sediment in high energy areas and so there is probably no
limitation of space (Peterson, 1991).
In many marine, sedimentary communities, deposit and detritus feeders compete for food
and suspension feeders compete for space (Levinton, 1979). Thus the large populations
inhabiting intertidal mudflats and, to a lesser extent intertidal sandflats, will have
inter- and intra-specific competition for food. Because of this, resource partitioning may
occur among certain deposit feeders to avoid competition as shown for the gastropod Hydrobia
and the amphipod Corophium which ingest different size food particles (Fenchel,
1972). Inter-specific competition may be relatively low in intertidal mud and sandflats
because of the restricted community diversity.
Recruitment and lifecycles
Most macrofauna are iteroparous in that they breed several times per lifetime. The
fecundity is closely linked to the limited food supply with temperature changes an
important controlling factor. Many polychaete worms including Nephtys spp. and
spionids release eggs and sperm into the water where, after fertilisation, the larvae
enter the plankton for a short time before settling to the substratum (Rasmussen, 1973).
The passive movement of these stages again reinforces the importance of understanding the
hydrographic regime to interpret the factors influencing the community structure.
Intertidal sand and mudflats
The presence of high densities of adult invertebrates may inhibit the recruitment of
potential colonising stages from the water (Olafsson et al, 1994). This may account
for juveniles occupying less favourable parts of the intertidal areas, for example
juvenile Arenicola and Nephtys settle at areas outside the optimal
distribution for the adults. However, many juveniles and adults are mobile and can enter
the water column and relocate themselves. Larvae from species such as Nephtys settle
in low energy areas and then migrate to the more favourable areas favoured by the adults
(Peterson, 1991). Recruitment is then linked with the hydrographic regime which allows the
dispersal and eventual settlement of metamorphosing larvae. This then allows for the
hydrographic concentration of new recruits to a population.
Although some sediment dwellers have a benthic and brooding mode of reproduction (e.g.
amphipods and oligochaetes), most are planktonic spawners (Rasmussen, 1973) and the
settlement of Nephtys caeca did not take place in the intertidal zone, suggesting
sublittoral larval recruitment. The number of larger individuals increased markedly with
decreasing level on the shore. Nephtys caeca is polytelic (which is discrete,
iteroparous) and on European coasts breeds in its second and subsequent years (Olive et
al, 1981). The spawning is highly synchronised, and an elevation of the water
temperature could be the triggering factor for gamete release (Olive, 1978). Nephtys
caeca has a diverse population structure which allows a better recovery from a poor
recruitment. Some species show spatial variation in their life cycles, for example,
different populations of Corophium volutator display one or two generations per
year depending on their location.
Subtidal mobile sandbanks
Severe exposure such as that occurring on subtidal mobile sandbanks restricts
diversity, by eliminating sedentary forms, especially bivalve molluscs, and encouraging
the numerical dominance of agile swimmers such as haustoriid amphipods and isopods. These
species have a short life span (r strategists) and the fauna is characterised by
its flexibility. The population dynamics of the fauna in exposed habitats may be based on
long term breeding success, e.g. 6-7 years for tellinids with a cohort produced which may
then dominate the population (Pearson & Barnett 1987). The opportunist
pollution-tolerant polychaete Capitella capitata (which is also an r strategist)
has both benthic and planktonic larvae and breeds throughout the year, this means it is
able to colonise impacted or stressed areas very quickly.
Subtidal mobile sandbanks are usually dependent on an input of colonising organisms and
have few species with benthic reproduction, thus any disruption to the delivering currents
will cause changes. In addition, some sandbanks are likely to be sinks of materials as
centres of gyres. The community of these areas in most cases will not contain rare species
given the dispersal mechanisms of the species and the nature of the areas. The larvae of
many benthic species e.g. Ophelia bicornis, Protodrilus spp., Pygospio elegans
and Phoronis spp. can differentiate between substratum types and settle upon the
preferred grade of sediment. The larvae of the reef forming polychaete Sabellaria
spinulosa seeks contact with the tubes of adults and will settle in these areas before
commencing metamorphosis, hence some of the biological components influencing other