Hydroids, ephemeral seaweeds and Littorina littorea in shallow eulittoral mixed substrata pools
Ecological and functional relationships
This biotope is dominated by species able to withstand the frequent disturbance caused by wave action. The fact that LR.H rockpools are shallow and have a mixed substratum means that sand and pebbles will be frequently moved around the rockpool. This is especially true in stormy weather when larger cobbles and boulders may be moved into the pool and when the pool may be flushed clean of sediment. This in itself means that the community is unlikely to be a climax community, but more a transient community dominated by ephemeral, rapidly growing species that are able quickly to dominate space created by wave energy. Furthermore, both the flora and fauna are likely to vary both spatially, i.e. between rock pools, and on a temporal basis, depending on the frequency, severity and timing of disturbance.Primary producers in this biotope are represented by ephemeral green algae such as Ulva sp. Ulva intestinalis can grow rapidly and is tolerant of a range of temperatures and salinities. Ulva intestinalis is also the preferred food of Littorina littorea (see below).
In terms of characterizing species, suspension feeders are the dominant trophic group in LR.H. The most common suspension feeders likely to be found in LR.H are the hydroid Obelia longissima and the common mussel Mytilus edulis. The acorn barnacle Semibalanus balanoides may also be common. Semibalanus balanoides actively feeds on detritus and zooplankton. Mytilus edulis actively feeds on bacteria, phytoplankton, detritus, and dissolved organic matter (DOM). Obelia longissima is a passive suspension feeder, feeding on small zooplankton, small crustaceans, oligochaetes, insect larvae and probably detritus. The branches of Obelia longissima may be used as substratum by Mytilus edulis pediveligers (Brault & Bourget, 1985). Other suspension feeders may include the barnacle Elminius modestus and the tubeworm Pomatoceros triqueter.
The grazing gastropod Littorina littorea feeds on range of fine red, green and brown algae including Ulva sp., Cladophora sp. and Ectocarpus sp.
Deposit feeding worms such as the sand mason Lanice conchilega and the lugworm Arenicola marina may be found if patches of sand are present in the pools. The sand mason is also capable of active suspension feeding.
The common shore crab Carcinus maenas is the largest mobile predator frequently associated with LR.H. Carcinus maenas is likely to move in and out of the rockpool feeding on plant and animal material including Semibalanus balanoides and Littorina littorea.
Seasonal and longer term change
Rockpools constitute a distinct environment for which physiological adaptations by the flora and fauna may be required (Lewis, 1964). Conditions within rockpools are the consequence of prolonged separation from the main body of the sea, and physico-chemical factors within them fluctuate dramatically (Huggett & Griffiths, 1986). Shallow pools such as those associated with LR.H are especially influenced by insolation, air temperature and rainfall, the effects of which become more significant towards the high shore, where pools may be isolated from the sea for a number of days or weeks (Lewis, 1964).
Water temperature in pools follows the temperature of the air more closely than that of the sea. In summer, shallow pools are warmer by day, but may be colder at night, and in winter may be much colder than the sea (Pyefinch, 1943). It is also possible that shallow pools may freeze over in the coldest winter months.
High air temperatures cause surface evaporation of water from pools, so that salinity steadily increases, especially in pools not flooded by the tide for several days. Alternatively, high rainfall will reduce pool salinity or create a surface layer of brackish/nearly fresh water for a period. The extent of temperature and salinity change is affected by the frequency and time of day at which tidal inundation occurs. If high tide occurs in early morning and evening the diurnal temperature follows that of the air, whilst high water at midday suddenly returns the temperature to that of the sea (Pyefinch, 1943). Heavy rainfall, followed by tidal inundation can cause dramatic fluctuations in salinity, and values ranging from 5-30 psu have been recorded in rockpools over a period of 24 hrs (Ranade, 1957). Rockpools in the supralittoral, littoral fringe and upper eulittoral are liable to gradually changing salinities followed by days of fully marine or fluctuating salinity at times of spring tide (Lewis, 1964).
Due to the frequent disturbances likely to affect this biotope, any seasonal changes are likely to be masked by changes caused by wave energy. Some species of hydroids demonstrate seasonal cycles of growth in spring/summer and regression (die back) in late autumn/winter, over wintering as dormant stages or juvenile stages (Gili & Hughes, 1995). Many hydroids are opportunists adapted to rapid growth and reproduction (r-selected), taking advantage of the spring/summer phytoplankton bloom and more favourable (less stormy) conditions (Gili & Hughes, 1995). Brault & Bourget (1985) noted that Obelia longissima
exhibited an annual cycle of biomass, measured as colony length, on settlement plates in the St Lawrence estuary. Colony length increased from settlement in June, reaching a maximum in November to March and then decreasing again until June, although the decline late in the year was attributed to predation, and data was only collected over a two year period. The ephemeral algae are also likely to experience an obvious decline in biomass over the winter months.
Habitat structure and complexity
The mixed substratum of the rockpool will give the habitat some heterogeneity since there is likely to be a mixture of sand, gravel, pebbles and cobbles. It is the surfaces of the larger pebbles and cobbles that are likely to be colonized by the algae, barnacles and tubeworms and hydroids although Obelia longissima
can also grow on coarse clean sand. Clumps of mussels, whether the shells are empty or not, will also provide a substratum for the hydroids and barnacles. The mussel matrix will bind sediment and the sediment trapped between the shells may provide shelter for cryptic species and small worms, for example.
No information was found regarding the productivity in LR.H although it is expected to be low. At any given time it is unlikely that there will be a well established community, regardless of species composition.
- Obelia longissima exhibits a typical leptolid life cycle consisting of a sessile colonial, vegetative hydroid stage, a free-living sexual medusoid stage, and a planula larval stage (see MarLIN review). In terms of reproduction and recruitment, Obelia longissima has a number of strategies.
- Obelia longissima can grow vegetatively and branch to form a network across the substratum. It can also reproduce by fission or mechanical fragmentation of the colony which may aid dispersal (Gili & Hughes, 1995). Hydroids can also form frustules or gemmules, which are thought to be resting stages, in response to stress (Gili & Hughes, 1995). These frustules are adhesive and stick to the substratum where they can form new colonies (Cornelius, 1995a; Kosevich & Marfenin, 1986).
- In terms of sexual reproduction, Obelia longissima is dioecious, producing male and female medusae. The medusoid stage lasts between 7 -30 days (Stepanjants, 1998). Eggs and sperm are released into the sea and fertilization is external, resulting in an embryo that develops into a typical planula larva (Cornelius, 1995a, b; Gili & Hughes, 1995). In Europe, the medusae of Obelia longissima are usually found in the water somewhere between April and July, depending on area (see MarLIN review). Assuming that all the medusae survive to release gametes, Cornelius (1990b) estimated that an average colony could potentially produce about 20,000 planulae, although he also suggested that only one of these planulae was likely to survive to form a colony which itself might survive to reproduce.
- Ulva intestinalis is a rapidly growing opportunistic species. It can be found in reproductive condition at all times of the year, but maximum development and reproduction occur during the summer months (Burrows, 1991). The life history consists of an alternation between haploid gametophytic and diploid sporophytic generations (see MarLIN review). The haploid gametophytes produce enormous numbers of biflagellate motile gametes which cluster and fuse to produce a sporophyte (diploid zygote). The sporophyte matures and produces large numbers of quadriflagellate zoospores that mature as gametophytes, and the cycle is repeated. Together spores and gametes are termed 'swarmers'. Mobility of swarmers belonging to Ulva intestinalis (studied as Enteromorpha intestinalis) can be maintained for as long as 8 days (Jones & Babb, 1968) and as a result, tend to have large dispersal shadows. Propagules have been found 35 km from the nearest adult plants (Amsler & Searles, 1980).
- Littorina littorea can breed throughout the year but the length and timing of the breeding period are extremely dependent on climatic conditions. Fertilization is internal and Littorina littorea sheds egg capsules directly into the sea during spring tides. Eggs are released on several occasions. Fecundity can be as much as 100,000 for a large female (27 mm shell height) per year. Larval settling time or pelagic phase can be up to six weeks.
- Spawning in Mytilus edulis is protracted in many populations, with a peak of spawning in spring and summer (see MarLIN review). Gametogenesis and spawning varies with geographic location, e.g. southern populations often spawn before more northern populations (Seed & Suchanek, 1992). The planktonic life can exceed two months in the absence of suitable substrata or optimal conditions (Bayne, 1965; Bayne, 1976a). Mytilus edulis recruitment is dependant on larval supply and settlement, together with larval and post-settlement mortality. Larval mortality is probably due to adverse environmental conditions, especially temperature, inadequate food supply, inhalation by suspension feeding adult mytilids, difficulty in finding suitable substrata and predation (Lutz & Kennish, 1992). Widdows (1991) suggested that any environmental factor that increased development time, or the time between fertilization and settlement would increase larval mortality. Jorgensen (1981) estimated that larvae suffered a daily mortality of 13% in the Isefjord, Denmark but Lutz & Kennish (1992) suggested that larval mortality was approximately 99%. Recruitment in many Mytilus sp. populations is sporadic, with unpredictable pulses of recruitment (Seed & Suchanek, 1992). Mytilus is highly gregarious and final settlement often occurs around or in-between individual mussels of established populations.
- Semibalanus balanoides is an obligate cross-fertilising hermaphrodite. It produces one brood per year of 5000 -10,000 eggs/ brood in mature adults but this varies with age and location (see MarLIN review). Copulation takes place in the UK from November to early December and nauplii larvae are released from the barnacle between February and April, in synchronisation with the spring algal bloom. Nauplii larvae are planktotrophic and develop in the surface waters for about two months. They pass through six nauplii stages before eventually developing into a cyprid larva. Cyprid larvae are specialized for settlement and peak settlement occurs in April to May in the west and May to June in the east and north of Britain although settlement and subsequent recruitment are highly variable.
Time for community to reach maturity
LR.H is subjected to frequent small disturbances and the associated community is characterized by relatively short lived and opportunistic species. As a consequence, the time taken for the community to reach 'maturity' is likely to be fairly rapid, i.e. less than a few years.
- Obelia longissima is capable of growing rapidly, budding and forming stolons that allow it to colonize space rapidly. Hydroids are often the first organisms to colonize available space in settlement experiments (Gili & Hughes, 1995). Rapid growth, budding and the formation of stolons allows hydroids to colonize space rapidly. Cornelius (1992) stated that Obelia longissima could form large colonies within a matter of weeks.
- Ulva intestinalis is opportunistic and capable of rapidly colonizing bare substratum, providing the substratum is suitable. Kitching & Thain (1983), for example, reported that following the removal of the urchin Paracentrotus lividus (that grazes on the Ulva sp.) from areas of Lough Hyne, Ireland, Ulva sp. grew over the cleared area and reached a coverage of 100% within one year.
- Littorina littorea is a slow crawler but because LR.H rockpools are likely to be surrounded by other rockpools, active immigration of snails is possible from the surrounding rocky shore where Littorina littorea may be abundant.
- The establishment of the Mytilus edulis community may take significantly longer (see Sensitivity). However, this species is not characteristic of the biotope. Furthermore, if Mytilus edulis are present in LR.H, they will usually be part of a larger mussel bed (SLR.MytX) and this will favour recruitment to the area. Recovery of Mytilus edulis may take at least 5 years, although in certain circumstances and under some environmental conditions recovery may take significantly longer.
- Bennell (1981) observed that barnacles were removed when the surface rock was scraped off in a barge accident at Amlwch, North Wales. Barnacle populations returned to pre-accident levels within 3 years. However, barnacle settlement and recruitment can be highly variable because they are dependent on a suite of environmental and biological factors (see MarLIN review), therefore populations may take longer to recover.
This review can be cited as follows:
Hydroids, ephemeral seaweeds and Littorina littorea in shallow eulittoral mixed substrata pools.
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=54&code=1997>