Many aspects of the reproduction, development and growth of Modiolus seem to be highly variable.

The spawning season is often ill defined, and may vary greatly with depth (Schweinitz & Lutz, 1976) and probably with latitude (water temperature). Populations in Strangford Lough showed no pronounced annual cycle, with a slow release of gametes throughout the year (Brown & Seed, 1977; Seed & Brown, 1975; Seed & Brown, 1977). The geographically very close population off the south east of the Isle of Man showed clear annual cycles in gonad development and in recruitment, with a likely maximal spawning in spring and summer, although probably occurring to some degree all year (Jasim & Brand, 1989). Populations in Scotland also showed variable reproductive seasonalities, and again some showed strong seasonality, with spawning occurring typically in the spring and early summer (Comely, 1978; Comely, 1981). Reproduction in populations in northern Norway and Sweden seems to be controlled by temperature, with a strong peak of spawning in June (Brown, 1984).

Little is known about larval stages but the planktonic stage is thought to be extremely long (Ocklemann, 1965), suggesting that recruitment may be possible from distant populations (Brown, 1984). Preliminary genetic work using enzyme electrophoresis on a population off the south east of the Isle of Man appeared to support this, since the results suggested that recruitment came from several different parent populations (Lewis, unpubl.).

Recruitment of juveniles (spat) is very variable, not only seasonally, but probably also very much between years in many populations. Most early work on recruitment was inferred from size distribution within populations, which is very difficult in such a long-lived and (for much of its life) slow growing species. Bimodal size distributions have usually been found. In some populations such as Strangford Lough, where there does seem to be regular recruitment, this is simply a result of size specific variation in growth and mortality (eg Seed & Brown, 1978), since young Modiolus tend to grow quickly in the early years (see below). This was also the case for Modiolus in two areas off the south east of the Isle of Man (Jasim, 1986; Jasim & Brand, 1989). Other populations do seem to undergo very irregular recruitment, however. Recent work on the Modiolus reef areas to the north east of the Isle of Man suggested that recruitment in many years has been negligible. Very irregular recruitment, with gaps of many years, has also been reported for Norwegian (Wiborg, 1946; Dahle, 1984 cited in Jasim, 1986) and Canadian populations (Rowell, 1967).

Intuitively it seems likely that enclosed areas, such as the Strangford Lough cSAC, and Scottish sea lochs and Voes, must be largely self sustaining, but this may not be the case for more open areas such as the Lleyn Peninsula and the Sarnau cSAC. Preliminary unpublished genetic work at Port Erin Marine Laboratory has suggested that the beds off the south east of the Isle of Man may recruit largely from other areas in a somewhat sporadic way.

Settlement can be very dense, as sometimes seems to occur in populations in the Bristol Channel. Recruitment to adult populations is very strongly limited by predation in the early years however; the Bristol Channel juveniles do not seem to recruit well, for example.

Studies by Anwar et al. (1990) showed that growth is very rapid in the first four to six years, at which age they reach a length of 35-40 mm, after which they are less vulnerable to attack by predators. Only the very largest crabs and starfish can open mussels over 50 mm in length, and large Modiolus are thought to be relatively predator free.

A wide variety of growth rates has been reported, with some relatively fast growing young populations (eg on rigs in the Bravo Forties oil field in the North Sea), where fast growing populations of animals up to 10 years old occur. Particularly slow growing populations have been reported in the Firth of Lorne (Comely, 1978), and in nearshore populations off the south east of the Isle of Man (Jasim, 1986), where average lengths of 70 mm or less are reached at an estimated age of around 20 years. In many populations animals seem typically to reach a length of 100 mm at ages of roughly 12-18 years (eg several populations is the West of Scotland, Shetland, Strangford Lough and around the Isle of Man; (Comely, 1978; Comely, 1981; Holt & Shalla, unpublished; Jasim, 1986; Seed & Brown, 1975)). Maximum sizes in British waters are generally thought to be around 200 mm (Anwar et al., 1990; Brown, 1976).

Rates of development of reefs are not known. There would appear to be some potential for spread of existing bioherms where these take the form of very dense raised beds, as off the Lleyn Peninsula, as a result of clumps of mussels dropping off from the edges, which are often quite discrete. This would undoubtedly be a very slow process taking probably many years per metre of spread. Spread or recovery of more infaunal types of reefs would presumably be slower still, although this is purely speculative.

Sexual maturity occurs at around 35-40, and the mussels are able to divert more resources to reproduction and less to growth (Anwar et al., 1990). The age at which the majority of animals reach sexual maturity has been reported as:

• 3-4 years (Isle of Man; Jasim, 1987);
• 5-6 years (Norway; Wiborg, 1946);
• 4 years, but reaching full gonad maturity at 7-8 years old (Canada; Rowell, 1967).

  Longevity and stability

Occasional mussels aged up to 35 years are reported in most British populations and ages in excess of twenty five years seem to be very frequent. Maximum ages are likely to be well in excess of 50 years (Anwar et al., 1990).

Dense reefs and beds are thought in general to be very stable in the long term, despite somewhat intermittent recruitment in some cases. This is based upon observations that reefs are consistently found in the same place over long time periods, but to what degree the Modiolus population structure, physical nature of the reefs, or the associated community structure might vary does not appear to have been studied. The variable nature of recruitment in at least some populations demonstrates that some variation in Modiolus population structure with time must occur, but this has not been described in any detail.

Modiolus is a filter feeder, although considerably less work has been done on mechanisms and influences on feeding rates than in Mytilus. It has been shown that Modiolus remove particles over a wide size range, and that considerable amounts of pseudofaeces as well as faeces can be produced (Navarro & Thompson, 1997). See Ecological Functioning below for further information.

No detailed information was found on this subject regarding Modiolus, although it is known that the boring sponge Cliona celata can badly damage the shells of old Modiolus (Comely, 1978).

The communities associated with Modiolus are known generally to be extremely rich and diverse. There are clearly variations in composition of associated species. No species are known to be limited in distribution only to Modiolus reefs.

Brown & Seed (1977) recorded 90 invertebrate taxa associated with Modiolus clumps in Strangford Lough, with most of the major groups well represented. Holt & Shalla (unpublished) found 270 invertebrate taxa associated with Modiolus reef areas to the north east of the Isle of Man, and suggested that this was likely to be an underestimate, particularly in terms of sponges and infauna.

In the Gulf of Maine it has been found that the diversity of other benthic species increased as Modiolus clump size and number increased (Ojeda & Dearborn, 1989). From limited data plus subjective observations it seems likely that this would be the case in British waters, and moreover that reef areas would have a more diverse fauna than non-reef areas, but at present this has not been conclusively demonstrated.

Apart from the infauna, the Modiolus community in Strangford Lough has been described as consisting of mainly three components (Magorrian, 1996), and this description probably applies to any Modiolus reef community:

A) Very dense aggregations of living and dead Modiolus shells which form the frame work in a single or multiple layers

B) A rich community of free living and sessile epifauna and predators.

C) A very rich and diverse small community which seeks shelter in the crevices between the Modiolus shells and byssus threads and flourishes on its rich sediment.

Sponges, ascidians, Alcyonium digitatum, Chlamys varia, Aequipecten opercularis, hydroids, and Ophiothrix fragilis are all frequent or abundant in some, but not all, Modiolus communities. Urchins, starfish and whelks are numerous on most. In shallower areas red seaweeds such as Phycodrys rubens and corallines may also be present. It has long been suggested that many Modiolus bed communities contain an infauna similar in composition to the Boreal Offshore Gravel Association of Jones (1950), equivalent to the ‘deep Venus’ community (Mackie, 1990) occurring in coarse sand/gravel/shell sediments at depths of 40-100m, with typical species including the urchin Spatangus purpureus and bivalves Glycymeris, Astarte sulcata, and Venus spp.

The MNCR marine biotope classification (Connor et al., 1997) lists many of the epifaunal major species associated with the four Modiolus biotopes which could represent biogenic reefs (see Table 1). Indeed, the distinctions between the biotopes are made partially on the basis of the associated communities.

In very sheltered sealochs and Shetland Voes with only slight tidal movement the MNCR has identified a community of ‘Modiolus modiolus beds with fine hydroids and large solitary ascidians’ (SCR.ModHas) on very sheltered circalittoral mixed substrata where there is often a high mud content. Decapods such as spider crabs Hyas araneus, Aequipecten opercularis, and brittlestars such as Ophiothrix fragilis, Ophiocomina nigra and Ophiopholis aculeata may be common. The biotope ModCvar (Modiolus modiolus beds with Chlamys varia, sponges, hydroids and bryozoans on a slightly tide-swept very sheltered circalittoral mixed substrata), so far reported only from Strangford Lough (a cSAC), is a richer version with far more sponges including several species of Mycale, Haliclona spp, Spanaplion armaturum and Iophon hyndmani, hydroids, and Chlamys varia. No descriptions of the infauna of these communities were found.

The other two relevant MNCR communities (MCR.ModT; Modiolus modiolus beds with hydroids and red seaweeds on tide-swept circalittoral mixed substrata and CMX.ModMx; Modiolus modiolus beds on circalittoral mixed sediment) are also similar to each other, being distinguished mainly by the presence in the latter of much sediment in which venerid bivalves typical of the ‘deep Venus’ infaunal community occur. Sponges such as Hemimycale columella, hydroids such as Sertularia argentea, Hydrallmania falcata and Abietinaria abietina, Alcyonium digitatum, barnacles, tubeworms Pomatoceros triqueter, bryozoans such as Alcyonidium mytili and ascidians such as Dendrodoa grossularia tend to be typical of the epifauna. These communities can probably be found in close proximity, as seems to occur off the Ards peninsula. The former community (MCR.ModT) appears to be widespread, being reported from the Shetland Voes, the Humber, the Bristol Channel, the Lleyn Peninsula (within the cSAC), Northern Ireland, including off the Ards Peninsula, and some of the Scottish Sealochs including within the Loch Duich, Long and Alsh cSAC. The latter is reported off North East England (this probably occurs within the Berwickshire and North Northumberland cSAC, though this is unclear on presently available information), off the Ards peninsula and south east of the Isle of Man.

The reefs north east of the Isle of Man seem to combine elements of both of the above communities (MCR.ModT and CMX.ModMx), with the addition of considerable numbers of Chlamys varia and smaller numbers of Chlamys distorta and Aequipecten opercularis. The more prominent free living epifauna identified by Holt & Shalla (unpublished) included whelks, topshells, nudibranchs, starfish, sea urchins, brittle stars, and many decapods. Sessile epifauna included many prominent hydroids and bryozoans, anemones, Alcyonium digitatum, serpulid worms, barnacles, saddle oysters and sponges. There were also several species of fish including numerous dragonets (Callionymus lyra) lesser spotted dog fish (Scyliorhinus caniculus), butterfish (Pholis gunnellus) and occasional sea scorpions (possibly Taurulus bubalis).

The possible role of Modiolus reef communities in providing a nursery refuge for other species is occasionally mentioned but does not appear to have been investigated. Dense growths of bushy hydroids and bryozoans could conceivably provide an important settling area for spat of bivalves such as Pecten maximus and Aequipecten opercularis, adults of which are often abundant in nearby areas.

Predators are significant mainly in young animals, and appear to be of great importance in determining survival of juveniles to adulthood. Predation in the early years is probably largely by crabs and starfish, which are very numerous on most Modiolus beds and reefs, and it is suspected that survival is greatly enhanced by juveniles living within the mass of adult’s byssus threads where predators can not easily get them (Anwar et al., 1990). Only the very largest crabs and starfish can open mussels over 50 mm in length, and large Modiolus are thought to be relatively predator free. Modiolus grows rapidly in the early years, which is thought to be important in enhancing survival to adulthood (see earlier).

No mention of competitors of Modiolus was found during this study. However, it is known that the brittle star Ophiothrix fragilis can occur in high densities in similar areas to Modiolus reefs; that it is prone to large population fluctuations;, and that high densities of Ophiothrix are strongly suspected to have reduced growth and recruitment of Sabellaria spinulosa reefs in the Bristol Channel (see also ‘food limitation').

Modiolus reefs may be very extensive, and often include many other filter feeders such as sponges, hydroids, bryozoans, soft corals, brittlestars, bivalves and ascidians, and so are probably of great importance in channelling organic material between the plankton and the benthos (usually referred to in the literature as ‘benthic-pelagic coupling’), although there appear as yet to have been no detailed publications on this aspect of Modiolus reefs in British waters.

The majority of the work on this subject has been concentrated in the Bay of Fundy by Wildish and co-workers and is briefly summarised here. Modiolus in this area forms ‘bioherms’, often with relatively low densities of actual Modiolus (see chapters I & II), but covering extensive areas. Mixing processes allow Modiolus to feed on live phytoplankton down to 100 m depth, and it is the largest contributor to secondary benthic production (Wildish & Fader, in press); biodeposition via faeces and pseudofaeces (up to 41 mg dry weight per animal per day, mainly in the form of large diatoms) is a major mechanism in providing organic material and other nutrients to deposit and suspension feeders (Navarro & Thompson, 1997). It has been suggested from flume tank experiments that seston concentrations will be depleted directly over dense Modiolus beds where water flow is low, and that this can be identified in the field by a decrease in mussel bed density (Wildish & Kristmanson, 1984; Wildish & Kristmanson, 1985). No comparable work was found on British Modiolus communities, although similar suggestions have been made for Mytilus.

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