Modiolus modiolus - Biology and ecological functioning
Reproduction, development and growth
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).
Larval development, settlement and recruitment
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,
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
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
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.
Parasites and diseases
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.
Richness and diversity
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
Community and habitat descriptions
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
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
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).
Possible roles as nursery grounds
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 adults 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').
Wider effects on the environment
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.