ABSTRACT1. Maerl beds occur worldwide and are formed by an accumulation of unattached calcareous red algae (Rhodophyta).2. Maerl-forming algae grow in a superficial living layer on sediments within the photic zone. 3. Maerl beds are spatially complex habitats with a high degree of species and trophic group diversity.4. The European Commission's 'Habitats Directive' mandates the conservation management of two of the main European maerl-forming species, Phymatolithon calcareum and Lithothamnion corallioides.5. Mediterranean maerl beds are to be considered for inclusion in national inventories of sites of conservation interest, as required by the SPABIM Protocol of the Barcelona Convention.6. In spite of their importance, and the requirement for their conservation management, European maerl grounds suffer a variety of anthropogenic perturbations including direct exploitation through extraction, fishing impacts and chemical pollution by organic matter and excess nutrients.7. The ecology of northeast Atlantic and Mediterranean maerl beds has received little attention, in contrast to other marine communities (e.g. kelp forests, sea-grass meadows). * Correspondence to: P.G. Moore, University Marine Biological Station Millport, Isle of Cumbrae, KA28 0EG, UK. E-mail: pmoore@udcf.gla.ac.uk y Authorship alphabetical: cite as BIOMAERL team z Coordinator 8. Key conservation and management measures proposed include: the recognition that maerl beds are non-renewable resources and cannot sustain direct exploitation; prohibitions on the use of towed gear on maerl grounds; moratoria on the issue of further permits for the siting of aquaculture units above maerl grounds; monitoring of existing exploited or impacted maerl beds; the designation of 'no-take' reserves; measures to limit the impacts that might affect water quality above maerl beds; a programme of monitoring of the 'health' of European maerl beds; an awareness campaign on the biological importance of maerl beds; a higher conservation status for maerl habitats and maerlforming species in European legislation; and further research on maerl ecosystems.

Forty stations within a 20 km2 Maltese maerl bed were sampled by grab to gather data on sediment granulometry and the percentage mass, sphericity, and morphotype of rhodoliths. Two stations were monitored between July 1996 and April 1998 to study temporal variation in species diversity and abundance of the epi- and endo-benthos. Maerl was commonest at 51–90 m depth with 20–39% live rhodoliths in central parts of the maerl bed, while the peripheral parts had less than 20% live rhodoliths. The most abundant rhodolith morphotypes were branching forms and those with a rugged surface. The maerl bed proved to have high species diversity with 244 animal and 87 algal taxa recorded; molluscs, crustaceans, and annelids were the dominant taxa in the endobenthos, and bryozoans and sponges in the epibenthos. Community composition, rhodolith morphology and sediment characteristics at the two sites were related to differences in the hydrodynamic regime resulting from seabed topographical heterogeneity.peer-reviewe

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Sex determination in the echiuran Bonellia viridis Roland0 has classically been regarded as depending primarily on the environment of the newly settled larvae. The majority of the sexually undifferentiated larvae settling on an adult female become males; the larvae which settle away from the adult female become females in most cases. Previous work on this problem is reviewed.The behaviour, including the time-course of settlement, and the development of the indifferent larvae of B. viridis in the presence and absence of an adult female is described. Evidence is provided that even in the absence of adult females there is an interaction between newly settled larvae such that up to 20% of larvae become attached to one another in pairs with masculinization of one partner, the other developing into a female.Using larvae cultured singly, it was shown that crude extracts of adult female proboscis and trunk body-wall and the pigmented secretion of an irritated female masculinize indifferent larvae, the vast majority of which would have developed into females in pure sea water. Solutions of the purified integumentary pigment, bonellin, gave inconsistent results. Our experiments prove conclusively that sex determination is metagamic, i.e., not fixed at fertilization but is the result of an interaction between genetic and environmental factors, in r 83 y0 of all larvae. The main environmental factor is a substance produced by the female. The other 17% are larvae whose sex is determined exclusively by their genetic make-up. These are syngamic males, females and intersexes. The problem of indefinitely undifferentiated larvae is discussed.