Viewed from space the oceans dominate the surface of the Earth – as Arthur C. Clarke once noted, ‘How inappropriate to call this planet Earth when it is quite clearly Ocean.’ In terms of climate, the oceans are the Earth's major heat storage and transport system. For example, the first three metres of the oceans alone have an equivalent heat capacity to the Earth's entire atmosphere. Ultimately it is the oceans that exert the strongest control over planetary climate change and so perhaps it is not surprising that the clues to past climate change and the symptoms of future climate change can be found in the ocean record. Palaeoceanographers have reconstructed past climatic conditions using marine temperature proxies recorded in diverse sources ranging from foraminiferal tests in deep-sea sediments to annually banded shallow-water tropical corals. For example, oxygen isotope records from shallow, warm-water corals have revealed a recent, long-term warming and/or freshening throughout tropical regions (see review by Grottoli & Eakin, 2007). As predicted by Druffel (1997), cold-water corals are now emerging as a key archive of intermediate water-mass history. Unlike sediment-based foraminiferal records, which can be disrupted by the sediment mixing activity of infauna (bioturbation), coral skeletons offer continuous, high-resolution archives. Unlike tropical corals they are not restricted to the shallow, euphotic zone at tropical latitudes.

This chapter brings a new dimension to the understanding of cold-water coral habitats by considering cold-water corals from the temporal perspective of the fossil record. We will focus on the calcified Scleractinia as their stony skeletons are widely preserved in ancient rocks. The early evolution and phylogeny of Scleractinia have been studied by comparing morphological characteristics of extant and extinct corals, their skeletal ultrastructure and, recently, by using molecular phylogenetics (see Section 2.5, p. 52). However, none of these approaches have so far provided a unified theory for the origin of the Scleractinia. The present-day coral reef ecosystem, both in shallow and deep waters, is a geologically young achievement and its fossil record is a fascinating story of extinctions and radiations mirroring dramatic changes in the Earth's climatic history. This chapter sheds light on the fossil record of the main habitat-forming cold-water coral genera Lophelia, Madrepora, Goniocorella, Oculina and Enallopsammia and it describes fossilisation processes that control the quality of preservation in this record.

Triassic dawn

What does the fossil record of the Scleractinia tell us about their ancestry?

To most people corals are synonymous with the bright, well-lit waters of tropical coral reefs. Yet in fact the majority of corals inhabit deep, cold waters across a diverse range of marine environments from inland fjords to the continental shelf, slope, offshore banks, seamounts and even the abyssal plain. While we have known about these cold-water corals for hundreds, or even thousands, of years it is only in the last ten years that research into the biology of the corals themselves, the ecology of the habitats they provide and the geology of the structures they form has gathered pace. Cold-water coral habitats are biodiversity rich. Recent work has revealed them as unique palaeoceanographic archives. Sadly all too many surveys have shown they have been damaged by human activity. In this book we have tried to summarise the many, varied and exciting developments in our understanding of cold-water corals. Research effort on cold-water corals is now increasing exponentially around the world and it has been challenging to compress this body of work into the pages of one book. Before we consider cold-water corals and some of these recent findings in more detail we begin with a brief historical summary and an outline of the research approaches used to study cold-water corals.

History

Early history and taxonomy

The history of modern research on cold-water corals goes back to the late eighteenth century.

The habitat-forming capacities of corals have fascinated generations of marine biologists. Shallow-water tropical coral reefs, so often called the ‘rainforests of the sea’, are home to the greatest vertebrate biodiversity on the planet with over 4000 species of reef fish in 179 families estimated to be found in the Indo-Pacific region alone (Myers, 1989). The ability of one species to provide habitat for others has been formalised through the concept of ‘ecological engineering’ (Jones et al., 1994; Wright & Jones, 2006) and we discuss this in relation to cold-water corals and some of their symbiotic associates later in this chapter. Different cold-water coral species develop habitats of varying physical sizes and life spans. For instance, gorgonians may grow close together, forming dense forest-like habitats, but after death these colonies will break down, unlike the reef frameworks left by colonial scleractinians that were discussed in the last chapter.

Cold-water coral reefs and coral carbonate mounds are morphological features formed through complex interactions between biological and geological processes under suitable hydrodynamic conditions. Such interactions are common in marine systems with biological activity having a profound effect on a number of sedimentological processes: from diatom stabilisation of mudflat surfaces, to the role of algae in carbonate grapestone formation and the development of deep-water bioherms. Marine organisms can strongly influence their sedimentary environments and where their activities are focused in one place, distinct biogeological structures may be formed. Such scenarios often involve enhanced sediment accumulation through substratum stabilisation and/or sediment trapping and usually benefit the organisms in question, for instance by giving them a preferential position from which to feed and isolating them from the effects of scouring bedload transport.

Such biogeological formations are not only common in the geological record, because they form persistent features, but also because such positive interactions are ubiquitous throughout Earth history.

A sound appreciation of the biology of coral habitats must be grounded in understanding both the geological and hydrographic contexts in which these habitats have developed. As sessile, structure-forming species dependent on water flow to supply food, exchange gametes and disperse larvae, corals are intimately related to near-seabed hydrography and, in turn, their occurrence on continental shelves, slopes and seamounts may help interpret flow and turbulence regimes in these dynamic regions of the world's oceans. In this chapter, we consider both the structural and functional biology of the major groups of cold-water corals described in Chapter 2. We outline the major hypotheses that have been advanced to explain coral occurrence and review currently available evidence. Finally, and perhaps most significantly, we note that many aspects of the basic biology of cold-water corals are either unknown or have only been examined very recently. For this reason elements of the chapter will necessarily be brief but will include notes on where we can expect advances as the scientific community capitalises on our improved geological understanding of cold-water coral distribution and occurrence and begins to address fundamental questions on seasonality, food supply, ecophysiology, growth and reproduction.

The ecosystems at the bottom of the deep sea are out of sight beneath the impenetrable grey of the waves. Any marine ecosystem, especially those of the deep sea, are remote from people's daily experience and may therefore be of little general interest. One of the greatest challenges facing deep-sea conservation is raising awareness of the hidden diversity and vulnerability of this, the largest and least-known environment on Earth. What is out of sight can all too often also be out of mind, but public perception of the threats facing some marine ecosystems, such as shallow-water tropical coral reefs, is relatively high – they are highly visual and popular holiday destinations for many people. But although out of sight and sometimes thousands of miles from the nearest land, cold-water corals and other deep-sea benthic ecosystems have been affected by human activity. Many, if not most, marine ecosystems have been affected by fishing and researchers regularly report deep-water trawl damage to cold-water coral and sponge habitats. Here we will describe present-day impacts, consider emerging threats and review what can and is being done to ensure their conservation.

Impacts

Fishing

Around the world, marine fisheries are declining.

What is a coral? Dictionary definitions vary, probably because the concept of the word coral is not a scientific one, but rather a layman's term. Like the omnibus words ‘bug’ and ‘worm’, these words mean different things to different people and professions. In all three cases, these words refer to a polyphyletic assemblage of organisms, not a natural unit of evolution. Nonetheless, over the last century the term coral has come to refer to seven cnidarian taxa that have been defined by Cairns (2007, p. 312) as: ‘Animals in the cnidarian classes Anthozoa and Hydrozoa that produce either calcium carbonate (aragonitic or calcitic) secretions resulting in a continuous skeleton or as numerous microscopic, individualised sclerites, or that have a black, horn-like, proteinaceous axis.’ This is admittedly a cumbersome definition, but is necessarily so in order to include the seven disparate coral taxa, which are listed inTable 2.1. This classification table also gives some common names used for all or part of these seven taxa, as well as the current number of valid Recent species. From this table we see that as of early 2007, there were approximately 5160 species of corals, 65% of which occur in water deeper than 50 m.

Corals are not restricted to shallow-water tropical seas. Of the approximately 5100 coral species alive today over half are found in deep waters. Cold-water corals can be found over a tremendous range of latitudes from tropical to polar regions and from the shallows to the deep sea. We have known about cold-water corals since the mid-eighteenth century, and pioneering oceanographic expeditions in the late nineteenth century frequently recovered cold-water corals in their dredge nets. But only since the 1970s, and particularly in the last ten years, as acoustic survey techniques have improved and been applied to wider areas of the continental shelf, slope, offshore banks and seamounts have we begun to reveal the true extent of cold-water habitats around the world. In this book we try to summarise what we know about cold-water corals and capture the excitement of a field that is now growing exponentially. For instance, a literature search for the terms ‘cold-water coral’ and ‘deep-sea coral’ over the 20 years up to 1996 returned less than 300 publications whereas the same search terms for the following 10 years revealed nearly 700 (see Fig. 1).

The scientific community's fascination with cold-water corals has developed for several reasons. As sessile, suspension feeders that produce complex, sometimes long-lasting, three-dimensional structural habitat they fall at a natural confluence of biology, hydrography and geology. A few species of scleractinian cold-water corals develop elaborate reef frameworks that have spawned many studies into the processes underlying cold-water coral reef and coral carbonate mound formation.