Chapter 18. Razorfish and scallops CHAPTER 18. RAZOR FISH AND SCALLOPS ALAN BUTLER CSIRO Marine and Atmospheric Research, Hobart, Tasmania 7001. Email: [email protected] Figure 1. Razorfish Pinna bicolor near seagrass, Posidonia sp. The individual in foreground shows some recent, rapid growth at the posterior margin of the shell and an epifauna mainly of serpulids but with a gastropod, a few bryozoans, and a small colony of a didemnid ascidian beginning to overgrow the bryozoans. The shell in the background is dead, may have been there for a decade, and has a well-developed epifauna, mainly of bryozoans with a couple of sponge colonies. Introduction This chapter outlines some ecological studies of bivalve molluscs in Gulf St Vincent (GSV) waters. It is eclectic, not attempting to cover all studies of bivalves, but mainly those of several large species—the ‘razor fish’ Pinna bicolor, and the scallops Mimachlamys asperrima and Equichlamys bifrons—which are conspicuous, and have proven instructive not only for understanding the ecology of these sorts of animals (sessile filter-feeders, with broadcast spawning, external fertilisation and a long larval period), but of the dynamics of the Gulf ecosystem and of the relationships between species. A vast amount has been written about the ecology of bivalves worldwide (e.g. Wilbur & Yonge 1964- 1966; Bayne 1976; Yonge & Thompson 1976; Purchon 1977; Wilbur 1983-1988; Suchanek 1985; Ludbrook & Gowlett-Holmes 1989; Beesley et al. 1998), and no attempt is made to summarise it here. 238 Chapter 18. Razorfish and scallops Razor fish ‘Razor fish’ are bivalve molluscs, not fish. The Pinnidae—pen shells, fan shells, wing shells, and commonly known in South Australia (SA) as razor fish—is a tropical and subtropical family (Butler 1998), only a few species occurring in temperate zones. Two, Pinna bicolor Gmelin and the less common Atrina tasmanica (Tenison Woods), occur in Gulf St Vincent (GSV). I discuss only the former here, because we have very little information on the latter. Pinna bicolor thrives in the SA gulfs, but, like a number of other subtropical organisms (mangroves, blue swimmer crabs), is not found much further south. Razor fish have a triangular or fan-shaped shell, up to 45 cm long, and live with the pointed, anterior end buried in the sediment and attached by byssus to stones and shell fragments, with the posterior gape at or above the surface of the sediment (Fig. 1). When they protrude above the surface, razor fish are quite conspicuous at low tide or to a diver in the subtidal. When the posterior margin is at sediment level, however, there is a risk of cutting your bare feet on the sharp edge of the shell—hence the name ‘razor fish’—this is not one of the ensid bivalves called ‘razor shells’ or ‘razor clams’ elsewhere. Scallops The family Pectinidae (scallops) are the familiar, disc-shaped, fairly symmetrical bivalves with a straight, usually “winged” hinge. Some are free-living, some attached by byssus either when young or throughout life, and some are cemented to a hard substratum or embedded in coral. Members of the family occur from shallow depths to 7 000 m and they extend from the equator to polar regions, though they are best represented in warmer waters. Most have separate sexes, and the free-living species are noted for swimming by clapping the valves with most of the mantle opening sealed, and expelling water near the hinge—i.e., they swim by jet propulsion (Beesley et al. 1998). Three species of scallops are (or were) common in the Gulf—the doughboy scallop Mimachlamys asperrima (Lamarck), the queen scallop Equichlamys bifrons (Lamarck), and Pecten fumatus Reeve. Pecten fumatus is the commercial scallop of southern Australia, and has been discussed by Olsen (1955) and Dix & Sjardin (1975). Another species of scallop (Mesoplepum tasmanicum) can be found beneath rocks, for example at Edithburgh Jetty. I discuss only the first two here. Distribution The distribution of Pinna bicolor in SA was described by Butler & Keough (1981), who reported that it is ‘patchy’ on large and small scales—they can be locally very abundant, but are also absent from apparently suitable sites, and within a site will be found in localised patches that change with time, apparently due in part to spatially patchy recruitment (Butler 1987). They are, however, widespread on sheltered shores or in deeper water (from a little above low-water down at least to 20 m). Within GSV, P. bicolor was recorded at many sites between 1964 and 1969 by Shepherd & Sprigg (1976). Their survey was revisited in 2000-2001 by Tanner (2005), who found Pinna at fewer sites. In particular the ‘Malleus-Pinna assemblage’ identified by Shepherd & Sprigg (1976) in the SE section of the Gulf was essentially missing in 2000-2001, only scattered individuals remaining. Pinna distribution remained relatively unchanged in the northern part of the Gulf. The doughboy scallop, M. asperrima, is widespread in the Gulf. They live attached by byssus to hard substrata such as rock, jetty pilings and notably the shells of Pinna bicolor (Fig. 2). They are commonly found in tight aggregations of up to 50 individuals. Typically, each individual has a coating of sponge on the shell, discussed further below. M. asperrima is not entirely sessile; sequential photographs show that they move, partly by progressively re-attaching the byssus (as mussels do) but also by breaking the byssus and swimming, which they will do when disturbed. Given their frequent attachment to the shells of Pinna or to other hard surfaces that elevate them from the seafloor, and noting from Tanner (2005) that many areas formerly occupied by Pinna and Malleus had become bare sand, it seems possible that the distribution and abundance of M. asperrima have been reduced along with those of Pinna (above). Queen scallops, E. bifrons, (Fig. 3) are medium-sized (up to 130 mm shell length), and found free-living on soft sediments at depths of 2-50 m. They will swim when disturbed, much more readily than M. asperrima. In many places, both species occur together. Equichlamys bifrons has been historically 239 Chapter 18. Razorfish and scallops widespread and abundant in the Gulf (Shepherd & Sprigg 1976) but its distribution and density seem to have been reduced in the past 30 years. Both Shepherd & Sprigg (1976) and Tanner (2005) refer only to ‘scallops’ but from my observations this must refer primarily to E. bifrons with an unknown but lesser proportion of P. fumatus. Tanner (2005) found that the abundance of scallops in the central eastern section of the Gulf had decreased markedly since Shepherd & Sprigg’s (1976) survey—from densities of 0.5-4 m-2 to densities far below 1 m-2. Only at a few locations near the western shore of the Gulf and one in the south-east were they still abundant. The Ecology of Razor Fish Pinnidae are unusual amongst bivalves and have some interesting adaptations. They have basically an ‘epifaunal’ bivalve design—like a mussel, adapted to living attached to a hard surface—but have adopted an ‘infaunal’ habit, living in soft sediment; this is associated with adaptations such as their unique mechanisms for clearing sediment from the mantle cavity (Butler 1998). Pinna bicolor is widely-distributed in the Indo- Pacific, from the equator to the latitude of southern Australia. Published work on its ecology comes from the warm temperate regions of southern Australia (Rosewater 1961; Shepherd & Sprigg 1976; Butler & Brewster 1979; Butler & Keough 1981; Butler 1998). It occurs in the Gulf from the intertidal to 20 m, but is locally patchy; typical densities are 2-5 m-2. Clearly from our observations, Pinna must be understood as existing in metapopulations (i.e. patchy systems with more or less dispersal between patches; e.g. Sinclair 1988; Hanski 1999). In such a system the nature of the dispersal between patches is crucial to understanding the dynamics within any patch as well as the whole population system (see Kritzer & Sale 2006). Pinna bicolor reaches sexual maturity by 1+ yr (Butler 1987). The sexes are separate and gonads mature seasonally (Butler & Keough 1981; Roberts 1984; Butler 1987). Pinna have external fertilisation—eggs and sperm are simply released into the water. For a spawning animal the chances of successful fertilisation must depend on the proximity of other Pinna spawning at the same time, as well as on other factors, especially water movement. This has been studied in scallops in SA (see below). We have no data on fertilisation success for Pinna, but it seems likely that a failure of fertilisation would become a significant problem when a population becomes sparse. They are commonly clumped (e.g. Fig. 4). This might be important in areas such as the SE section of the Gulf, where Tanner (2005) found the former ‘Malleus-Pinna assemblage’ had disappeared and that only few, scattered Pinna remained. Following fertilisation, a veliger larva develops. It will presumably be denser than water and will sink unless it swims actively. Many Pinnidae, including P. bicolor, have teleplanic larvae, capable of being transported for long distances (Scheltema 1977; Scheltema & Williams 1983) but this does not imply that they always are; the patterns of currents, and larval behaviour, could limit the distance travelled. In addition to dispersal itself, the feeding and mortality of larvae are important, and virtually unknown in Pinnidae. Food supply may be crucial for a plankton-feeding larva and, in the habitat of P. bicolor, nutrients are limited and plankton not generally very dense. There are, however, variations in plankton density and these may be important in determining the survival of larvae. Thus, a combination of limited, irregular dispersal and variable larval survival may explain what we clearly do know for this species, namely that recruitment to local populations is highly variable (Butler & Keough 1981).