Abyssal foraminifera attached to polymetallic nodules from the eastern Clarion Clipperton Fracture Zone: a preliminary description and comparison with North Atlantic dropstone assemblages

Abstract

We examined encrusting organisms on seven polymetallic nodules from the eastern Clarion Clipperton Fracture Zone (CCFZ, ∼4070 m water depth, eastern equatorial Pacific). Apart from occasional sponges and a single bryozoan, all the organisms were foraminifera or foraminifera-like protists. A total of 75 morphotypes (presumed to be morphospecies) was recognised, with between 9 and 19 being present on individual nodules. Additional species were observed during shipboard examination of the nodules, bringing the total number of species to 86. The assemblage was dominated by a variety of mat-like formations, clusters of patches, isolated domes, broad trails, anastomosing networks and branched or unbranched tubular structures that either lay flat against the nodule surface or projected away from the surface. These forms were interpreted as monothalamous foraminifera (monothalamids). Most have mainly agglutinated walls but a few are predominately organic. Some can be assigned to the Komokiacea (notably the genus Chondrodapis) or families such as the Hemisphaeramminidae ('domes'), while others (e.g. many of the mats and patch-like forms) are difficult to place into existing monothalamid groupings. Some of the branching and anastomosing tubes resemble the genus Rhizammina. The most easily recognisable morphotypes include Telammina, in which tiny chambers are linked by extremely thin tubes to form a network, and sinuous orange tubes that incorporate sponge spicules and can be assigned to the genus Saccorhiza based on the occasional presence of a proloculus. Polythalamous foraminifera are also fairly common. They include various calcareous species (mainly Cibicides spp.), as well as agglutinated forms such as Hormosina, ?Placopsilina and trochamminaceans. Similar assemblages, including some morphospecies that are clearly identical to ours, have been described previously from somewhat deeper sites (4500–5000 m) in the CCFZ. In order to explore distributions at a global scale, we compared our Pacific nodule assemblages to foraminifera attached to ice-rafted dropstones from several deep seamounts in the abyssal northeast Atlantic Ocean, mainly on the Porcupine Abyssal Plain (4630–4680 m depth) with additional material from the BIOTRANS area (3796–4351 m). These hosted superficially similar assemblages of mats, tubular forms, komokiaceans and polythalamous calcareous and agglutinated foraminifera. However, apart from the sometimes extensive development of Telammina networks, there were no morphospecies in common between the Pacific and Atlantic assemblages. These preliminary observations, based on limited material, suggest that most of the foraminiferal morphospecies encrusting hard substrates are widely distributed at regional scales in the abyssal Pacific but not necessarily at global scales. The study of abyssal encrusting assemblages poses considerable challenges. Priorities for the future include the development of reliable methods for distinguishing living and dead individuals, and molecular approaches to clarifying the taxonomic affinities of novel morphotypes.