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During a recent expedition to Ascension Island in the southern Atlantic Ocean, a polyplacophoran of the genusAcanthochitonawas collected, corresponding to the first recorded species of this molluscan class from this remote island. A scanning electron microscope study reveals a close affinity toAcanthochitona subrubicunda, which was so far only known from the Cape Verde Archipelago. Due to the lack of comparable type material the new finding warrants a confirmed identification and is only tentatively attributed toAcanthochitona subrubicunda. It is hypothesized that the class Polyplacophora was either overlooked during earlier surveys or colonized the island in more recent time. If the latter scenario took place, a transport by ships is thinkable.

Polyplacophoran molluscs (chitons) are phylogenetically ancient and morphologically constrained, yet multiple living species are often found co-occurring within widely overlapping ecological niches. This study used two sets of experiments to compare interspecific variation among co-occurring species in the North Atlantic (Ireland) and separately in the North Pacific (British Columbia, Canada) chiton faunas. A complementary review of historical literature on polyplacophoran physiology provides an overview of the high level of metabolic variability in this group of ‘living fossils’. Species examined in de novo experiments showed significant variation in oxygen consumption both under air-saturated water conditions (normoxia), and in response to decreasing oxygen availability (hypoxia). Some species demonstrate an ability to maintain constant oxygen uptake rates despite hypoxia (oxyregulators), while others oxyconform, with uptake rate dependent on ambient oxygen tension. These organisms are often amalgamated in studies of benthic communities, yet show obvious physiological difference that may impact their response or tolerance to environmental change.

The present study describes the species composition and vertical distribution of chitons in tide pools and on exposed rock areas of the intertidal zone of Samara, Guanacaste, Pacific Costa Rica. Nine different species of chitons were recorded, and their densities and sizes were determined using quadrate sampling. Physical (period of emersion, temperature and salinity range) and biotic (presence and diversity of food resources) factors were examined to assess their influence on distribution and abundance of each species.Ischnochiton disparwas the predominant species with densities of up to 200 ind m−2, representing 85.4% of all sampled individuals (N = 4193). The size ofI. disparwas positively correlated with the size of the inhabited boulders. Two species (I. disparandStenoplax limaciformis) showed segregation in size:I. disparbeing negatively andS. limaciformispositively correlated to the distance from the shore. Two of the nine species (Chiton stokesiiandAcanthochitona hirundiniformis) occurred mainly on rocky areas outside the tide pools with variable amounts of exposure to air during each tidal cycle. In both species the tide pool specimens were significantly smaller than those collected in the rocky areas. Distribution of all other species was restricted to the tide pools and species had their peak abundance in the zones with more moderate conditions and a higher diversity of algae, resulting in the highest chiton diversity in these areas.

A cold-shock technique was used to determine radula production rates for the chitonsAcanthopleura hirtosaandPlaxiphora albida, and for the limpetPatelloida alticostata, which replaced their radular teeth at rates of 0.40, 0.36 and 0.51 rows d−1, respectively. These rates are far slower than those determined previously for non-iron-mineralizing molluscs, suggesting that the improved working life of the teeth afforded by iron-mineralization acts to significantly reduce replacement rates. In addition, inductively coupled plasma-atomic emission spectroscopy has been used to determine the quantity of iron and other elements comprising the radula of each species. These data, used in conjunction with the radula production rates, reveal thatA. hirtosa,Plaxiphora albidaandPatelloida alticostatahave daily radula mineralization requirements for iron of 3.06, 4.12 and 0.55 μg, respectively. Such information is vital for continuing studies related to the cellular delivery of ions and subsequent biomineralization of the tooth cusps in chitons and limpets.

Iron-containing precipitates, present within the organic matrix of the poriferanIrcinia oroswere identified as crystalline lepidocrocite (γ-FeOOH) by electron diffraction and57Fe Mössbauer spectroscopy. The crystals were located specifically within the spongin filaments, suggesting that the binding and subsequent accumulation of iron by macromolecules present in these filaments were responsible for mineralization. Iron biomineralization did not appear to serve any specific structural role, but may be involved in biological processes such as detoxification.It is well recognized that Porifera are active in the biomineralization of calcium carbonate and silica, producing beautiful and intricate skeletons (Simpson, 1984). However, the formation of other biominerals is less well documented. Iron biomineralization has been reported in a few species of keratose sponges (Töwe & Riitzler, 1968; Vacelet et al., 1988), and the mineral has been identified as lepidocrocite (γ-FeOOH). Lepidocrocite is a relatively uncommon biomineral, having previously been identified as only a minor phase in chiton teeth (Webb et al., 1989; Lowenstam & Weiner, 1989).

Valve structure was investigated using light and scanning electron microscopy and each valve was found to comprise four layers; periostracum (dorsal), tegmentum, articulamentum and hypostracum (ventral). The penetration of the valves by the aesthetes within channels is described and three distinct types of aesthete channels are identified, namely, multiple branch aesthete channels, slit ray aesthete channels and jugal sinus aesthete channels. The distribution of the different aesthete channels within the various valves is defined and their relationship with the valve external sculpturing is described. The process of valve growth by marginal increment is discussed in relation to the formation of the aesthetes.

The description of a new species of the genus Chrysochromulina, C. chiton, includes for the first time some evidence from the electron microscopy of sections in addition to the techniques previously used. Important diagnostic characters are the exceptionally large scales, which are shown to be of two kinds, the larger being saucer-shaped, and the smaller plate-like with a narrow rim flexed towards the upper surface; the structure and arrangement of the two types of scale on the body are described. Anatomical facts are given for the first time for the internal structure of the haptonema; in this species this organ consists of three concentric membranes surrounding a ring of seven fibres or tubes, the centre of the haptonema being hollow; this combination of characters distinguishes the organ fundamentally from a flagellum. Microanatomical facts are also given for the following major cell organs: the nucleus, the chromatophores including the pyrenoids, mitochondria, putative golgi material, muciferous bodies, the flagellar bases (preliminary observations only). Other distinguishable cytoplasmic components include the surface membrane, unsaturated fat bodies, vesicles of various kinds and granular protoplasm in the interstices.

Chrysochromulina minor and C. kappa have been re-investigated by means of electron microscopy of thin sections to add details of the microanatomy of pyrenoids and haptonemata, and by anoptral contrast light microscopy to study pyrenoids in living cells. In both species the pyrenoid is in the form of a diverticulum projecting from the centre of the inner face of a plastid and, in C. minor, strongly flexed to lie along it. In C. kappa the pyrenoid is commonly enveloped by the nucleus which may conceal it entirely from view in life. Some details of the behaviour of the surface membranes of plastids and pyrenoids in relation to that of the nucleus are given. The haptonema structure in both species is shown to be comparable to that of others in which this appendage is much longer, though an occasional variant with eight instead of seven central fibres or tubes has been encountered in C. kappa and is demonstrated. The presence of ‘peculiar’ Golgi structure is reported for both species and demonstrated for C. kappa. Some direct evidence indicating an internal origin of scales from vesicles is demonstrated in C. minor. Finally a summary is given of salient structural criteria for all the described species attributed to this genus from the marine plankton, the closest agreement as regards pyrenoid structure in the two species under investigation being with C. chiton.