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By cDNA sequencing we have achieved the first, and complete, hemocyanin sequence of a bivalve (Nucula nucleus). This extracellular oxygen-binding protein consists of two immunologically distinguishable isoforms, here termed NnH1 and NnH2. They share a mean sequence identity of 61%, both contain a linear arrangement of eight paralogous, ca.50-kDa functional units (FUs a-h), and in both isoforms the C-terminal FU-h possesses an extension of ca. 100 amino acids. The cDNA of NnH1 comprises 11,090 bp, subdivided into a 5'utr of 75 bp, a 3'utr of 791 bp, and an open reading frame for a signal peptide of 19 amino acids plus a polypeptide of 3389 amino acids (M r = 385 kDa). The cDNA of NnH2 comprises 10,849 bp, subdivided into a 5'utr of 47 bp, a 3'utr of 647 bp, and an open reading frame for a signal peptide of 16 amino acids plus a polypeptide of 3369 amino acids (M r = 387 kDa). In contrast to other molluscan hemocyanins, which are highly glycosylated, the bivalve hemocyanin sequence exhibits only four potential N-glycosylation sites, and within both isoforms a peculiar indel is present, surrounding the highly conserved copper-binding site CuA. Phylogenetic analyses of NnH1 and NnH2, compared to the known hemocyanin sequences of gastropods and cephalopods, reveal a statistically sound closer relationship between gastropod and protobranch hemocyanin than to cephalopod hemocyanin. Assuming a molecular clock, the last common ancestor of protobranch and gastropods lived 494 million ± 50 million years ago, in conformity with fossil records from the late Cambrian.

Measurement of the respiration of Nucula nitidosa and N. nucleus determined that N. nucleus had a respiration rate approximately a third greater than that of N. nitidosa, 215·28 and 135·64 μl O2 gdfw−1 h−1, respectively. This was calculated to be equivalent to a metabolic rate of 0·648 J individual−1 24 h−1 for N. nitidosa and 1·752 J individual−1 24 h−1 for N. nucleus. Estimation of the production of N. nucleus, from its respiration rate, revealed that for comparable populations, N. nucleus was approximately a third more productive than N. nitidosa, 30 kJ g dry flesh weight (dfw)−1 m−2 y−1 as opposed to 20 kJ gdfw−1 m−2 y−1. Examination of the Kleiber's constant (β) obtained for each species, demonstrated that for N. nitidosa β fell in the range 0·75–1 and that for N. nucleus β fell in the range 1–1·25. This suggests, in combination with other data, that N. nucleus adopts an ‘exploitative’ functional strategy as opposed to N. nitidosa, which can be regarded as adopting a ‘conservationist’ functional strategy. Observations on the hypoxic tolerance of both N. nitidosa and N. nucleus revealed that N. nucleus had a hypoxic tolerance about twice that of N. nucleus. The mean survival time±standard error for N. nitidosa was 3·53±0·18 d in contrast to 7·72±0·21 d for N. nitidosa. The hypoxic tolerance of either species was not related to body size and was independent of any possible effects of starvation. These results are discussed with reference to their potential effects to determine the distribution of N. nitidosa and N. nucleus.

Ecological speciation probably plays a more prominent role in diversification than previously thought, particularly in marine ecosystems where dispersal potential is great and where few obvious barriers to gene flow exist. This may be especially true in the deep sea where allopatric speciation seems insufficient to account for the rich and largely endemic fauna. Ecologically driven population differentiation and speciation are likely to be most prevalent along environmental gradients, such as those attending changes in depth. We quantified patterns of genetic variation along a depth gradient (1600-3800m) in the western North Atlantic for a protobranch bivalve (Nuculaatacellana) to test for population divergence. Multilocus analyses indicated a sharp discontinuity across a narrow depth range, with extremely low gene flow inferred between shallow and deep populations for thousands of generations. Phylogeographical discordance occurred between nuclear and mitochondrial loci as might be expected during the early stages of species formation. Because the geographic distance between divergent populations is small and no obvious dispersal barriers exist in this region, we suggest the divergence might reflect ecologically driven selection mediated by environmental correlates of the depth gradient. As inferred for numerous shallow-water species, environmental gradients that parallel changes in depth may play a key role in the genesis and adaptive radiation of the deep-water fauna.

The adoral sense organ in bivalve molluscs is a paired ridge of specialized epithelium positioned laterally at the base of the labial palps near the mouth opening, clearly distinguishable from the surrounding epithelia. Six species of the protobranch order Nuculoida, and one species of Solemyoida, were investigated by light microscopy concerning presence, gross anatomy, and innervation of the adoral sense organ. The organ was described in detail for Nucula nucleus and N. nitidosa by transmission electron microscopy; in these two species, the organ was characterized as a pseudostratified epithelial thickening with specialized cells bearing a specialized microvillar border and a basal matrix with a lamellar layer. Three types of bipolar primary receptor cells were recognized and these were reconstructed for N. nucleus. Most of the receptor cells had 2 cilia orientated parallel to the cell surface; in addition, there were 2 types of supporting cells and 1 type of basal cell. The surrounding epithelial cells were narrow with short microvilli and lacked cilia. The homology of the organ within protobranch bivalves was suggested by a character-complex of: (1) position, (2) dimension of the epithelium, (3) innervation, (4) pseudostratified construction, (5) dimension of the specialized microvillar border, (6) thickening of the basal matrix, and (7) presence of specialized cell types such as receptor cells. Despite the estimated high number of protobranch species there is only scant information available on the adoral sense organ from 24 species of 8 genera. Structures and receptor types that are similar to those found in the adoral sense organ are widespread in molluscs and other invertebrate groups; this may indicate a plesiomorphy of these characters rather than an apomorphy for the protobranch clade. Therefore, the adoral sense organ may be of minor phylogenetic value above the level of protobranch orders.