Pseudoikedella achaeta ( Zenkevitch, 1958 )

Saiz-Salinas, J. I., 2007, Sipunculans and echiurans from the deep Angola Basin, Journal of Natural History 41 (45 - 48), pp. 2789-2800 : 2795-2796

publication ID	https://doi.org/ 10.1080/00222930701782187
persistent identifier	https://treatment.plazi.org/id/03BF363F-8431-FFA2-6989-E402F67FE404
treatment provided by	Felipe
scientific name	Pseudoikedella achaeta ( Zenkevitch, 1958 )
status

Treatment

Pseudoikedella achaeta ( Zenkevitch, 1958) View in CoL

Material. Sta. AT no. 319, one; Sta. AT no. 327, one; Sta. EBS no. 344, two juveniles; Sta. EBS no. 350, one .

Description. Trunk 9–30 mm long and 3–6 mm wide. Proboscis missing. Ventral setae absent. Opaque and contracted in the anterior part of the trunk, whereas translucent in the posterior part. Internally about 20–30 bands of longitudinal musculature could be observed in the contracted part of the trunk. A single gonoduct with terminal gonostome. Paired anal vesicles long, thin, and tubular without ramifications.

Remarks. The general appearance of the worms, and especially the single gonoduct with a terminal gonostome, show these specimens belong to Pseudoikedella achaeta . Most of the previous records of the species came from the North Pacific and the Tasman Sea at depths of 3450–5540 m ( Zenkevitch 1958, 1966; Zenkevitch and Murina 1976). Murina (1978) and Saiz Salinas et al. (2000) reported further specimens from Antarctic waters at 1300– 3490 m. The species has been recently recorded in the North Atlantic by Biseswar (2005), therefore this finding is an extension of its range towards the deep waters of the South Atlantic.

Gradient analysis

All ordination methods and their constrained counterparts were performed by using the CANOCO package with the species list and the four abiotic variables investigated (working zone, type of dredge, depth of the samples, cryptic life inside tubes). Methods based on the linear response model ( PCA and RDA) perform better than those that rely on the unimodal response ( CA, CCA). RDA is especially recommended when the samples are representative of a narrow environmental gradient and taxa appear to respond in a linear fashion to the environmental gradients. The first two RDA axes capture 52% of the cumulative variance of the faunal data, a slightly larger proportion of the variance than is explained by both CCA and detrended CCA. Forward selection and unrestricted Monte Carlo permutation tests indicate that only two ( WZ, dredge) of the four abiotic variables previously introduced make statistically significant contributions to explaining the variance in the faunal data. The most significant of these is WZ, which captures 34% of the total variance explained by the whole set of abiotic variables, followed by type of dredge (22%). A Monte Carlo permutation test of the trace statistics, which gives an overall test of the effect on the faunal data of the two variables selected as active, resulted in a P value of 0.16 .

The result of RDA, incorporating the two forward-selected abiotic variables, is given in Table II and illustrated in Figure 2 View Figure 2 as a sites–indicator taxa–abiotic triplot. Two neat latitudinal gradients ( EBS and AT gradients) from the right to the left of the plot are revealed for each type of dredge used in this faunal survey. The first abiotic variable, working zone, arranges all the sampling sites from the south to the north of the Angola Basin, whereas the type of dredge separates the fauna well according to the different sampling procedure. The importance of the two forward-selected abiotic variables is shown by their corresponding long vectors in the plot of Figure 2 View Figure 2 .

Simultaneous plots of species in the ordination diagram of Figure 2 View Figure 2 reflect their preferences with respect to sampling sites and abiotic vectors. From this plot prospective indicator taxa could be identified that might be sensitive to some sampling sites. Again, the length of the species arrows indicates the importance of the species in the final faunal survey. Thus, most species move along axis 1, such as Nephasoma diaphanes , N. flagriferum , Apionsoma murinae , and Onchnesoma steenstrupii steenstrupii . The reasons are different, for example the first species, N. diaphanes , is ubiquitous throughout all WZ, but locally abundant in the sampling sites of WZ VI. Nephasoma flagriferum is only present in WZ VI, whereas Apionsoma murinae was only collected at different sites of WZ IV, V, and VI. Onchnesoma steenstrupii steenstrupii also has a long arrow in the final plot of Figure 2 View Figure 2 , but along the positive part of axis 1. This indicates its preference for sampling site no. 318 which was sampled by the EBS dredge. Only N. abyssorum abyssorum showed preferences along the positive part of axis 2 ( Figure 2 View Figure 2 ), indicating its ubiquitous presence in all WZ with local abundant peaks at some sampling sites of WZ III, IV, and V. Echiuran species are quite rare and occasional in the faunal survey and do not exhibit any indicator value in the final plot.

Spearman’s rank correlation coefficients and Type I error probabilities between the spatial distribution of the species identified and the abiotic variables investigated are presented in Table III. Only two abiotic parameters were significantly correlated with relatively few species of the faunal survey. WZ is significantly correlated with those species that move along the negative part of axis 1 in the ordination plot, suggesting the existence of a latitudinal gradient across the Angola Basin. On the other hand, only one species of sipunculan, Nephasoma diaphanes , is positively correlated with the availability of horny void tubes of polychaetes, which are remarkably abundant in the sampling sites of WZ VI .