Fractal analysis highlights analogies in arenaceous tubes of Sabellaria alveolata (Metazoa, Polychaeta) and agglutinated tests of foraminifera (Protista)

Abstract

Bioconstructions of Sabellaria alveolata (Polychaeta Sabellariidae) from southern Sicily (Central Mediterranean) were sampled and analysed through a multidisciplinary approach in order to unravel the construction pattern of arenaceous tubes and explore possible analogies existing between the worm tubes and the agglutinated tests of benthic foraminifera (Protista). Scanning Electron Microscopy and Energy Dispersive Spectroscopy analyses were carried out on entire tubes as well as sectioned ones. Results show that arenaceous tubes are built following a rigorous architectural framework, based on selection and methodical arrangement of the agglutinated grains, and show surprising analogies with the test microstructure previously observed in agglutinated foraminifera. The grain distribution detected in both model species bioconstructions was analysed using a fractal numerical model (Hausdorff fractal dimension). Collected data show that in both organisms the grains were distributed according to a fractal model, indicating that the evolutionary process may have led to finding the same optimal constructive strategy across organisms with an independent evolutionary history, notwithstanding different geometrical scales. Furthermore, in sectioned tubes we observed microplastic fragments agglutinated within the arenaceous wall and in the inter-tube area. This unexpected finding shows that marine animals can be affected by microplastic pollution not only in soft tissues, but also engineered hard structures, and suggests the problem is more pervasive than estimated so far.

Fig 1. SEM images in secondary electrons of Sabellaria alveolata bioconstruction from site SB.

1. Horizontal view showing the typical honeycomb structure; 2. Vertical view highlighting the internal structure of tubes covered by a smoothed pavement.

Fig 2. Back-scattered electron (BSE) image of aggregated tubes (the same of Fig 1; photo 1) horizontally sectioned at the bottom.

The coloured crosses indicate the spots for standardless microanalyses; the corresponding EDS spectra as well as the elemental maps (silicon-Si in violet, aluminium-Al in yellow and calcium-Ca in light blue) are reported below.

Fig 3. BSE image of aggregated tubes (the same of Fig 2; photo 2) vertically sectioned (6A).

For further details in this figure, see also caption of Fig 2.

Fig 4. BSE images of microplastic particles and compositional spectra found in sectioned tubes from both sites.

Fig 5. Mathematical distribution of the agglutinated grains in Sabellaria alveolata and Karreriella novangliae according to the fractal dimension of Hausdorff.