The "Bald Disease" of the Sea Urchin Paracentrotus lividus: Pathogenicity, Molecular Identification of the Causative Agent and Therapeutic Approach

Abstract

In recent decades, various species of Mediterranean sea urchins, including Paracentrotus lividus, have been subject to widespread seasonal episodes of mass mortality whose causative agents are still unclear. In particular, P. lividus is subject to late winter events of mortality, due to a disease manifested by a massive loss of spines and the presence of greenish amorphous material on the tests (i.e., the sea urchin skeleton consisting of spongeous calcite). Documented mortality events show a seasonal epidemic diffusion and might produce economic losses also in aquaculture facilities, besides the environmental constraints to its diffusion. We collected individuals showing conspicuous lesions on the body surface and reared them in recirculated aquaria. Samples of external mucous were collected along with coelomic liquids and cultured to isolate bacterial and fungal strains, further submitted to molecular identification through the amplification of prokaryotic 16S rDNA. In addition, pools of infected sea urchins were reared in recirculated tanks after short baths in a formulated therapeutic compound and their survival rates were compared to non-treated individuals for variable periods. Here, we aimed at a redescription of the etiopathogenetic nature of the parasites and tested the efficacy of a possible treatment, to be proposed for aquaculture purposes.

Keywords: bacteria; bald disease; echinoid necrosis; epidemics; sea urchin.

Figure 1

Percent of diseased individuals found in the samples monthly collected in the bay of Naples. In August no collections were performed, for technical reasons.

Figure 2

Specimens bearing external symptoms of bald disease at various stages. (a) Initial appearance of the disease is indicated by a slight loss of spines in the aboral region. (b) Lateral side contains a first appearance of whitish spots and loss of spines. (c) Extensive loss of spines and enlargement of diseased areas on the test. (d) Appearance of blue spots and initial corrosion of the test on the lateral side. (e) Extensive loss of spines and widening of the green spots. (f) Appearance of green spots and of a corroded area (dark spot) on the test observed under the stereomicroscope at 30×.

Figure 2

Specimens bearing external symptoms of bald disease at various stages. (a) Initial appearance of the disease is indicated by a slight loss of spines in the aboral region. (b) Lateral side contains a first appearance of whitish spots and loss of spines. (c) Extensive loss of spines and enlargement of diseased areas on the test. (d) Appearance of blue spots and initial corrosion of the test on the lateral side. (e) Extensive loss of spines and widening of the green spots. (f) Appearance of green spots and of a corroded area (dark spot) on the test observed under the stereomicroscope at 30×.

Figure 3

Mortality rates recorded in treated (single bath of 15 min) and control specimens, as averages of three replicates of five individuals each. The readings were performed three times during the recovery period, with clean seawater in recirculated tanks. Averages were evaluated on three replicates of five individuals, along with standard deviations among replicates (indicated by vertical bars).