Abstract :
[en] The echinoderm immune system is based on circulating cells called coelomocytes, some of which are pigmented. Red-pigmented coelomocytes in sea cucumbers have long been assumed to be haemocytes carrying haemoglobin. Our recent work challenges this century-old paradigm. In the European species Holothuria forskali, red coelomocytes contain no haemoglobin; instead, they accumulate carotenoids, leading us to reclassify them as carotenocytes. Interestingly, these cells appear to perform immune functions similar to the naphthoquinone of sea urchin red spherule cells, including reactive oxygen species scavenging and antimicrobial activity. This suggests that analogous immune strategies may exist across echinoderm classes through distinct molecular pathways.
To explore whether this finding extends to other holothuroids, we studied coelomocyte pigmentation across diverse species and ecological niches. Several tropical species (H. notabilis, H. scabra, H. edulis, Pearsonothuria graeffei and Bohadschia atra) exhibit brown-pigmented coelomocytes abundant in the Polian vesicle. High-performance liquid chromatography coupled with mass spectrometry revealed proportions of carotenoids dominated by diatoxanthin or fucoxanthin which may confer golden-brown pigmentation. This contrasts with the Polian vesicle of H. forskali, which contains predominantly reddish carotenoids, namely astaxanthin and canthaxanthin. These results demonstrate that coelomocyte pigment composition varies among sea cucumber species. Further studies are required to determine whether these differences are driven by genetic or ecological factors, such as diet, and to reassess the classification of red-pigmented coelomocytes in other holothuroids. In conclusion, this work highlights unrecognised diversity in echinoderm immune cells and calls for a re-evaluation of the nature and functions of red-pigmented immune cells in sea cucumbers.
