Abstract
The ultrastructure of the digestive gland of several sea hare species that produce different colored ink (Aplysia californicaproduces purple ink,A. julianawhite ink,A. parvulaboth white and purple ink, whileDolabrifera dolabriferaproduces no ink at all) was compared to determine the digestive gland’s role in the diet-derived ink production process. Rhodoplast digestive cells and their digestive vacuoles, the site of digestion of red algal chloroplast (i.e., rhodoplast) inA. californica, were present and had a similar ultrastructure in all four species. Rhodoplast digestive cell vacuoles either contained a whole rhodoplast or fragments of one or were empty. These results suggest that the inability to produce colored ink in some sea hare species is not due to either an absence of appropriate digestive machinery, that is, rhodoplast digestive cells, or an apparent failure of rhodoplast digestive cells to function. These results also propose that the digestive gland structure described herein occurred early in sea hare evolution, at least in the common ancestor to the generaAplysiaandDolabrifera. Our data, however, do not support the hypothesis that the loss of purple inking is a synapomorphy of the white-ink-producing subgenusAplysia.
Highlights
The release of bright purple ink from the ink glands of many species of sea hares (Gastropoda: Opisthobranchia: Anaspidea) is a major component of their defensive arsenal against predators [1,2,3,4]
That released from the ink gland has two major components: aplysioviolin (APV) which is 65% of the dry weight of the bright purple ink released by the best studied ink producing sea hare, Aplysia californica Cooper [6], and a high molecular mass protein that has been named escapin [7, 8]
Rhodoplast digestive cells were found in the digestive glands of all species studied (Figures 1(a), 1(d), 1(e), 1(g), and 1(h))
Summary
The release of bright purple ink from the ink glands of many species of sea hares (Gastropoda: Opisthobranchia: Anaspidea) is a major component of their defensive arsenal against predators [1,2,3,4]. APV is derived from the red algal photosynthetic pigment, r-phycoerythrin (PE), minus its low molecular mass protein This produces phycoerythrobilin (PEB) that is methylated in the ink gland to form APV [6, 7, 9,10,11,12,13]. Like similar high molecular mass protein from related sea hare species, is capable of producing antipredator responses as well as provide both antimicrobial and antitumor activity ([4], see [14] for a review). This ink protein appears to be synthesized in the RER cells of the ink gland [14]. Additional compounds in ink released from the ink gland produce a powerful feeding response, phagomimicry [4, 7, 15]
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