Secondary metabolome and its defensive role in the aeolidoidean Phyllodesmium longicirrum, (Gastropoda, Heterobranchia, Nudibranchia)

Abstract

Phyllodesmium longicirrum is the largest aeolidoidean species known to date, and extremely rich in terpenoid chemistry. Herein we report the isolation of a total of 19 secondary metabolites from a single specimen of this species, i.e., steroids 1-4, cembranoid diterpenes 5-13, complex biscembranoids 14 and 15, and the chatancin-type diterpenes 16-19. These compounds resemble those from soft corals of the genus Sarcophyton, of which to date, however, only S. trocheliophorum is described as a food source for P. longicirrum. Fish feeding deterrent activity was determined using the tropical puffer fish Canthigaster solandri, and showed activity for (2S)-isosarcophytoxide (10), cembranoid bisepoxide 12 and 4-oxochatancin (16). Determining the metabolome of P. longicirrum and its bioactivity, makes it evident that this seemingly vulnerable soft bodied animal is well protected from fish by its chemical arsenal.

Keywords: Nudibranchia; Phyllodesmium longicirrum; chemical defense; chemoecology; natural compounds.

Figure 1

Secondary metabolites isolated in this study from P. longicirrum.

Figure 2

Structures of secondary metabolites from P. longicirrum as described by Coll et al. in 1985 [13].

Figure 3

Significant ¹H,¹H COSY correlations as found in compound 1.

Figure 4

Secosterols [22,24] related to 3β,5α,6β-trihydroxy-9-oxo-9,11-secogorgostan-11-ol (1) from P. longicirrum.

Figure 5

Conformational structure of 1 (key NOESY correlations are indicated with blue arrows; coupling constants crucial for the determination of the orientation of the 3-OH group are shown).

Figure 6

Structure of cembranoid 5. ¹H,¹H spin systems (A, B and C) are indicated in bold, arrows show key HMBC correlations.

Figure 7

Compound 5 and the most closely related cembranoids from soft corals.

Figure 8

Proposed configuration and selected NOE correlations of bisepoxide 12 (key NOE correlations are indicated with blue arrows).

Figure 9

Structures of bisglaucumlids A–C (23–25).

Figure 10

Proposed configuration of the eastern part (rings B, C and D) of isobisglaucumlides B and C (14 and 15; ROESY correlations are indicated with blue arrows).

Figure 11

Effect of Phyllodesmium metabolites in different concentrations on predation by Canthigaster solandri (n = 8–40, depending on availability). Experiments were repeated twice with compounds 10–12 in all tested concentrations, twice with 16 at 1% and 2% and three times at 0.5%. Mean values with SD are displayed. Significance of deterrence was shown with Fisher´s exact test (P < 0.05 for 10, 12 and 16, calculated for each trial separately). Control pellets were 100% eaten for each trial.

Figure 12

Phylogenetic tree of octocorals relevant as putative food sources for Phyllodesmium spp. Phylogram of Sarcophyton and Lobophytum, based on a consensus phylogram of McFadden et al. [54]. Only the Sarcophyton clade with species investigated with regard to secondary metabolites are given in detail. The two other clades from the original phylogram are only indicated; Lobophytum and the mixed clade consisting of Sarcophyton and Lobophytum species. Numbers indicate the number of specimens that represent the respective branch. Note that the single specimen of S. cherbonnieri groups with 4 specimens of S. glaucum. S. glaucum is not monophyletic, but is represented with several independent clades. The dots at the terminal branches of S. glaucum in the tree indicate that secondary metabolites are described from this species, but it is not known, from which clade. Species in bold indicate that secondary metabolites were described.