Species-specific metabolites mediate host selection and larval recruitment of the symbiotic seastar shrimp

Abstract

In marine environments, host selection, defining how symbiotic organisms recognize and interact with their hosts, is often mediated by olfactory communication. Although adult symbionts may select their hosts detecting chemosensory cues, no information is available concerning the recruitment of symbiotic larvae which is a crucial step to sustain symbioses over generations. This study investigates the olfactory recognition of seastar hosts by adult Zenopontonia soror shrimps and the recruitment of their larvae. We examine the semiochemicals that influence host selection using chemical extractions, behavioural experiments in olfactometers, and mass spectrometry analyses. After describing the symbiotic population and the embryonic development of shrimps, our results demonstrate that asterosaponins, which are traditionally considered as chemical defences in seastars, are species-specific and play a role in attracting the symbiotic shrimps. Adult shrimps were found to be attracted only by their original host species Culcita novaeguineae, while larvae were attracted by different species of seastars. This study provides the first chemical identification of an olfactory cue used by larvae of symbiotic organisms to locate their host for recruitment. These findings highlight the importance of chemical communication in the mediation of symbiotic associations, which has broader significant implications for understanding the ecological dynamics of marine ecosystems.

Figure 1

Description of the model organisms investigated and localization: (A)  map of Mo’orea Island highlighting the collection area (white rectangle); (B) zoom of map (A) showing the precise collection site of Culcita novaeguineae in red, and the collection area of Acanthaster planci in orange. (C–E) Different morphotypes of the symbiotic shrimps Zenopontonia soror, with a white arrow highlighting the symbionts. (F) Zoeal larval stage of Z. soror. (G) The host C. novaeguineae in its environment and (H) A. planci on a coral colony. (A,B) Modified from ©Google Earth, version 9.181.0.1. Scale bars represent 0.2 km in (A); 1.6 km in (B); 0.5 cm in (C); 0.6 cm in (D; 0.5 cm in (E); 100 µm in (F); 10 cm in (G,H).

Figure 2

Description of the symbiotic population and the embryonic development of Zenopontonia soror: (A) occurrence (%) and symbiotic load of seastar shrimps per host individual. (B). Two symbiotic Z. soror in association with their host Culcita novaeguineae: one transparent shrimp close to the mouth (black arrow) and one coloured morphotype was observed inside the mouth (white arrow). (C)  Developmental cycle of the shrimp Z. soror. I. Pigmented egg that represents either an unfertilized egg or one of the first development stages. II. Pigmented egg with a large quantity of yolk and larger blastomeres. III. Pigmented egg with a beginning of embryo development (white arrow). IV. Egg with a slight growth of the embryo. V. Pigmented egg with embryo development presenting a discernible antero-posterior axis. VI. Pigmented egg presenting a black vesicle corresponding to the development of an eye (white arrow) and decrease in the quantity of yolk. VII. Advanced developed embryo showing a development of the eye, segmented body and a reduction of the pigmented yolk. VIII. Developed embryo with very little yolk content and developed body and complex eyes displaying ommatidia. IX. First zoeal larval stage. X. Adult gravid Z. soror with eggs (white circle). Scalebar represents 0.5 cm in (B); 140 µm for C I to − C IX and 700 µm in (C) X.

Figure 3

Relative abundances (mol-%) of the asterosaponins of the three potential host seastars: (A) Culcita novaeguineae, (B) Acanthaster planci and (C) Linckia laevigata.

Figure 4

Drifting time of Zenopontonia soror larvae test under different chemical cues: (A) Z. soror larvae drifting time (s) inside the olfactometer device during the different experimentations. For each experiment, the red spot corresponds to the meantime values and the red line represents the standard deviation. The grey dots represent the measures of the larvae drifting time, with small random horizontal shifts to visually separate them. The number n above each box represents the number of larvae observed in each experimentation. All experiments are abbreviated as the following (i) WXW: seawater VS seawater (negative control); (ii) CCXW: C. novaeguineae conditioned water VS seawater; (iii) SCXW: Saponins from C. novaeguineae VS seawater; (iv) SAXW: Saponins from A. planci VS seawater. (B) Results of the multiple comparisons of mean (Tukey) test. Significant results (P value, 0.01) are underlined and bolded.

Figure 5

Summary diagram of the mechanisms involved in host recognition, host switch and larval settlements in the symbiotic association between Culcita novaeguineae and Zenopontonia soror. Scale bars represent 110 µm in A; 3,6 cm in B; 250 µm in C, D and F; 2 cm in E and G.

Figure 6

Experimental devices used in behavioural experimentations: (A) olfactometer used for adults Z. soror. (B) low-flow olfactometer used for Z. soror larvae. C1 chemical cue 1, C2 chemical cue 2, M mix of both cues, MZ monitoring zone, P peristaltic pump, V valve composing the valve regulation system, E exit. (C) Real experimental device used for adult Z. soror. (D) Zoom of the Y-tube (corresponding to the monitoring zone) used for adult Z. soror, with the visualization of the flow laminarity using fluorescein and (E) real low-flow olfactometer used for Z. soror larvae, with the visualization of the flow laminarity using food colourants. MZ = monitoring zone. Scale bars represent 13.3 cm in (C); 3.3 cm in (D); 5 cm in (E).