Variable host responses mediate host preference in marine flatworm-snail symbioses

Abstract

Host preference of symbionts evolves from fitness trade-offs. However, it is often unclear how interspecific variations in host response traits influence this evolutionary process. Using the association between the polyclad flatworm Paraprostatum echinolittorinae and its intertidal snail hosts on the Pacific Coast of Panama, we assessed how a symbiont's host preference is associated with varying host defenses and post-infestation performances. We first characterized the prevalence and intensity of worm infestation in five snail hosts (Tegula pellisserpentis, Nerita scabricosta, N. funiculata, Planaxis planicostatus, and Cerithium stercusmuscarum). We then used manipulative experiments to test flatworm's host choice, hosts' behavioral rejection of flatworms, and hosts' growth and survival following the infestation. In the field, flatworms were orders of magnitude more prevalent and dense in T. pellisserpentis, N. scabricosta, N. funiculata than P. planicostatus and C. stercusmuscarum, although the three former hosts were not necessarily more abundant. The results from our laboratory host selection trials mirrored these patterns; flatworms were 3 to 14 times more likely to choose T. pellisserpentis, N. scabricosta, N. funiculata over P. planicostatus and C. stercusmuscarum. The less preferred hosts frequently rejected flatworms via mantle contractions and foot withdrawals, which reduced the infestation rate by 39%-67%. These behaviors were less frequent or absent in the preferred hosts. Flatworm infestation variably influenced host performances in the field, negligibly affecting the growth and survival of T. pellisserpentis and N. funiculata but reducing the growth of P. planicostatus. Flatworms thus preferred less defended hosts that can also support higher worm densities without being harmed. Stable isotope analysis further revealed that flatworms are unlikely to feed on snail tissues and may live as a commensal in their preferred hosts. Our study demonstrates that host response traits can modulate a symbiont's host choice and calls for more explicit considerations of host response variability in host preference research.

Fig 1. Symbiotic flatworm Paraprostatum echinolittorinae (Platyhelminthes: Turbellaria: Polycladida) and its snail hosts.

(A) A flatworm inside the mantle cavity of snail host (Tegula pelllisserpentis), as indicated by a white arrow. Five species of intertidal snails common on the Pacific Coast of Panama including (B) Nerita scabricosta, (C) N. funiculata, (D) Tegula pelllisserpentis, (E) Cerithium stercusmuscarum, and (F) Planaxis planicostatus.

Fig 2. Prevalence and density of flatworm infestation in the field and flatworms’ host choice in the laboratory.

(A) Proportion of snail individuals infested by flatworms in the field, (B) box plot of flatworm infestation density (no. worm per snail) in the field, and (C) box plot of infestation probability (i.e., a proportion of individual worms selecting a particular host species) measured during the host selection experiment. In box plots, mean values for different snail groups are represented by red diamond symbols. Different letters indicate a significant difference based on post hoc multiple comparisons (Holms-corrected P < 0.05).

Fig 3. Host rejection behaviors and flatworm infestation success.

Frequencies of different snail and flatworm responses during experimental infestation. The responses were classified as: the host rejects and repels the worm (red pink), the host rejects the worm, but the worm successfully infests the host (orange), the host does not reject the worm, but the worm does not infest the host (olive green), and the host does not reject the worm, and the worm successfully infests the host (light blue). Snail and flatworm responses were tracked over the allotted time period of 3 minutes using 18 different individuals for each species.

Fig 4. Effect of flatworm infestation on snail growth performances.

Box plots of (A) percentage increase in snail dried tissue mass (DTM) and (B) growth in the shell aperture lip (mm) for Tegula pellisserpentis, Nerita funiculata, and Planaxis planicostatus with (turquoise box) and without (dark orange box) flatworms during the field experiment. Mean values for different treatment groups are represented by red diamond symbols. The asterisk (*) indicates a significant difference (P < 0.05).

Fig 5. Stable isotope analysis of flatworm and snail tissues.

Tissue δ ¹³C and δ ¹⁵N of flatworms and their snail host species: Nerita funiculata (diamond), N. scabricosta (square), and Tegula pellisserpentis (triangle). Different colors represent tissues sampled from flatworm (emerald green), snail gill (light blue), and snail mantle and foot muscle (purple pink). Sample size was n = 4−8 for each snail species × tissue category. Error bars indicate 1 SE.