Ontogeny of settlement behaviours in response to Grammatophora marina diatom biofilms in the marine polychaete, Platynereis dumerilii

This study demonstrates that the marine polychaete *Platynereis dumerilii* exhibits developmentally programmed, age-dependent increases in settlement behaviors, such as crawling speed and trajectory straightness, in response to *Grammatophora marina* diatom biofilms.

The Gist

Imagine the ocean floor as a bustling city, and the tiny marine worms called Platynereis dumerilii as babies who have spent their entire lives swimming in the open sky (the water column). Eventually, these babies need to "move out" of the sky, find a house on the ground, and grow up. This big life change is called settlement.

But how do they know where to move? They can't just pick a random spot. They need a signal, like a "For Sale" sign or a friendly neighbor waving them over. In the ocean, that signal is often a biofilm—a slimy, sticky layer of microscopic algae (like Grammatophora marina) that grows on rocks and seagrass.

This paper is like a detective story about how these baby worms grow up, learn to recognize their new neighborhood, and decide when to move in. Here's the breakdown in simple terms:

1. The "Swimming Baby" Phase (Too Young to Care)

When the worms are very young (about 2 days old), they are like toddlers who don't understand the concept of "home." They just swim around in circles, looking for fun.

• The Experiment: The researchers put these babies in a tank with a "slimy" patch (the algae biofilm) and a "plain" patch (just water).
• The Result: The babies didn't care. They swam over the slime just as happily as they swam over the plain water. They were too young to know that the slime meant "food and a good place to live."

2. The "Teenage Discovery" Phase (Learning the Ropes)

As the worms get a little older (around 3.5 to 4 days), they start to grow legs (called parapodia) and muscles that let them crawl. This is like a teenager getting their driver's license; they are ready to explore the ground.

• The Change: Suddenly, they start noticing the slime. When they touch the algae, they stop swimming and start crawling.
• The Behavior: Imagine a person walking through a forest. When they are just wandering (swimming), they take a winding, zig-zag path, looking at everything. But when they find a trail (the biofilm), they start walking in a straighter, more direct line. The older worms did exactly this: their paths became straighter and more purposeful when they touched the algae.

3. The "Adult Decision" Phase (Choosing the Best House)

By the time the worms are 5 or 6 days old, they are fully "competent." They know exactly what they are looking for.

• The Choice: When given a choice between a plain rock and an algae-covered rock, the older worms overwhelmingly chose the algae. They didn't just touch it; they stayed there.
• The Speed: Interestingly, when they were on the algae, they moved slower. Think of it like finding a delicious buffet. When you're hungry and looking for food, you might rush around. But once you find the buffet, you slow down, savor the food, and settle in. The worms slowed down on the algae because they were "savoring" the perfect spot to grow up.

The "Traffic Light" Analogy

You can think of the worm's development like a traffic light system for behavior:

• Red Light (Young Larvae): "Keep swimming! Don't stop for anything." They ignore the biofilm.
• Yellow Light (Mid-Stage): "Slow down and look around." They start touching the biofilm but might still swim away if they aren't sure.
• Green Light (Old Larvae): "Go ahead and park!" They recognize the biofilm, slow down, crawl in a straight line, and commit to staying there.

Why Does This Matter?

The researchers used high-tech cameras and computer software to track these tiny movements, almost like a GPS for microscopic worms. They found that this ability to recognize the "right" neighborhood isn't just luck; it's programmed into their DNA. As they grow, their brains and senses mature, allowing them to switch from "swimming mode" to "crawling mode" at the exact right time.

The Big Takeaway:
Just like humans need to learn when to leave home and find a job, these marine worms have a built-in biological clock that tells them when to stop swimming and start crawling. They use the "slime" of the ocean floor as a map, and as they get older, they get better at reading that map, ensuring they settle in a place where they can thrive. This study helps us understand how the tiny building blocks of the ocean ecosystem find their way home.

Technical Summary

1. Problem Statement

Settlement—the transition from a swimming planktonic larva to a crawling sessile juvenile—is a critical bottleneck in the life cycle of marine invertebrates, dictating ecosystem health and recruitment success. While microbial biofilms are known to act as positive settlement cues, the specific behavioral processes and ontogenetic changes that precede final attachment remain poorly understood.

• Knowledge Gap: Most studies focus on settlement rates after 24 hours of exposure, potentially missing transient behavioral traits (e.g., exploration, speed changes, track straightness) that occur immediately prior to settlement.
• Specific Context: The marine polychaete Platynereis dumerilii is a key model organism for neurobiology and evolution. Previous research established that the diatom Grammatophora marina induces high settlement rates in P. dumerilii, but the mechanism of how larvae detect, interact with, and decide to settle on these biofilms across different developmental stages was unknown.

2. Methodology

The study employed a high-resolution video tracking pipeline to quantify behavioral changes in P. dumerilii larvae across five developmental stages (2 to 6 days post-fertilization, dpf) in response to G. marina biofilms.

• Biological Material:
  • Larvae: Cultured at the University of Exeter. Stages tested: 2-dpf (mid-trochophore), 3-, 3.5-, 4-dpf (mid-nectochaete), and 5-, 6-dpf (late-nectochaete).
  • Biofilm: G. marina (CCAP 1027/1) cultured on autoclaved ACLAR film slips. Negative controls used F/2 media without diatoms.
• Experimental Setup:
  • Imaging: Custom-built infrared (IR) recording stage (850 nm light) to avoid phototaxis interference, using a U3-3680XCP-NIR-GL camera at 20 fps.
  • Assays: Two types of optical cuvettes were used:
    1. Dual-chamber: Separated "slip" (biofilm/control) and "no-slip" regions to analyze local behavior.
    2. Single-chamber: Both slip types present in one chamber to test choice preference.
• Data Analysis Pipeline:
  • Software: FIJI (ImageJ) with the TrackMate 7 plugin for automated spot detection and tracking.
  • Behavioral Metrics:
    • Mode Classification: Tracks were classified as "swimming" (>0.24 mm/s) or "crawling" (≤0.24 mm/s) based on empirical speed thresholds.
    • Tortuosity: Calculated as a measure of track straightness (lower values = straighter).
    • Relative Settled State (RSS): A metric tracking the normalized number of larvae on the slip over time to measure colonization rates.
  • Statistics: Generalized Linear Models (GLM) with beta and Gaussian distributions, ANOVA, and Tukey's HSD post-hoc tests using R.

3. Key Contributions

• Developmental Behavioral Pipeline: Established a robust, automated pipeline for distinguishing swimming vs. crawling and quantifying trajectory metrics in marine larvae, transferable to other species.
• Ontogenetic Shift Mapping: Mapped the precise developmental window where P. dumerilii transitions from a purely swimming, non-responsive state to a competent, biofilm-responding state.
• Decoupling Cues: Differentiated between physical contact cues and potential chemical cues by analyzing behavior in the absence of direct contact (swimming over biofilms) versus direct contact (crawling).

4. Key Results

A. Developmental Progression of Competency

• 2-dpf Larvae: 100% swimming; no crawling observed. Unresponsive to biofilms; tracks remained tortuous (helical) regardless of surface.
• 3-dpf Larvae: Initiation of crawling (12% of time), but no significant preference for biofilms over controls.
• 3.5–6 dpf: Significant increase in crawling proportion. Larvae showed a strong preference for G. marina over controls starting at 3.5-dpf, becoming statistically significant at 4-dpf.

B. Behavioral Metrics on Biofilms

• Crawling Proportion: Increased significantly with age on biofilms. Older larvae (4–6 dpf) were significantly more likely to crawl on G. marina than on control slips.
• Crawling Speed:
  • Crawling speeds decreased on biofilms compared to control surfaces (indicating surface exploration/attachment behavior).
  • However, absolute crawling speed increased with larval age (older larvae moved faster than younger ones).
• Track Tortuosity (Straightness):
  • Swimming: Tortuosity decreased (tracks became straighter) with age, independent of the presence of biofilms.
  • Crawling: Tracks on G. marina biofilms were significantly straighter (lower tortuosity) than on control slips or no-slip areas. This suggests larvae switch from a "searching" (tortuous) mode to an "exploratory/committing" (straight) mode upon finding the cue.

C. Settlement Dynamics (RSS)

• Colonization Rate: Larvae aged 3.5–6 dpf showed a rapid increase in RSS on G. marina slips, indicating they remained on the surface after contact.
• Choice Preference: In single-chamber assays, larvae >3.5 dpf significantly preferred G. marina over control slips. At 4–6 dpf, roughly 50% of larvae settled on the biofilm, while the other 50% remained swimming or unsettled.

D. Mechanism of Induction

• No significant changes in swimming speed or tortuosity were observed before physical contact with the biofilm. This suggests that physical contact (mechanoreception or surface-bound chemical cues like EPS) is the primary trigger for the switch to crawling, rather than volatile chemical cues in the water column.

5. Significance and Conclusion

• Behavioral Repertoire: The study reveals that settlement in P. dumerilii is not a binary switch but a graded ontogenetic process. Competency begins around 3.5 dpf, characterized by a shift from helical swimming to straighter crawling upon contact with specific biofilms.
• Sensory Hierarchy: The results support a hypothesis of shifting sensory priorities:
  • Early stages (1–3 dpf): Prioritize light/pressure (vertical positioning).
  • Competent stages (3.5+ dpf): Prioritize mechanoreception and chemoreception (surface exploration and settlement).
• Evolutionary Insight: The straightening of tracks upon biofilm contact suggests an adaptive strategy to maximize surface exploration area before committing to metamorphosis.
• Methodological Impact: The developed tracking and analysis framework provides a standardized tool for investigating larval settlement behaviors across diverse marine taxa, facilitating future studies on the effects of environmental stressors (flow, temperature) and neurobiological mutants on settlement.

In summary, the paper demonstrates that P. dumerilii larvae possess a developmentally programmed behavioral repertoire that allows them to distinguish suitable settlement cues (G. marina) from non-suitable surfaces via physical contact, leading to distinct changes in movement speed and trajectory straightness that facilitate successful recruitment.