Planula - A Sensory Voyage
Josh Pysanczyn
The phylum Cnidaria consists of over 9000 species, found only in freshwater and marine aquatic environments. Cnidarians come in an abundance of sessile and mobile forms, from the deadly Hydrozoan Portuguese man o’ war (Physalia physalis) to the great Anthozoans, the reef builders: corals and sea anemones. Unlike their medusal counterparts, polypoid corals and sea anemones share an evolutionary bond to the reef in their adult stage, which has driven a divergence in their mode of reproduction.
A healthy reef can house a diverse range of cnidarians. Photo: Unsplash.
Corals
Around three-quarters of all zooxanthellate corals – that’s corals that host photosynthetic endosymbiotic algae - are hermaphroditic, the remainder are gonochoric, consisting of separate male and female colonies or, in solitary species, separately sexed individuals (Veron 2000). Both stony (Scleractinian) and soft (Alcyonacean) corals can reproduce asexually, through budding and fragmentation, or sexually through spawning or brooding. To learn more about the different reproductive modes of corals, check out last week’s article ‘Coral 101: Spawning’.
When reproducing sexually, corals release free-swimming planula larvae into the water column. Both brooded and spawned planulae seemingly respond differently to environmental cues. What I would like to do is explore the sensory voyage that coral planulae undertake, discussing the ways in which these mobile genetic units use a variety of different environmental cues to determine a perfect location for settlement.
Let’s take a closer look at corals. Photo: Unsplash.
Sensory Voyage
Depending on the species, and even the geographical variance among species, planulae take on two modes of transport, they can either crawl epibenthically across the surface of the reef (Paz-Garcia et al. 2007) or, in some cases, drift for up to 100 years amongst the plankton! (Richmond 1987).
Though planktonic by nature, planula larvae are densely covered with exterior cilia and associated pores. These are suggested to play a role in several aspects of their settlement dynamics, from increasing mobility within the water column or benthos (Tranter et al. 1982), to the detection of chemical cues from other species (Stömberg el al. 2019). In fact, scientists have now shown that planulae have what is called an apical tuft – a small bundle of cilia which is suggested to not only be related to chemoreception, but also functions like an aircraft propeller, driving the planula forward (Stömberg el al. 2019).
As it turns out, the ultrastructural morphology of planulae may in fact contribute significantly to the assessment of their environment and choice of settlement location. Let’s look at the physical (abiotic) and biological (biotic) cues that planulae use to determine substrate suitability:
Abiotic:
Hydrostatic Pressure
Light
Sedimentation
Temperature
Water Flow
Biotic:
Chemical
Acoustic
To spare you a lengthy essay, I thought I’d focus on three very important cues: Light, Acoustic and Chemical.
Coral reefs are bustling environments and animals in them receive a wide range of sensory cues. Photo: Pexels.
Light
In all zooxanthellete corals, light is essential. The symbiotic algae that reside within each polyp require light to photosynthesis, and in turn translocate up to 95% of their metabolic products to their coral host (Muscatine 1990). It is hardly surprising then, that whilst planulae are driven by the currents, they are shown to actively avoid areas that are exposed to damaging UV radiation (Gleason et al. 2006) and undertake what is called a ‘step-down photophobic response’... basically, they aggregate in lighter areas (Sakai et al. 2020).
Acoustic
Although indeed driven by the currents, there is mounting evidence that planula larvae use acoustic cues to actively navigate their way around the oceans. Planulae are shown to not only detect and swim towards the sounds of coral reefs – even when the sound emanated from above (Vermeij et al. 2010), but also that they prefer louder, healthier reefs (Lillis et al. 2016). Scientists from Exeter and Bristol (UK) are currently investigating the mechanisms by which planulae detect sound in their environment and hope to uncover the full benefits that acoustic enrichment may have regarding coral larvae recruitment on reefs.
Chemical
Once the planulae have discovered a visually and/or acoustically attractive location, they move vertically downwards towards the reef. Here, chemical cues give the larvae precise invitations on where to settle and metamorphosis into the primary polyp. Early studies have shown that chemicals in the insoluble portion of the cell wall of crustose coralline algae (CCA) induce settlement and metamorphosis in stony corals (Morse et al. 1988, 1994; Morse and Morse 1991). However, more recent investigations have highlighted the complexity of this relationship: highlighting different associated chemicals (Kitamura et al. 2007), species-specific interactions (Diaz-Pulido et al. 2010), and the importance of various microbial biofilms on CCA (Webster et al. 2004).
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