By Michael Yue
Imagine walking along the warm sandy bleach from the coast of Washington – with every step, your feet sink into the sand, feeling the cool tingly grains between your toes. However, you step into something slimy. You immediately move away from the source – anticipating the sharp agonizing stings of a sea jelly. But seconds later, with yours eyes closed with fear; there was nothing – no pain at all. What was it? As you look closer, there laid a glop of goo; the strange organism was small, cylindrical, and transparent. The gelatinous form resembled a sea jelly, but clearly it lacked tentacles and stinging cells. What is this strange organism? It is a salp! What in the world are salps?
Salps are barrel-shaped, planktonic tunicates from the family, Salpidae. These gelatinous and translucent organisms are common in equatorial, temperate, and/or cold seas, living near the surface1. They move through the water by contraction of their bodies, creating vortices for jet propulsion2. For obtaining nutrients, they capture tiny plankton from suspension feeding3. Remarkably, salps are quite small as individuals vary from only micrometers to centimeters in dimension! These transparent creatures are able to make beautiful salp chains within the sea during the colonial phase of their life cycle, however. Salps are sometimes mistaken as sea jellies by people, which may be due to the similar appearance to a sea jelly because of their simple and transparent body form4.
Interestingly, salps are structurally more closely related to vertebrates, than they are to sea jellies4.Salps are very important for evolutionary inferences of early vertebrates. Salps appear to have a form ancestral to vertebrates. Because of this, they are used by scientist as an evolution model for primitive vertebrates. Scientists speculate that their primitive nervous system gave rise to the central nervous systems of an ancestral vertebrate4.
Even though salps may appear rather simple (or even like a glop of goo), these remarkable invertebrates can display mesmerizing complexity, e.g., vast salp chains, bioluminescence and optical deception.
Salps have a fascinating life cycle. Like a lone warrior, a salp starts off a solitary life. In their solitary phase they are often referred to as oozoids5. After feeding and surviving well in pelagic waters, an oozoid utilizes its energy for asexual reproduction. The salp produces blastozooids, forming a chain of salps. A single oozoid can produce an army of efficient suspension feeders. Interestingly, salps start off as a female which then matures into a male. Because of this, young chains of blastozooids are fertilized by older chains during mating season 6. If successful, an attached viviparous oozoid embryo will develop in the fertilized female6. Thus, it renews the cycle of alteration between solitary and colonial phase5.
Surprisingly, when salps form vast aggregate chains, communication is not distorted. Try to imagine yourself as a professor, who attempts to communicate to hundreds of students during a lecture – clearly, there will be some degree of confusion between classmates. For salps, they use a bioluminescent cascade signal2. The first oozoid of the chain is the messenger. When the salp is going to communicate, it creates bioluminescence (which produces light) 2. Adjacent blastozooids capture the light signal with a specialized cell called a photoreceptor. This energy is then transformed to the next bastozooid. The process is repeated until the very last blastozooid receives the message. This elegant systematic way of communication prevents confusion and aids in cooperation2.
So, at the moment, we know that an oozoid can remarkably produce an army of blastozooids and communicate efficiently by light signals. However, what if I tell you that they are also masters of disguise, the stealth of the pelagic seas? Due to their small, gelatinous and clear body form, it is difficult for predators to see them7 – salps appear invisible like a ghost or a ninja. In addition, maybe they are even experts in physics? Since salps stay near to the surface, light polarizes8. This effect causes salps to appear cloaked within the background of the sea.
So, next time when you walk along a sandy beach, there might be glop of goo. However, if that glop of goo is a salp, don’t become deceived by the appearance because they are like a sea jelly in disguise, a master of survival, and a stealthy ninja in the sea!
1 Hereu, C. M., & Suarez-Morales, E. (2012). Checklist of the salps (Tunicata, Thaliacea) from the Western Caribbean Sea with a key for their identification and comments on other North Atlantic salps. Zootaxa, 3210, 50-60.
2 Mackie, G. O., & Bone, Q. (1977). Locomotion and propagated skin impulses in salps (Tunicata: Thaliacea). The Biological Bulletin, 153(1), 180-197.
3 Alldredge, A. L., & Madin, L. P. (1982). Pelagic tunicates: unique herbivores in the marine plankton. Bioscience, 32(8), 655-663.
4 Yount, J.L. (1954). The taxonomy of the Salpidae (Tunicata) of the Central Pacific Ocean. Pacific Science, 8(3), 276-330.
5 Deibel, D., & Lowen, B. (2012). A review of the life cycles and life-history adaptations of pelagic tunicates to environmental conditions. ICES Journal of Marine Science, 69(3), 358-369.
6 Lucas, C. H., & Dawson, M. N. (2014). What Are Jellyfishes and Thaliaceans and Why Do They Bloom? In Jellyfish blooms (ed. by K.A. Pitt and C.H. Lucas ), pp. 9–44. Springer, Dordrecht.
7 Hongli, L., Long, Z., Zhidong, T., & Yaolin, J. (2014). Dynamic Behaviors of Holling Type II Predator-Prey System with Mutual Interference and Impulses. Discrete Dynamics in Nature & Society, 2014, 1-13.
8 Sönke, J. (2011). Polarization sensitivity as a contrast enhancer in pelagic predators: lessons from in situ polarization imaging of transparent zooplankton. Philosophical Transactions of the Royal Society B: Biological Sciences, 366(1565), 655-670.