Viscous Eggs and Sperm May Slow Down Dilution

underwater photograph of red sea urchin on pebbles

Biological Strategy

Collector urchin

Jeanette Lim

Image: Paul Blair / Flickr / CC BY NC ND - Creative Commons Attribution + Noncommercial + NoDerivatives

Modify Material Characteristics

The materials found in living systems are variable, yet often made from the same basic building blocks. For example, all insect exoskeletons consist of a material called chitin. Because material resources are limited, each material within or used by a given living system must frequently serve multiple purposes. Therefore, living systems have strategies to modify materials’ softness, flexibility, and other characteristics. To ensure survival, the benefits of these modifications must outweigh the living system’s energy and material expenditure to generate them. For example, spiders store the liquid components of spider silk in a gland, converting them into silk thread when needed. Some threads have different characteristics, such as elasticity and UV reflectance, than others.

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Regulate Reproduction or Growth

Reproduction and growth are two physiological processes that occur in all living systems. There are situations when conditions are right for both, and other situations when continuing either harms the living system because both have a very high energy cost. Reproduction and growth are unique in that both can stop until conditions improve, although stopping either for an extended time can cause problems. An example of regulating reproduction is a process called delayed implantation or embryonic diapause found in some mammals, such as otters. An otter’s embryos sometimes temporarily cease developing and won’t develop further until the female senses that conditions are suitable.

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Expel Solids

Living systems must often discharge solids–such as seeds, eggs, pollen, and mineral salts–for reproduction, protection, or other reasons. Solids can be easily moved when a living system applies force to them. But creating that force requires energy, so living systems must either have efficient strategies worth the energy investment or use an outside force (such as gravity). For example, a tropical mistletoe positions its pollen so that the weight of a bird landing nearby triggers the pollen’s release onto the bird’s forehead. The bird then fertilizes other mistletoes that it visits.

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Distribute Solids

Solids include resources such as food, seeds, spores, and leaves, which living systems must sometimes move to accomplish various activities such as pollinating, feeding their young, or building shelters. To effectively distribute solids, living systems must use strategies appropriate to the sizes and types of solids and the media in which they exist (air, water, or another medium). This may entail strategies beyond simply holding and carrying solids. For example, some seeds are carried by the wind to new locations for germination. Others–like the burdock plant’s hooked seeds–cling to animal fur to be transported.

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Attach Temporarily

Living systems must sometimes, temporarily, stay in one place, climb or otherwise move around, or hold things together. This entails attaching temporarily with the ability to release, which minimizes energy and material use. Some living systems repeatedly attach, detach, and reattach for an extended time, such as over their lifetimes. Despite being temporary, these attachments must withstand physical and other forces until they have achieved their purpose. Therefore, living systems have adapted attachment mechanisms optimized for the amount of time or number of times they must be used. An example is the gecko, which climbs walls by attaching its toes for less than a second. Other examples include insects that attach their eggs to a leaf until they hatch, and insects whose wings temporarily attach during flight but separate after landing.

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Echinoderms (Sea Stars, Sea Urchins)

Phylum Echinodermata (“spiny skin”): Sea stars, brittle stars, sea cucumbers, sea urchins

Echinoderms get their name from their tough, spiny outer skin, which is clearly visible in some species, like sea urchins. They are known for their five-point radial symmetry and specialized water vascular system, which depends on changes in water pressure to extend or contract tiny tube feet. Pumping water through this system allows echinoderms to eat, move, or cling to surfaces, but it doesn’t generally enable very rapid movement. Some even have the ability to regrow body parts, which comes in handy if a limb is nibbled off by one of the ocean’s very fast-moving predators.

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Viscous eggs and sperm from sea urchins may slow down dilution and enhance fertilization rates by forming sticky clumps.

Sea urchins are spiny marine animals that reproduce through broadcast spawning: they gather together into groups where females release millions of eggs and males release billions of sperm into the open water where the two can mix, potentially meet, and fertilize.

The exact conditions that trigger spawning in different urchin species are as yet unknown, but environmental cues like water temperature and food availability appear to play a role in triggering group spawning in some urchin populations.

Once released from the urchin’s body, sperm and eggs are at the whim of the surrounding water and its motion. If they become too diluted from dispersing too rapidly and widely, the chance of fertilization could decrease dramatically. Some species of sea urchins appear to have strategies that can help control dispersion of eggs and sperm and potentially make fertilization more likely. For instance, the eggs and sperm are negatively buoyant, meaning they tend to sink. This may prevent them from being swept away too quickly when first released.

In addition, the eggs and sperm have high viscosity. This property affects how much substances stay together or disperse: high viscosity substances tend to stick together (e.g., glue or molasses), while low viscosity substances flow and separate easily. Because of their high viscosity, urchin eggs and sperm can form concentrated clumps or sticky strings that stay close to the urchin and gradually diffuse over time. This could increase the chance that sperm and eggs encounter each other. Furthermore, this viscosity can change depending on how masses of sperm or eggs flow. When flow speeds within a flowing stream of sperm or eggs vary greatly (which can happen when fluids travel through a narrow pipe, for example), their viscosity decreases. This behavior is called shear-thinning. For an urchin, having eggs or sperm be less viscous while they travel through a reproductive duct and out an external pore could mean that less energy is needed to expel the eggs or sperm from its body.

To see urchin spawning in action, check out this video from the Plankton Chronicles.

Image: Ratha Grimes / CC BY - Creative Commons Attribution alone

Image: Diver vincent / CC BY SA - Creative Commons Attribution + ShareAlike

Image: Bernard Picton / CC BY SA - Creative Commons Attribution + ShareAlike

This species of urchin, Echinus esculentus, was not included in the mentioned studies, but the general process of sperm release is similar among urchins.

Last Updated April 20, 2018

References

“Evidence reported in this paper suggests that the shear-thinning of gametes may provide a performance advantage for these sea urchins. High viscosity of gametes at low shear rates may decrease gamete dispersal upon release and, under certain flow conditions, allow gametes to form strings and clumps on the surface of the urchin. Depending upon the morphology of the surface, these clumps or strings may be retained and fertilization may occur within these clumps or strings. Conversely, low viscosity of gametes at high shear rates decreases the power required to extrude gametes through the gonoduct during spawning.” (Thomas 1994:263)

Journal article

Physical properties of gametes in three sea urchin species

Journal of Experimental Biology, 194: 263–284 | 01/09/1994 | Thomas FIM

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Reference

“Although dilute sperm are viable for only minutes (Chia and Bickell 1983), viable sperm continue to diffuse away from concentrated clumps for at least several hours (Meidel and Yund 2001).” (Yund and Meidel 2003:796)

Journal article

Sea urchin spawning in benthic boundary layers: Are eggs fertilized before advecting away from females?

Limnology & Oceanography, 48(2): 795–801 | 17/03/2003 | Yund PO; Meidel SK

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Reference

“…extensive spawning as shown by fertilization rates near 100% and a dramatic drop in gonad mass only occurred in the large natural population around the onset of thermal stratification and the spring phytoplankton bloom.” (Gaudette et al. 2006:1485)

Journal article

Spawning events in small and large populations of the green sea urchin Strongylocentrotus droebachiensis as recorded using fertilization assays

Limnology & Oceanography, 51(3): 1485–1496 | 15/05/2006 | Gaudette J; Wahle RA; Himmelman JH

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Reference

Journal article

Egg longevity and time-integrated fertilization in a temperate sea urchin (Strongylocentrotus droebachiensis)

The Biological Bulletin, 201(1): 84-94 | 01/08/2001 | Meidel SK; Yund PO

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