Unit 3: Life Cycles There are many different ways to classify marine animals. Some of the most basic have to do with how they develop as embryos; how they reproduce; and how their offspring develop. The embryos of only a few groups of animals, including humans, exhibit deuterostome development; we share this trait with such unlikely organisms as starfish and sea urchins! (Deuterostomes are also the only organisms that can have identical twins, because the fate of embryonic cells is indeterminate.) Marine organisms have also developed a wide variety of reproductive strategies to deal with the environmental conditions they face; for examples, if mates are scarce, or hard to reach, it might be better to be a simultaneous hermaphrodite: that way anyone you encounter is a possible mate. The offspring of marine organisms display a similar diversity of life stages, appearances, and behaviors. Because they exist in an environment that is wet, without the threat of desiccation, many marine organisms have external reproduction, releasing their gametes directly into the water; the resulting zygotes travel on the ocean currents from the moment of their conception. Many other organisms release free-swimming larvae that drift as part of the plankton for days or weeks before metamorphosing. For organisms that are slow-moving or sessile (unmoving or attached to a substrate) as adults, these drifting eggs and larvae provide the only means of dispersing their populations to other habitats.
Unit 3 Time Advisory
It should take you approximately 4 hours to complete this unit.
☐ Subunit 3.1: 0.5 hour
☐ Subunit 3.2: 2 hours
☐ Subunit 3.3: 1.5 hours
Unit3 Learning Outcomes
Upon successful completion of this unit, the student will be able to:
- Distinguish between protostome and deuterostome development: know which groups of animals go through which type of development and know some of the major features of each type of development. - Answer questions about different types of reproductive systems (including some of the organisms that use each) and demonstrate an understanding of the assumed advantages of each. - Answer questions about complex vs. simple life cycles, including the effects of these cycles on populations and dispersal potential.
3.1 Identify Protostome and Deuterostome Development
- Reading: Marietta College’s The Online Biology Book:
“Deuterostomes and Protostomes”
Link: Marietta College’s The Online Biology Book: “Deuterostomes
Instructions: Read this section on the differences between these two basic types of development.
3.2 Types of Reproductive Strategies
- Reading: MarineBio’s “Marine Life Cycles”
Link: MarineBio’s MarineBio: “Marine Life
Instructions: Read the entirety of this web page. It will cover the topics outlined in subunits 3.2.1-3.2.3 and 3.3.1.
3.2.1 Sexual vs. Asexual Reproduction 3.2.2 Planktotrophic vs. Lecithotrophic Larvae 3.2.3 Gonochorism (Two Sexes) vs. Hermaphroditism Note: This material was covered in the reading for subunit 3.2.
3.2.4 Sex Change
- Reading: Odyssey Expeditions: Jason Buchheim’s “A Quick Course in
Icthyology”: “Hermaphroditic Fish” and Davidson College: Professor
Aaron Rice’s “Terminology Relevant to Sex-Change”
Link: Odyssey Expeditions: Jason Buchheim’s “A Quick Course in
(HMTL) and Davidson College: Professor Aaron Rice’s “Terminology
Instructions: In “Ichthyology,” read the section on hermaphroditism in fish, focusing in particular on sex change. Please note that sex change occurs in a number of different marine species (including some snails, oysters, and shrimp), not just fish. Read the page on “Terminology” to understand the size-advantage hypothesis (originally put forth by Michael Ghiselin), the model that is accepted as the best explanation for the advantages of changing sex.
3.2.5 Parasitic Males
- Web Media: Sundance Channel: Isabella Rossellini’s “Green Porno”
Seasons 2 and 3 and “Seduce Me” Season 1
Link: Sundance Channel: Isabella Rossellini’s “Green
Porno” (Adobe Flash)
Seasons 2 and 3 and “Seduce
Me” (Adobe Flash)
Instructions: This is an optional resource. From the Green Porno webpage, scroll down to the right-hand tabs that give you a choice of watching episodes of “Seduce Me” Season 1 or “Green Porno – All.” Watch these clever, scientifically accurate, and raunchy episodes on squid, shrimp, anchovy, elephant seals, starfish, whales, anglerfish, barnacles, salmon, and cuttlefish. Please note, however, that the episode on “Limpets” confuses true limpets (which look like the ones in the video but do not stack) with slipper-shell limpets (also called slipper-shell snails, genus Crepidula), which are not closely related to true limpets but do pair or stack up and also change sex.
- Reading: Palomar College: Professor W. P. Armstrong’s Wayne’s
Word, An On-Line Textbook of Natural History: “Mysterious Males of
the Deep-Sea Anglerfish”
Link: Palomar College: Professor W. P. Armstrong’s Wayne’s Word, An
On-Line Textbook of Natural History: “Mysterious Males of the
Instructions: Read this section (“Mysterious Males”) to learn about another reproductive strategy employed by organisms living in environments where other individuals are very scarce.
3.2.6 External vs. Internal Fertilization
- Web Media: National Geographic’s “Coral Reef Spawning”
Link: National Geographic’s “Coral Reef
Instructions: Watch this short (3 minutes) video to see external fertilization in action and also learn about the adaptations of organisms to ensure reproductive success, even as they cast their sperm and eggs upon the waters.
3.3 Types of Life Cycles 3.3.1 Simple and Complex Life Cycles Note: This topic is covered in the reading under subunit 3.2.
3.3.2 Planktotrophic vs. Lecithotrophic Larvae - Reading: The Saylor Foundation’s “Planktotrophic vs. Lecithotrophic Larvae” The Saylor Foundation does not yet have materials for this portion of the course. If you are interested in contributing your content to fill this gap or aware of a resource that could be used here, please submit it here.