Bio Lab: An Introduction to copepods

Plankton are defined by their habit, not by their species. A traditional definition: an aquatic organism that cannot swim against the current.
- They are typically small, often microscopic, and cannot make progress against currents in their body of water.
- In this course, we focus on saltwater plankton.
Plankton are classified into two major groups by how they obtain food:
- Phytoplankton: autotrophs that photosynthesize. Common examples include algae and diatoms. They form the basis of the oceanic food chain and play a critical role in life on Earth.
- Photosynthesis converts solar energy into carbon-containing compounds (e.g., glucose).
- There are so many phytoplankton in the oceans that they account for about $50\%$ of the carbon-containing compounds on the planet.
- Zooplankton: consumers that obtain energy by eating other plankton. Some feed on phytoplankton; others feed on other zooplankton.
- Includes organisms such as copepods (which spend their whole life as plankton) and crab larvae (which are planktonic only when young).
- Zooplankton serve as a food source for larger organisms like fish, jellyfish, and blue whales.
Images referenced (for context): PHYTOPLANKTON, ZOOPLANKTON, OSTRACOD, COPEPOD.

The species studied this semester is Tigriopus californicus, a zooplankton copepod in the Family Harpactidae.
Classification hierarchy (from broad to specific):
- Domain: $Eukarya$
- Kingdom: $Animalia$
- Phylum: $Arthropoda$
- Subphylum: $Crustacea$
- Class: $Maxillopoda$
- Order: $Harpacticoida$
- Family: Harpacticidae
- Genus: Tigriopus
- Species: californicus
Copepods are arthropods in the domain Eukarya (constructed of eukaryotic cells) and the kingdom Animalia.
The Phylum Arthropoda translates to "jointed-foot"; key features include an exoskeleton and multi-jointed legs. Arthropods are the largest group of organisms on the planet.
Crustaceans (subphylum Crustacea) have especially hard outer shells.
Copepods in the Class Maxillopoda are typically small crustaceans that feed with modified head appendages called maxillae.
Order Harpacticoida: all species are copepods, with the majority living on or near the bottom; however, Tigriopus californicus lives in the water column.
Harpacticoids are characterized by short first antennae and a major joint halfway down the body, giving flexibility.
Taxonomic note: T. californicus is currently listed in the Family Harpacticidae, but this classification is under debate; many Harpacticidae members are freshwater-adapted, and T. californicus may be reclassified in the future.
Summary description: small animal with hard exoskeleton, jointed legs, head-based feeding appendages, short first antennae, and a major body joint.

Geographic range: commonly along the coast of California; found along the western United States from the Baja Peninsula to southern Alaska.
Typical habitat: tide pools along the shoreline.
Tide pools: bodies of water that remain after the tide recedes; often on rocky shores where rocks form depressions that hold water.
Environmental characteristics of tide pools:
- Stressful due to rapid fluctuations in temperature and salinity as water volume changes.
- Temperature and salinity can change quickly with evaporation or rainfall, especially when the tide recedes and later returns.
Human-caused stressors in tide pools:
- Run-off of pesticides and fertilizers from nearby agriculture can alter chemical composition.
- Oil spills and beach pollutants can affect the ecosystem.
Implication: tide pool organisms must be highly resilient to a wide range of environmental stressors.

Reproduction begins with females carrying eggs in an external egg sac attached to the body.
Nauplius stage: the egg hatches into a Nauplius, which looks very different from the adult.
Naupliar progression: there are 6 naupliar stages (N-I through N-VI).
- Each naupliar stage lasts about $1-2\ \text{days}$ .
- Each stage adds one or two segments to the abdomen and/or increases body size.
Transition to copepodid stages: after N-VI, the nauplius metamorphoses into the copepodid stage.
Copempodid stages: there are 6 copepodid stages (C-I through C-VI), after which the individual reaches the adult copepod stage.
- Each copepodid stage lasts about $2-4\ \text{days}$ , dependent on environmental conditions (details in the literature are limited).
- As stages advance, more appendages appear and more body segments are added.
Sex determination cue: by the 5th copepodid stage (C-V), sex can be determined; some literature suggests it may be distinguishable as early as the C-IV stage.
Sexual dimorphism: males have larger antennules and a larger body size than females.
Overall note: while much is known about similar copepod species, a complete stage-by-stage description for T. californicus (size changes, precise morphology per stage) is not fully documented.
Visual reference: general life cycle diagram provided for Calanoid copepods (note: similar order but not identical to T. californicus).

Reproductive challenge: internal fertilization is difficult due to a hard external exoskeleton.
Solution: mating strategy involves timing with molts:
- Males locate females in the later copepodid stages and hold onto them until after the female molts and the new exoskeleton is soft.
- Once the female molts to the adult stage, she mates with the male and stores the sperm for multiple egg clutches over her lifetime.
- The male, after releasing the female, moves on to seek another mate.
Mating system characteristics:
- A female’s sole mate is often a single male; males may mate with multiple females over a short period.
- Burton (1985) found that males mate with, on average, $2.5$ females in $72\ \text{hours}$ .
Open questions and findings:
- How does the male choose a particular female? What are the costs of holding onto a female?
- Is there competition among males for mates?
- In some copepod species, males preferentially mate with the oldest females to maximize mating opportunities; some species can recognize sisters and avoid mating with them.
- Preliminary data from Dr. Fisher's lab at UNC suggests male T. californicus prefer older females and avoid mating with siblings, mating with non-siblings instead. These findings are preliminary with small sample sizes