G7 How Many Arms Do Starfish Have?_02

Bella’s eyes began to fill with tears. It was not the first time she had a castle swept away by the ocean, but it did not stop the waterworks that fell from her eyes. “I guess we can start another one further up on the beach,” Jaime said.

Andre had his arm around Bella in efforts to comfort her and make a plan on what to do next to stop her tears, then he felt her wiggle away from him and lunge toward what was left of their castle. “What is that?” We all looked in the moat as the water washed away and saw what appeared to be a three-armed starfish. Jaime said, “Look! There are two more tiny limbs that are growing”.

Bella looked confused, after all how can starfish have arms that are all different sizes.

Andre was pretty sure Jaime was a year or two older than him, so he paused to see if he knew how such a thing was possible. Jaime quickly spoke up; “We learned about this last year, and it is pretty interesting stuff.” Jaime continued, “It’s one of the ways it reproduces.” Jaime’s class had talked about starfish and their ability to reproduce sexually through meiosis and asexually through mitosis.

The starfish that washed up currently has one small arm where you would expect to see an arm the same length as the other four. What you are witnessing is the starfish’s ability to regenerate. In order to do so, they undergo cell division. Cell division is a cellular reproductive process where cells divide. There are two ways that cells divide. One way is through meiosis (sexual) and the other through mitosis (asexual). This asexual reproduction is what occurs in starfish when they lose limbs and allows them to regenerate new limbs as needed.

Mitosis explains how we grow from a tiny baby into a full-sized adult. It also explains how our body can repair itself. No, we can’t grow a new arm, but a cut will heal, and a broken bone will grow back together. The production of new cells in such instances is achieved by mitosis, an asexual process.  Unlike meiosis, mitosis undergoes only one round of cell division instead of the two rounds found in the meiosis process. Within the cell division process, mitosis produces 2 identical daughter cells during the division process. Asexual reproduction is a type of reproduction which does not involve the fusion of gametes or change in the number of chromosomes. The daughter cells that arise by asexual reproduction from a single cell or from a multicellular organism inherit the genes of that parent. Mitosis explain how a starfish can regrow a lost arm.

Starfish are also able to reproduce sexually through meiosis. Meiosis is a process where a single cell divides twice to produce four cells (egg in a female and sperm in a male), each containing half the original amount of genetic information. This division produces gametes, or sex cells, which are the sperm (in a male cell) and egg cells (in a female cell). During meiosis one cell divides twice to form four daughter cells. Meiosis occurs in both plants and animals. This process passes the genetic information from one generation to the next. It explains why you look like your parents and not like the family dog.

Andre and Bella never realized starfish were so interesting. They knew that they needed energy from food and even that they had a way to repair their bodies but understanding how it works is a whole different story. Bella couldn’t help but wonder what other animals and plants can reproduce both sexually through meiosis and asexually through mitosis. 

With the waterworks at an end and new understandings of science in hand, they walked back to where their parents had established camp. It was time for lunch, which of course is when Bella explained to everyone about their morning building sandcastles and learning about how some animals can reproduce. Andre’s Mom looked at him and smiled. “Thank you,” she said softly, acknowledging Andre’s efforts in providing a fun day for his little sister.  Andre nodded and explained he too had a good time building sandcastles and he also learned a lot about the science of reproduction from his new friend, Jaime.