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Clams

 

Hard Clams

 

Anatomy

 

Life Cycle

 

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 The  life cycle of the hard calm                      Mercenaria mercenaria

In general, most calms reproduce in wild similar the Northern Quahog

Introduction

The hard clam can live up to 30 years and grow to (on average) 4 inches in length. 

 But, like all living things, they have to start somewhere.

Hard clams begin their adult lives as males.  In successive years they may change their sex and produce eggs.

Sperm cells are much smaller than eggs and require less metabolic energy to produce.  Larger clams grow at a much slower rate so more metabolic energy can be devoted to the production of sex cells, thus the switch to the larger eggs.

By the time hard clams reach harvestable size, there is about a 1 to 1 ratio of males to females.

Reproduction

In the spring when the bay water temperature rises above 50°F,  clams come out of "hibernation" and begin to feed on available phytoplankton.  In late spring into early summer when the water temperature rises above 68°F the adults begin to spawn.

The males release sperm and females release eggs through their excurrent siphons into the bay. (left image)

Fertilization takes place in the water column as sperm and egg unite.  (right image)

As the fertilized egg develops the cells divide and the

 embryo grows.

Pictured from left to right are:

  The first cell division, the four-cell embryo and

  the eight cell embryo.

 

As the embryo grows, the next stage is a ball of cells called the morula

 

It is derived from the Latin word for mulberry, which it resembles.

With further development we see the growth of hair-like cilia and the embryo becomes a free-swimming larva known as a trocophore

The image on the left depicts the early larval stage while the image on the right shows the fully developed larva

Development to this stage could take from 18 to 48 hours depending upon water temperature

Next, the larva begins to develop a shell and foot. In this stage, the larva is called a veliger.

The D-shaped or strait-hinged veliger (left image) is so called because the body side where the hinge is forming is straight while the open valve side is rounded.

The larval shell of the of the umbonate veliger (right image) has the characteristic triangular shape of the hard clam.

The last phase of the veliger stage is called the The pediveliger (ped = foot) stage.

This is the final stage prior to settlement and eventual metamorphosis to juveniles.

Pediveligers have a well-developed foot that extends from the shell.

Finally, as a juvenile, the calm has now grown enough and is ready to continue life in the bay bottom.  The weight of the shell prevents drifting and the clam settles to the bottom (sets) and uses its foot to dig in and extends its siphons to feed.

Development to this stage from the trocophore stage may last from 8 days to 2 weeks depending upon water temperature.

Fun Fact - In New Jersey, the minimum size of hard clams that may be harvested is 1 1/2 (1.5) inches in length.  Clams can reproduce at about 1 to 1 1/4 inch in length which gives them "protected time" to breed.

Fun Fact - The hard clam is active and grows in water temperature between 50° and 85° Fahrenheit.  Above or below those temperatures they shut down (hibernate).

 



Credits

Rhode Island Sea Grant http://seagrant.gso.uri.edu/G_Bay/HabitatEco/Shellfishing/quahog_dev.html

The Northern Quahog, The biology of Mercenaria mercenaria by Michael A. Rice