Have you seen a clam reproduce? We caught it on video during our study on the fertility on soft-shell clams.
Downeast Institute (DEI) scientists conducted an observational study on clam reproduction by examining fertility, or “fecundity”, of female clams. The study addressed six questions:
- What time of year do clams reproduce (spawn), and does it depend on location along the coast?
- What size are clams when they begin to reproduce?
- Are clams similar to lobsters, oysters, and sea urchins in that they keep reproducing into old age?
- Do large female clams produce more eggs than smaller females, or is it the other way around?
- Do female clams of different sizes produce eggs of different sizes?
- Does the tidal position where clams grow (high, mid, or low intertidal) affect clam reproduction?
Answering these key questions provides information shellfish managers can use to help the fishery adapt to warming ocean waters and the associated increased levels of predation (Beal et al. 2018, Beal et al. 2020).
Beginning in early May 2020, before seawater temperatures reached 50° F, and continuing through mid-July, different sized clams were removed from the high, mid and low tidal heights at a flat in southern Maine (Freeport) and one in downeast Maine (Jonesport) each week.
Comparing clam reproduction from these two distinct regions of the coast helps scientists understand if there are geographic differences in clam spawning. Comparing clams from the three tidal zones helped determine if spawning is affected by a clam’s location within the tidal gradient, since spawning may be influenced by the length of time seawater covers the clam.
After being removed from the flats, the clams were carefully transported to DEI in Beals, where they were induced to spawn using techniques that were developed in our shellfish hatchery over thirty years.
Once at DEI, the clams from one location were arranged by size in three rows (one for each tidal height) in preparation for the spawning process. To encourage clams to spawn, DEI simulated thermal shock, which is a natural process that also induces spawning in the wild.
First, the clams were bathed in cold, filtered seawater (55.4°F) drawn from the waters surrounding DEI near Black Duck Cove in Beals. Clams remained in the cold seawater bath for an hour, and were watched with flashlights for signs of spawning.
The tray was then drained and filled with warm seawater (75.2°F). The clams stayed at least two hours in the warm seawater bath, and were continually watched for signs of spawning. Clams spawn by releasing either eggs or sperm through their excurrent, or outgoing, siphon.
When female clams began to spawn they were removed from the tray and placed into a separate dish of seawater to ensure an accurate count of the eggs she produces. Scientist Dr. Brian Beal measured the volume of seawater containing the eggs.
Once the volume was determined and recorded, three seawater samples were taken and placed on a microscope slide. The total number of eggs from each was counted under a microscope. Egg size (diameter) was also measured.
The number of eggs in each of the three slides was averaged, and the original volume of seawater was factored in to estimate a total egg count for each clam. Technicians recorded the length of the clam, as well as the region, tidal height and date it was collected.
This data showed clam fecundity (number of eggs) and egg size for a given size clam over time, specific to each region and tidal height.
Initial results show that:
- Clams from the southern region started to spawn by May 27, when seawater temperatures were over 52°F, while eastern clams started to spawn by June 2, when seawater temperatures were over 48°F.
- Southern Maine clams spawned in a burst over four consecutive weeks, from 27 May to 17 June.
- During those four weeks, 55 females spawned and the eggs from each counted.
- Eastern Maine clams spawned over a more protracted time period, during ten consecutive weeks from 2 June to 3 August.
- During those ten weeks, we were able to induce 143 females to spawn.
- In both regions the peak of spawning occurred when seawater temperatures were 55°F.
- At peak spawning in Southern Maine, two-inch clams released an average of 2.1 million viable eggs, compared to three-inch clams which released 3.6 million eggs.
- At peak spawning in Eastern Maine, two-inch clams released an average of 1 million viable eggs, while the three-inch clams produced 2.6 million eggs.
The results from this 2020 study help us answer our six questions about clam fecundity:
1) Clams spawn in the late spring and early summer. Clams from southern Maine coastal communities begin to spawn earlier, by about two weeks, then clams from downeast Maine.
2) The smallest clam that was able to be induced to spawn was about 1.5 inches in length.
3) Clams are similar to lobsters, oysters, and sea urchins in that they keep reproducing into old age.
4) The larger the female clam, the more eggs she produces.
5) Different sized female clams do not produce eggs of different sizes. All clam eggs were about 70 microns, or about 0.003 inches, regardless of the size of the female.
6) The area of the mudflat (high, mid, or low intertidal) may affect clam reproduction. This research suggests that clams in the low intertidal (i.e. where they are covered by seawater longest) spawn first in a population.
Funding to support the research was provided by Maine Sea Grant.