Identifying and comparing native shrimps of NYC with a non-native Palaemon macrodactylus

Juan A. Mejia

Abstract - Palaemon macrodactylus is a native shrimp to Southeast Asia estuaries but recently it has been found in the estuaries of New York City (NYC) (Warkentine 2010). There are various differences that the Palaemon macrodactylus has when compared to Palaemonetes vulgaris, Palaemonetes intermedius, Palaemonetes pugio, and finally Crangon septemspinosa. Macrodactylus averaged a higher weight and length than the other shrimp that were measured which include P. vulgaris and Crangon, P. intermedius and P. pugio were not measured.

IntroductionThe first time that Palaemon macrodactylus was found in eastern US waters was

in 2001, 2002 (Warkentine 2010). But these P. macrodactylus shrimp have been in almost every continent and sea (Warkentine & Rachlin, 2012). They could be identified as different species because their anatomy is different. These differences lie around that one of the pereiopods (the 2nd pair) of macrodactylus were very long, extending well beyond the others. Measured and weighted macrodactylus came out to be very big when compared to the other shrimp that seemed small and tiny. One the clear examples are P. vulgaris next to P. macrodactylus. Besides their obvious physical differences a very important feature in distinguishing P. macrodactylus from other shrimps is the palp that is attached in their mandibles. Simple measurements were taking of the shrimp which involved measuring their length and them weighting them.

MethodsBarbara Warkentine and Joseph Rachlin collected the shrimps used in the summer

of 2014. The shrimps were differentiated using an Olympus microscope because the differences between them are not so obvious to the naked eye. The shrimp were kept in a 75% ethanol solution for preservation. When the shrimp were to be examined a petri dish was washed with water and then filled with a small amount of water, then the shrimp being examined was put into the dish and under the light microscope. High magnification was used to examine very small features that set the shrimps apart like the double row of setae on the ventral side of the rostrum of P. macrodactylus. The measuring of the length and weight were only done for C. septemspinosa, P. macrodactylus, and P. vulgaris. The length were measured using a ruler to the nearest 0.05 cm, placing the telson of the shrimp on the tip of the ruler that reads 0 cm, and then locating where the tip of the rostrum end up on the ruler. Not all shrimp in the jar they were kept on were measure because some had broken rostrums or broken middle edge. The weighting of the shrimp were done using a Mettler P163 balance to the nearest 0.001 g using weighting paper under the shrimp, the paper weight was then subtracted from the total weight to find out the weight of the shrimp only. The Log10 of the lengths and weights were taken and then plotted on a graph with a regression line. Fig. A includes the regression line of Crangon; Fig. B is macrodactylus, and Fig. C is vulgaris.

ResultsCrangon septemspinosa was observed to have a dorsally flattened body along

with a very small rostrum that barely extends from the eyes. It is also a feature of Crangon to have subchelate claws on the first pair of legs. Palaemonetes intermedius was observed to have four teeth under the rostrum that went all the way to the tip, and one dorsal tooth behind the eye socket. P. vulgaris was observed to be very similar to P. intermedius, but some differences is that P. vulgaris has two dorsal teeth on the rostrum behind the eye socket. P. Pugio was found to be very different from P. intermedius and P. vulgaris in that the rostrum did not have teeth that reached the tip and it had 2-3 teeth on the ventral side of the rostrum. Palaemon macrodactylus was different from the Palaemonetes group in that it had three dorsal teeth beyond the eye socket. Also one of the most noticeable features that were observed was that the second pair of pereiopods was notably larger than the others, sometimes twice as long. Also macrodactylus was observed to have 3-segmented palps on each mandible, which are located behind the maxilla and the maxilliped. Another unique feature that was observed was the double row of setae located on the ventral side of the rostrum, P. vulgaris only has one row of setae.

After the shrimps were measured and a regression line was made it was clear the relationship of length and mass of these animals. Figure A, showing the relationship in the C. Septemspinosa with an R2 of 0.97 and sample size being 26. Figure B is the graph for P. macrodactylus; it has the lowest R2, which comes out to be 0.89. It has a sample size of 38. Figure C has the regression line of P. vulgaris and also the highest R2 which is 0.99, it’s sample size is 9.

July 2014, P. Vulgaris July 2014, C. Septemspinosa

July 2014, P. Macrodactylus

Total in jar = 8 Total in jar = 29 Total in jars = 40

Measured (n=8) Measured (n=26) Measured (n=38)Average length= 2.68 cm Average length= 2.39 cm Average length= 3.85 cmAverage weight= 0.1744 g Average weight= 0.1627 g Average weight= 0.5659

Species Number of Shrimps % OccurrencePalaemon macrodacttylus 40 51.9Palaemonetes vulgaris 8 10.4Crangon septemspinosa 29 37.7Total 77

Figure A

Figure B

Figure C

DiscussionThe regression line of P. vugaris is the one with the highest R2 . One possible reason is the size of the sample size or another being that they are easy to measure lengthwise since the rostrum extends well beyond the eye sockets and it’s clear where the tip is located. C. Septemspinosa on the other hand was more difficult to measure because the rostrum is between the eyes and it’s hard to locate sometimes. Nevertheless the R2 is still very high which means that length and weight relationship is very rigid. Another thing is that most of the shrimps lie between length 2.00 and 2.50, which translates to Log length 0.3 and 0.4 respectively. Very few shrimps go beyond this mark, which could mean these shrimp experience a selection that promoted the higher survival rate of these sizes.

Figure B, which shows the regression line for the P. macrodactylus has the lowest R2, which is 0.89. The sample seems to stretch well all over the regression line; there’s no concentration of the shrimp to a range of size. This could be because there’s a good sample size that would fill in otherwise gaps if it were a small sample size. However the value of the R2 is only so because two different datas were merged together, they were taken a week separate from each other. The first data set for the P. macrodactylus was the first time I measured a shrimp so the data for that day could have come out to be not very representative of the overall sample, or population. On the other hand the second data set for P. macrodactylus was in fact taken and it had a very fitting regression line with an R2

of 0.96, as it can be seen in Figure D. When these two data sets were added together it averaged out the R2 value. This shows that measuring shrimp develops through time and that these’ shrimps weight are all too dependent on length.

Figure D

Bibliography

Warkentine, B. E., & Rachlin, J. (2010). The First Record of Palaemon macrodactylus (Oriental Shrimp) from the Eastern Coast of North America. Northeastern Naturalist , 91-102.Warkentine, B., & Rachlin, J. (2012). Palaemon macrodactylus Rathbun 1902 (Oriental Shrimp) in New York: Status Revisited. Northeastern Naturalist , pp. 173-180.